Compendium Pleasant Current Events Williams, Linear Technology Corpora

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Compendium Pleasant Current Events Williams, Linear Technology Corpora

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Application Note April 2002 Current Sources Fiber Optic Lasers
Compendium Pleasant Current Events Williams, Linear Technology Corporation INTRODUCTION large group fiber optic lasers powered current. Laser drive supplied current source with modulation added further along signal path. current source, although conceptually simple, constitutes extraordinarily tricky design problem. There number practical requirements fiber optic current source failure consider them cause laser and/ optical component destruction. Design Criteria Fiber Optic Laser Current Sources Figure shows conceptual laser current source. Inputs include current output programming port, output current clamp enable command. Laser current sole output. This block diagram deceptively simple. practice, laser current source must meet number practical requirements, some quite subtle. successful design thorough understanding individual system requirements. Various approaches suit different sets freedoms constraints, although must address some basic concerns.
IOUT PROGRAM TYPICALLY 2.5V IOUT CLAMP TYPICALLY 2.5V ENABLE (THIS FUNCTION BUILT INTO CIRCUIT BLOCK) IOUT LASER TYPICALLY 250mA 2.5A
Protection features must included prevent laser optical component damage. laser, expensive delicate device, must protected under conditions, including supply ramp down, improper control input commands, open intermittent load connections "hot plugging." Detailed Discussion Performance Issues useful expand above cursory discussion clarify design goals. such, each previously called issue treated greater detail below.
Required Power Supply available power supply should defined. single rail supply presently most common desirable. Supply tolerances, typically ±5%, must accounted for. System distribution voltage drops result surprisingly rail voltages point load. Occasionally, split rails available, although this relatively rare. Additionally, split rail operation complicate laser protection, particularly during supply sequencing. "Laser Protection Features" below additional comment. Output Current Capability power lasers operate less than 250mA. Higher power types require 2.5A. Output Voltage Compliance Current source output voltage compliance must able accomodate laser's forward junction drop additional drops drive path. Typically, voltage compliance 2.5V adequate.
AN90
Figure Conceptual Laser Current Source Deceptively Simple. Practical System Issues Laser Vulnerability Necessitate Careful Design
There basic sets concerns laser current sources: performance protection. Performance issues include current source's magnitude stability under conditions, output connection restrictions, voltage compliance, efficiency, programming interface power requirements.
registered trademarks Linear Technology Corporation.
AN90-1
Application Note
Efficiency Heat build fiber optic systems often concern space limitations. Accordingly, current source efficiency issue. current, linear regulation often adequate. Switching regulator based approaches necessary higher current. Laser Connection some cases, laser float ground; other applications require grounded anode cathode operation. Grounding anode seemingly mandates negative supply single rail operation retained switching regulator techniques employed. Output Current Programming Output current programming port voltage. voltage derived from potentiometer, filtered PWM. Typically, range 2.5V corresponds 250mA 2.5A. point accuracy usually within 0.5%, although better tolerances readily achievable. Output current stability, discussed below, considerably tighter. Stability current source should well regulated against line, load temperature changes. Line load induced variations should held well within 0.05%, with typical temperature drifts 0.01%. Judicious component choice considerably improve these figures. Noise Current source noise, which modulate laser output, must minimized. Typically, noise bandwidth 100MHz interest. linearly regulated current source inherently noise usually presents problems. Switching regulator based current sources require special techniques maintain noise. Transient Response current source does need fast transient response cannot overshoot programmed current under circumstances. Such overshoots damage laser associated optical components.
Detailed Discussion Laser Protection Issues
Overshoot noted above, outputs overshooting nominal programmed current destructive. possible combination improper control input power supply turn characteristics must accounted for. Also, spurious laser current under condition impermissible. Note that portions current source circuitry have undesired unpredictable responses during supply ramp up/down, complicating design. Enable enable line allows shutting current source output off. enable line also used hold current output during supply ramp preventing undesired outputs. This tricky because enable signal circuitry powered same supply that runs laser. enable signal must reliably operate independent power supply turn-on profile. Optionally, enable function self-contained within current source, eliminating necessity generate this signal. Output Current Clamp output current clamp sets maximum output current, overriding output current programming command. This voltage controlled input potentiometer, filtered PWM. Open Laser Protection unprotected current source's output rises maximum voltage load disconnected. This circumstance lead "hot plugging" laser, potentially destructive event. Intermittent laser connections produce similar undesirable results. current source output should latch load disconnects. Recycling power clears latch only load been established.
preceding discussion dictates considerable care when designing laser current sources. delicate, expensive load, combined with uncertainties noted, should promote aura thoughtful caution.1 following circuit examples (hopefully) maintain this outlook while simultaneously presenting practical, usable circuits. variety approaches shown, keeping with broad area
Note "For Fools Rush Where Angels Fear Tread." essay criticism, Pope. 1711.
AN90-2
Application Note
application. designs directly utilized serve starting points specific cases. Basic Current Source Figure basic laser current source, supplies 250mA This circuit requires both laser terminals float. amplifier controls laser current maintaining shunt voltage potential dictated programming input. Local compensation amplifier stabilizes loop 0.1µF capacitor filters input commands, assuring loop never slew limits. This precaution prevents overshoot programming input dynamics. enable input turns current source simultaneously grounding Q1's base starving amplifier's input while biasing input high. This combination also insures amplifier smoothly ramps desired output current when enable switches low. enable input must addressed external "watchdog" which switches after power supply been verified within operating limits. Because external circuitry operate from same supply current source, enable threshold threshold assures enable input will dominate current source output supply voltages during power turn This prevents spurious outputs unpredictable amplifier behavior below minimum supply voltage. High Efficiency Basic Current Source preceding circuit uses Q1's linear regulation close feedback loop. This approach offers simplicity expense efficiency. Q1's power dissipation approach under some conditions. Many applications permit this some situations require heating minimization. Figure minimizes heating replacing with step-down switching regulator. switched mode power delivery eliminates almost transistor's heat. figure shows similarities Figure linear approach, except LTC1504 switching regulator's addition. useful liken switching regulator's input (VCC), feedback (FB) output (VSW) transistor's collector, base emitter. This analogy reveals circuits have very similar operating characteristics, with switched mode version enhancing efficiency. LTC1504's output filter introduces phase shift, necessitating attention loop compensation. amplifier's local rolloff similar Figure although phase leading feedback elements (0.01µF 0.033µF capacitors) required good loop damping. other respects, including enable programming input considerations, this circuit's operation identical Figure Grounded Cathode Current Source Figure allows laser's cathode grounded, sometimes required, sensing anode current. utilizes device with 500mA output capability programmable output current limit. senses output current across shunt, with limiting controlled circuit's current programming input. unity-gain follower with respect laser, allowing positive input serve laser voltage clamp input. laser voltages below VCLAMP input, appears current source, controlled current programming
INPUT 2.5V 250mA 10k* 0.1µF 80.6K* 2N3904 ENABLE
10K* LT1006
FZT-849
100k
1000pF
METAL FILM RESISTOR 1N4148 LASER
AN90
Figure Basic Current Source Requires Off-Ground Operation Laser Terminals. Amplifier Controls Current Comparing Shunt Input. Biasing Enable Until Supply Verified Prevents Spurious Outputs
AN90-3
Application Note
22µF
ENABLE
2N3904
SHDN
LTC1504A
IMAX
100µF
METAL FILM RESISTOR 47µH, SUMIDA CD54-470 1N4148
0.01µF
LASER
0.033µF
LT1006 0.02µF
AN90
INPUT 2.5V 250mA
10k* 0.1µF
80.6k*
10k*
Figure Switching Regulator Replaces Figure Providing Higher Efficiency. Feedback Control Enable Input Considerations Before
ENABLE INPUT 0VIN 2VIN 250mA VSINK VCLAMP 0VIN 2.5VIN 0VOUT 2.5VOUT LT1970 1N5817 VSRC SENSE SENSE
1N4001 2N3904 FZT-749 LASER 1N4148
AN90
Figure LT1970 Power Ampifier/Current Source Permits Grounding Laser Cathode, Although Requiring Split Rails. Appropriate Modifications would Allow Grounded Anode Operation. Enable Input Must Biased Until Supplies Verified
AN90-4
Application Note
input's setting. laser voltages equaling above VCLAMP input, voltage source, controlled VCLAMP's value. This permits VCLAMP input limit maximum voltage across laser terminals. enable function operates similarly previous descriptions capacitors restrict output movement safe, well damped speeds. diode shunting laser prevents reverse bias during power supply sequencing. 1N5817 protects against uncontrolled positive outputs negative supply drops sequences slowly. This circuit's simplicity laser connection versatility (appropriate modifications permit grounded anode operation) attractive A1's negative supply requirement detrimental. negative supply complicates external "watchdog" circuitry required enable input. worst case, simply available host system. Single Supply, Grounded Cathode Current Source Figure preserves Figure grounded cathode operation while operating from single supply. This circuit reminiscent Figure with notable exception. Here, differential amplifier senses across shunt laser anode, permitting cathode grounding. A2's gain-scaled output feeds back loop closure. Loop compensation enable input considerations related previous examples and, before, could replaced with switching regulator. Fully Protected, Self-Enabled, Grounded Cathode Current Source Figure elements repeated Figure additional comment them necessary. However, three features appear, allowing this circuit operate fully protected self-contained fashion. circuit monitors power supply "self-enables" when supply within limits, eliminating "enable" port external "watchdog" previously required. settable current clamp open laser protection prevent laser damage. self-enable designed around LT1431 shunt regulator. highly desirable property maintaining predictable open collector output when operating below minimum supply voltage. initial turn supply voltage very (e.g., 1V), LT1431's output does switch current flows Q3's base. turns preventing Q1's base from receiving bias. Additionally, circuit's current programming input pulled down A1's input driven. This arrangement ensures that laser cannot receive current until turns off. Also, when does off, A1's output will cleanly ramp desired programmed current. resistor values LT1431's "REF" input dictate device will when VSUPPLY passes through This potential ensures proper circuit operation. Supply start-up waveforms appear Figure Trace nominal rail, ramps before arriving During this interval, LT1431
INPUT 2.5V 250mA
0.02µF
LT1006
FZT-849
0.02µF ENABLE
2N3904
METAL FILM RESISTOR 1N4148 1N5712 LASER 22.2k*
LT1789
AN90
Figure Differentially Sensed Shunt Voltage Allows Grounded Cathode Laser Drive with Single Supply. Loop Enable Input Considerations Derive from Previous Figures
AN90-5
Application Note
INPUT 2.5V 250mA 4.7k CLAMP ADJUST LT1004 2.5V 2.5k 0.02µF
LT1013
FZT-849
1.5k* 2.4k*
0.02µF COLL
LT1013
2N3906
2N3904
CURRENT CLAMP LT1789 22.2k* MBR0520
LT1431 1N4001 SELF-ENABLE 2.5V
1000pF 2N5060 METAL FILM RESISTOR 187k* 1N4148 1N5712 LASER 100k* LTC1696
0.1µF
AN90
OPEN LASER PROTECTION
Figure Figure Circuit, Augmented with "Self-Enable," Monitors Power Supply, Operates when VSUPPLY Current Open Laser Clamps Protect Laser
(trace follows ramp, biasing A1's output (trace uncontrolled during this period, Q1's emitter (trace however, Q3's conduction cannot pass disturbance. result, laser conducts current (trace during this time. When supply (trace ramps beyond (just before photo's vertical division), LT1431 switches (trace switches circuit "self-enables." A1's output (trace ramps with Q1's emitter (trace laser current (trace slaved movement. This action prevents undesired current laser during supply turn regardless unpredictable circuit behavior supply voltages. Supply turn only time laser current must controlled. Response programming input changes must similarly well behaved. Figure shows laser current response (trace programming input step (trace Damping clean, with hint overshoot. circuit also includes open laser protection. current source operates into open load laser), will
produce maximum voltage laser output terminals. This circumstance lead "hot plugging" laser, potentially destructive event. Intermittent laser connections produce similar undesirable results. LTC1696 overvoltage protection controller guards against open laser operation. This device's output latches high when feedback input (FB) exceeds 0.88V. Here, biased that laser output voltage exceeding 2.5V forces LTC1696 high, triggering shunt current away from laser. resistor supplies holding current diodes insure current flows output. Figure details events with properly connected laser supply turn Trace supply, trace laser voltage, trace LTC1696 output trace laser current. waveforms show laser voltage (trace rising about supply turn (trace Under these normal conditions, LTC1696 output (trace stays laser current (trace rises programmed value.
AN90-6
Application Note
Figure shows what happens when circuit turned into open laser connection. Trace assignments identical previous photo. supply turn (trace laser voltage (trace transitions beyond 2.5V open laser threshold. LTC1696 output (trace goes high, latches current flows shunted laser line (trace Once this occurs, power must recycled reset LTC1696-SCR latch. laser been properly connected, circuit will repeat protective action. Open laser protection restricted turn will also laser connection lost time during normal circuit operation. final protection feature Figure current clamp. prevents uncontrolled programming inputs from being transmitted clamping them settable level. associated components form clamp. Normally, A2's input above circuit's programming input (Q2's emitter voltage), A2's output high off. programming input exceeds A2's input level, swings low, comes amplifier feedback controls Q2's emitter "clamp adjust" wiper potential. This clamps A1's input "clamp adjust" setting, preventing laser current overdrive. Clamp action need
5V/DIV 5V/DIV 2V/DIV 1V/DIV
2V/DIV
50mA/DIV
20mA/DIV
1ms/DIV
AN90
1ms/DIV
AN90
Figure Figure Waveforms During Power Supply Application (Trace Traces LT1431 Outputs, Respectively. Q1's Emitter (Trace Provides Power Gain. Feedback Sets Laser Current (Trace Self-Enable Circuit Prevents Spurious Outputs (Trace During Supply Ramp from Corrupting Laser Current (Trace
Figure Figure Output (Trace Responding Trace Input Step. Trace Laser Current Controlled Damping, Overshoot
5V/DIV 5V/DIV 2V/DIV 2V/DIV 5V/DIV 5V/DIV 100mA/DIV 1mA/DIV 500µs/DIV
AN90
500µs/DIV
AN90
Figure Figure Open Laser Protection Does During Normal Turn Trace Supply, Trace Laser Voltage, Trace LTC1696 Output Trace Laser Current. LTC1696 Overvoltage Threshold Exceeded, Unbiased (Trace Laser Conducts Current (Trace
Figure Open Laser Protection Circuit Responding Open Laser Turn Trace Assignments Identical Previous Figure. Laser Line (Trace Excursion Beyond Overvoltage Threshold Causes LTC1696 Output (Trace Biasing Clamp Open Laser Line. Current Flows Laser Line, Trace (Note 100x Increase Measurement Sensitivity Figure
AN90-7
Application Note
particularly fast effective, because A1's 10k0.02µF input filter. Figure 11's traces show clamp response programming input overdrive. When programming input (trace exceeds clamp's preset level, Q2's emitter (trace does same, causing A2's output (trace swing down. feedback controls Q2's emitter clamp level, arresting voltage applied 10k-0.02µF filter. filter band limits abrupt clamp operation, resulting smooth corner A1's positive input (trace A1's clamped input dictates similarly shaped clamped laser current (trace clamp remains active until programming input falls below "clamp adjust" setting. forms detail dynamic response. Trace input step arrives filtered form A1's positive input (trace loop produces trace faithfully profiled laser current output. shown, circuit externally controlled enable function, although Figure "self-enable" feature used. Similarly, Figure current clamp open laser protection employed this circuit. This circuit's switched mode energy delivery provides high efficiency high power output noise issue. Residual harmonic content related switching regulator operation appears laser current. resultant level modulation laser output troublesome some applications. Figure shows about 800µAP-P switching regulator related noise laser current output.2 This disturbance composed fundamental ripple switching transition related harmonic. This 0.05% noise below most optical system requirements, although following circuit achieves substantially lower noise figures. 0.001% Noise, Grounded Cathode Current Source
500µs/DIV
AN90
2V/DIV
1V/DIV 5V/DIV 1V/DIV 100mA/DIV
Figure Figure Current Clamp Reacting Programming Input Overdrive. Waveforms Include Programming Input (Trace Emitter (Trace Output (Trace Input (Trace Laser Current (Trace When Programming Input Exceeds Clamp Threshold, Swings Abruptly (Trace Causing Q2's Emitter (Trace Clamp. A1's +Input (Trace Remains Clamp Level, Maintaining Safe Laser Current (Trace
2.5A, Grounded Cathode Current Source Figure derived from Figures provides 2.5A grounded cathode laser. control amplifier, output current efficiently delivered LT1506 switching regulator senses laser current shunt. Loop operation similar descriptions given Figures with feedback coming from Frequency compensation differs from previous figures. Stable loop operation achieved local roll augmented lead networks associated with Midband lead provided feeding back lightly filtered (1k-0.47µF) version LT1506 output activity. High frequency lead, arriving 3300.05µF pair, optimizes edge response. Figure 13's wave-
previous circuit's 0.05% noise content suits many optical system applications. More stringent requirements will benefit from Figure 15's extremely noise content. This grounded cathode, circuit only 20µAP-P noise, about 0.001%. Special switching regulator techniques used attain this performance. Substantial noise reduction achieved limiting edge switching speed regulator's power stage.3 Voltage current rise times switches controlled LT1683 pulse width modulator. LT1683's output stage operates local loops which sense control their edge times. Transistor voltage information back 4.7pF capacitors; current status derived from 0.033 shunt also back. This arrangement permits control chip transistor switching times, regardless power supply load changes. transition rates resistors (RVSL RCSL) associated with
Note Noise contains regularly occurring coherent components. such, switching regulator output "noise" misnomer. Unfortunately, undesired switching related components regulated output almost always referred "noise." Accordingly, although technically incorrect, this publication treats undesired output signals "noise." Reference Note Reference details this technique.
AN90-8
Application Note
0.12µF 0.68µF 0.47µF 0.05µF
BOOST LTC1506
15µH
MBR330 300µF
FROM OPTIONAL OPEN LASER PROTECTION CIRCUITRY OUTPUT. FIGURE
1000pF
LT1006
INPUT 2.5V 2.5A
0.33µF
2N3904
ENABLE
OPTIONAL CURRENT CLAMP FIGURE METAL FILM RESISTOR COILTRONICS UP2C-150 1N4148 1N5712, UNLESS NOTED LASER
LT1789
OPTIONAL SELF-ENABLE FIGURE
AN90
22.2k*
Figure Switched Mode Version Figure 2.5A Output. Feedback Loop Compensation Accomodates Switching Regulator Delay. Clamp, Protection Self-Enable Circuits Optional
5V/DIV 1mA/DIV COUPLED LEVEL
0.5V/DIV 0.5A/DIV 10ms/DIV
AN90
500ns/DIV
AN90
Figure 2.5A Current Source Waveforms Input Step (Trace A1's Input Filter (Trace Smooths Step, Resulting Trace Similarly Shaped Laser Current
Figure High Power Current Source Noise Includes Switching Regulator Fundamental Ripple Harmonic Content. 800µAP-P Noise About 0.05% Output
AN90-9
Application Note
THIS POINT
33µF
10µF LT1054 VOUT
68µF
LT1683 GATE RVSL RCSL GATE PGND 0.033 SHDN 1N5712 0.01µF FROM OPTIONAL OPEN LASER PROTECTION OUTPUT. FIGURE 1000pF 4.7pF 33µF 4.7pF 22µH 22µH 1000pF
3.9k SYNC 1200pF
330µF
330µF
0.02µF
1000pF METAL FILM RESISTOR COILTRONICS UP-2B 0.01µF COILTRONICS VP3-0780 1N4148 MBR330, UNLESS NOTED LASER 49.9k* 49.9k*
10k*
LT1006
ENABLE
OPTIONAL SELF-ENABLE FIGURE
SANYO OS-CON 2N3904 OPTIONAL CURRENT CLAMP FIGURE
0.02µF
AN90-10
AN90
IRLZ34
Figure 0.001% Noise, Laser Current Source Grounded Cathode Output. Clamp, Protection Self-Enable Circuits Added
Application Note
LT1683 controller. practice, these resistor values adjusting them minimize output noise. remainder circuit forms grounded cathode laser current source. drive whose rectified output filtered sections. Because T1's secondary floats, laser cathode shunt declared circuit ground. shunt returned T1's secondary center tap, completing laser current flow path. This arrangement produces negative voltage corresponding laser current shunt's ungrounded end. This potential resistively summed with positive voltage current programming input information. A1's output feedback controls LT1683's pulse width drives closing loop laser current. Loop compensation band limiting Q3's collector, aided single lead network arriving from L1-L2 junction. Some circuit details merit attention. LT1683's supply input pins from LT1054 based voltage multiplier. This boosted voltage provides enough gate drive ensure Q1-Q2 saturation. Damper networks across T1's rectifiers minimize diode switching related events output current. Finally, this circuit compatible with "self-enable" laser protection features previously described. Appropriate connection points appear figure. speed controlled switching times result spectacular decrease noise. Figure shows just 20µAP-P noise, about 0.001% laser current. Fundamental ripple residue switching artifacts visible against measurement noise floor.4 0.0025% Noise, 250mA, Grounded Anode Current Source This circuit, similar previous one, uses edge time control achieve exceptionally noise output. intended lower power lasers requiring grounded anode operation. LT1533, version previous circuit's LT1683, internal power switches. These switches drive T1's rectified filtered secondary produces negative output, biasing laser. laser's anode grounded current path T1's secondary completed shunt. This configuration makes T1's center voltage positive proportional laser current. This voltage compared current programming input. biases closing loop around LT1533. Loop compensation provided local bandwidth limiting Q2's collector damping feedback capacitors. This circuit's 2.5µAP-P noise qualifies most demanding applications. Figure shows residual switching related noise approaching measurement noise floor. enable function operates previously described. Additionally, this circuit compatible with Figure "self-enable" laser protection accessory circuits. Changes necessitated grounded anode operation appear schematic.
20µA/DIV COUPLED LEVEL
2µs/DIV
AN90
Figure Figure 15's Output Noise Measures 20µAP-P, About 0.001%. Coherent, Identifiable Components Include Fundamental Ripple Residue Switching Artifacts
Note Reliable wideband current noise measurement these levels requires special techniques. Appedix "Verifying Switching Regulator Related Noise" Appendix "Notes Current Probes Noise Measurement," details.
AN90-11
Application Note
3.9k
22nH PGND
820pF 0.01µF 300µH 33µH
LT1533
FROM OPTIONAL OPEN LASER PROTECTION. FIGURE OPTIONAL SELF-ENABLE FIGURE ENABLE RCSL RVSL 0.01µF SHDN
LT1533 SHUTDOWN
35.7k* 100k*
2N3904
1N5712 0.033µF LTC1696 PIN. FIGURE
LT1006
METAL FILM RESISTOR COILTRONICS CTX300-2 COILCRAFT DT1608C-333B COILCRAFT B07T COILTRONICS CTX02-13834 1N4148 BAT85 LASER INPUT 2.5V 250mA 10k*
2N3904
LT1006
OPTIONAL OPEN LASER PROTECTION. FIGURE REMAINDER CIRCUITRY. DELETE 2N5060
80.6k* 0.1µF
1000pF 10k*
AN90
OPTIONAL CURRENT CLAMP FIGURE
Figure 0.0025% Noise, Grounded Anode Laser Current Source 250mA Version Figure
5µA/DIV COUPLED 100mA LEVEL
2µs/DIV
AN90
Figure Figure 17's 2.5µAP-P Switching Related Noise Detectable Measurement Noise Floor
AN90-12
33µF
33µF
Application Note
Noise, Fully Floating Output Current Source Figure retains preceding example's noise also fully floating output. Either laser terminal grounded without effecting circuit operation. This feature realized feedback controlling transformer primary current relying interwinding coupling maintain regulation.5 This coupling varies slightly with operating point, limiting output current regulation about
FZT-849
schematic shows LT1533 noise switching regulator driving LT1533, while retaining controlled edge time characteristics, forced duty cycle grounding "duty" pin. Current flows through shunt into T1's primary. LT1533 open collector power outputs alternately chop primary current ground. Primary current magnitude, hence shunt voltage, Q1's bias. Q1's bias, turn, A1's output, which represents
3.3V
100µF 820pF LT1533 DUTY 22nH PGND 300µH 33µH
33µF
33µF
3.3V
1000pF LTC1696 7.15k* OPEN LASER PROTECTION 8.76k*
LT1789 9.5k*
ENABLE OPTIONAL SELF-ENABLE CIRCUIT, FIGURE CHANGE 1.5k RESISTOR LT1431
0.1µF
LT1006
2N3904 0.2µF OPTIONAL CURRENT CLAMP CIRCUIT. FIGURE
METAL FILM RESISTOR COILTRONICS CTX300-2 COILCRAFT DT1608C-333B COILCRAFT B07T COILTRONICS CTX02-13834 1N4148 INPUT 1.5V 150mA LASER
AN90
Figure Switched Mode, Noise Current Source Floating Output, Permitting Grounding Laser Anode Cathode. Open Laser Protection Included; Circuit Compatible with Current Clamp Self-Enable Options
Note have engaged this stunt before serve variety purposes. References
AN90-13
Application Note
difference between output current programming input A2's amplified version shunt voltage. This loop enforces shunt voltage proportionate current programming input value. this way, current programming input sets primary current, determining secondary current through laser. Current programming input scaling calibrated differential amplifier A2's gain setting resistor. primary side feedback's lack global feedback mandates current regulation compromise. Figure 20's plot laser current programming input voltage shows conformance over nearly entire range. error below 10mA, nonideal transformer behavior, normally insignificant because below typical laser threshold current. Line regulation, also degraded sensing scheme, still maintains about 0.05%/V. Similarly, load regulation, over 1.8V compliance voltage, typically This circuit's floating output complicates inclusion laser protection "self-enable" features described Figure text they accommodated. Open laser protection, shown Figure accomplished biasing LTC1696 from T1's center tap. laser opens, loop forces marked rise T1's center tap, latching LTC1696's output high. This skews A1's inputs, sending output shutting drive ceases. Because LTC1696 output latches, power must recycled reset circuit. laser been connected, latch will again, protecting laser from "hot plugging" intermittent connections. "self-enable" current clamp options added accordance with notations schematic. Anode-at-Supply Current Source Figure 21's current source useful where laser anode committed power supply. sensing Q1's emitter, closes loop which forces constant current laser. Local compensation input band limiting stabilize loop. This circuit also includes inherent "self-enable" feature. LT1635 operates supply voltages down 1.2V. Above 1.2V LT1635's comparator configured section (C1) holds circuit output until supply voltage reaches Below 1.2V supply, Q1's base biasing prevents unwanted outputs. Figure details operation during supply turn supply ramp (trace output current (trace disabled. When supply reaches (trace goes low, permitting A1's output (trace rise. This biases laser current flows (trace LT1635 operates supply voltages 1.2V. Below this level, spurious outputs prevented junction stacking band limiting Q1's base. Q1's base compo-
LASER CURRENT MILLIAMPERES
TYPICAL LASER THRESHOLD CURRENT
INPUT PROGRAMMING VOLTAGE
AN90
Figure Laser Current Input Programming Voltage Floating Current Source. Conformance within over Nearly Entire Range. Error Below 10mA, Nonideal Transformer Behavior, Below Typical Laser Threshold Current
AN90-14
Application Note
nents also prevent unwanted outputs when supply rises rapidly. Such rapid rise could cause uncontrolled outputs before amplifier feedback loop established. Figure shows circuit events during rapid supply rise. Trace shows supply's quick ascent. Trace C1's output, responds briefly goes some time after supply moves past (trace produces uncontrolled output about 100µs. combination Q1's base line filters this response insignificant levels laser current (trace flows.
OPTIONAL CURRENT CLAMP FIGURE 3.3V INPUT 1.5V 150mA 90.9k* 10K* 0.1µF
CURRENT SOURCE
3.3V
LT1635
BAT85
2N3904 2N2222
3.3V 200mV LT1635 1N5711 SELF-ENABLE 0.2µF
10µF
16.5k* OPTIONAL OPEN LASER 8.87k* PROTECTION LTC1696 PIN. FIGURE
METAL FILM RESISTOR 1N4148, UNLESS NOTED LASER
AN90
Figure Circuit Laser Anode Committed Supply, Inherent Self-Enabled Operation. LT1635 Functions 1.2V, Although Self-Enable Feature Holds Output Until Power Supply Exceeds Current Clamp Open Laser Protection Optional
2V/DIV 2V/DIV 1V/DIV 2V/DIV
1V/DIV 0.5V/DIV 50mA/DIV 10ms/DIV
AN90
10mA/DIV 50µs/DIV
AN90
Figure Output Current (Trace Held Until Supply (Trace Ramps Past Self-Enable Comparator (Trace Operates Above 1.2V; Base (Trace Biasing Prevents Output Below 1.2V
Figure Rapidly Rising Supply (Trace Produces Current Output (Trace Despite A1's Transient Uncontrolled Output (Trace (Trace Reacts Properly A1's Inactive Loop Cannot Respond. Q1's Base Line Components Preclude Spurious Current Output (Trace
AN90-15
Application Note
slew retarded input loop compensation yield clean dynamic response with overshoot. Figure trace input step. This step, filtered A1's input (trace represented well controlled laser current output trace Current clamping open laser protection options annotated schematic. Additionally, higher output current possible increased supply voltages, although Q1's dissipation limits must respected.
1V/DIV
1V/DIV
50mA/DIV
5ms/DIV
AN90
Figure Output Current (Trace Responds Cleanly Filtered Version (Trace Trace Input Step
Note: This application note derived from manuscript originally prepared publication magazine.
REFERENCES Hewlett-Packard Company, "Model 6181C Current Source Operating Service Manual," 1975. Williams, "Designing Linear Circuits Single Supply Operation," "Floating Output Current Loop Transmitter," Linear Technology Corporation, Application Note September 1985, Williams, Fourth Generation Backlight Technology," "Floating Lamp Circuits," Linear Technology Corporation, Application Note November 1995, 40-42. Linear Technology Corporation, "LT1182/LT1183/ LT1184/LT1184F CCFL/LCD Contrast Switching Regulators," Data Sheet, 1995. Grafham, "Using Current SCRs," General Electric AN-200.19, January 1967. General Electric Co., "SCR Manual," 1967. Williams, Monolithic Switching Regulator with 100µV Output Noise," Linear Technology Corporation, Application Note October 1997. Pope, Essay Criticism," 1711.
AN90-16
Application Note
APPENDIX SIMULATING LASER LOAD Fiber optic lasers delicate, unforgiving expensive load. This poisonous brew when breadboarding with high likelihood catastrophe. much wiser alternative simulate laser load using either diodes electronic equivalents. Lasers look like junctions with typical forward voltages ranging from 1.2V 2.5V. simplest simulate laser stack appropriate numbers diodes series. Figure lists typical junction voltages various currents popular diode types. MR750 suitable currents ampere range, while 1N4148 serves well lower currents. Typically, stacking three diodes allows simulating laser given current range. Diode voltage tolerances variations temperature current changes limit accuracy, although results generally satisfactory. Electronic Laser Load Simulator Figure laser load simulator powered battery. eliminates diode load junction voltage drop uncertainty. Additionally, desired "junction drop" voltage conveniently with indicated potentiometer. Electronic feedback enforces establishment maintenance calibrated junction drop equivalents. potentiometer sets voltage A1's negative input. responds biasing Q1's drain voltage controls Q2's base and, hence, Q2's emitter potential. Q2's emitter back closing loop around amplifier. This forces voltage across equal potentiometer's output voltage under conditions. capacitors stabilize loop Q2's base resistor ferrite bead suppress parasitic oscillation. 1N5400 prevents Q1Q2 reverse biasing load terminals reversed.
MR750 (25°C) TYPICAL JUNCTION CURRENT TYPICAL JUNCTION VOLTAGE 0.5A 1.0A 1.5A 2.0A 2.5A 0.68V 0.76V 0.84V 0.90V 0.95V
1N4148 (25°C) TYPICAL JUNCTION CURRENT TYPICAL JUNCTION VOLTAGE 0.1A 0.2A 0.3A 0.83V 0.96V 1.08V
Figure Characteristics Diodes Suitable Simulating Lasers. Appropriate Series Connections Approximate Laser Forward Voltage
100k 100k "JUNCTION DROP" 2.5V 100k LT1004 2.5V 0.01µF D45VH10 (HEAT SINK)
LT1077
VN2222L
1N5400
0.001µF
AN90 FA02
FERRITE BEAD, FERRONICS #21-110J
Figure Floating, Battery-Powered Laser Simulator Sets Desired "Junction Drop" Across Output Terminals. Amplifier Feedback Controls Q2's Potentiometer Voltage
AN90-17
Application Note
APPENDIX VERIFYING SWITCHING REGULATOR RELATED NOISE Measuring switching regulator related current noise levels discussed text requires care. microamp amplitudes wide bandwidth interest (100MHz) mandates strict attention measurement technique. theory, simply measuring voltage drop across shunt resistor permits current determined. practice, resultant small voltages required high frequency fidelity pose problems. Coaxial probing techniques applicable probe grounding requirements become severe. slightest incidence multiple ground paths ("ground loops") will corrupt measurement, rendering observed "results" meaningless. Differentially configured coaxial probes offer some relief from ground loop based difficulties there inherently better approach.1 Current transformers offer attractive measure noise while eliminating probe grounding concerns. types current probes available: split core closed core. split core "clip types convenient have relatively gain higher noise floor than closed core types.2 closed core transformer's gain noise floor advantages particularly attractive wideband, current measurement. Figure B1's test setup allows investigation closed core transformer's capabilities. transformer specified flat gain over wide bandwidth, well shielded enclosure coaxial output connection. 5mV/ output feeds noise x100, input amplifier. amplifier's terminated output monitored oscilloscope with high sensitivity plug-in. pulse driving known resistor value ("R") provides simple source calibrated current into transformer. 10k, resultant pulsed current 100µA. Figure B2's oscilloscope photo shows test setup response. waveform crisp, essentially noise free agrees with predicted amplitude. More sensitive measurement involves determining test setup's noise floor. Figure taken with current flowing transformer, indicates noise limit about 10µAP-P. Most this noise x100 amplifier. preceding exercise determines test setup's gain noise performance. This information provides confidence necessary make meaningful level current measurement. Figure taken with Figure B1's 100k, sources only 10µA transformer. This
TYPE OSCILLOSCOPE
TYPE 5mV/mA PULSE INPUT TEKTRONIX CLOSED CORE CURRENT PROBE #CT-1 HP-461A AMPLIFIER X100 NOISE AMPLIFIER
AN90 FB01
TERMINATOR
0.5V/mA
100µA 100k 10µA
Figure Noise Measurement Instrumentation Includes Resistors, Closed Core Current Probe, Noise Wideband Amplifier Oscilloscope
Note This denigrate level voltage probing methods. Their practice well refined directly applicable appropriate circumstances. Appendix Reference tutorial. Note Appendix "Notes Current Probes Noise Measurement," detailed comparison.
AN90-18
Application Note
comparable previously determined noise floor trace, clearly delineated against noise limit, indicates 10µA amplitude. This level agreement qualified this test method obtain text's quoted noise figures. Isolated Trigger Probe performance limits noted above were determined with well defined, pulsed input test signal. Residual switching regulator noise much less specific profile. oscilloscope encounter problems triggering illdefined, noise laden waveform. Externally triggering 'scope from switching regulator's clocking solves this problem introduces ground loops, corrupting measurement.3 possible, however, externally trigger 'scope without making galvanic connections circuit, eliminating ground loop concerns. This accomplished coupling field produced switching regulator magnetics. probe which does this simply choke terminated against ringing (Figure B5). choke, appropriately positioned, picks residual switching frequency related magnetic field, generating isolated trigger signal.4 This arrangement furnishes 'scope trigger signal with essentially measurement corruption. probe's physical form appears Figure good results, termination should adjusted minimum ringing while preserving highest possible amplitude output. Light compensatory damping produces Figure B7's output, which will cause poor 'scope triggering. Proper adjustment results more favorable output (Figure B8), characterized minimal ringing well defined edges.
100µA/DIV
10µA/DIV
100ns/DIV
AN90 FB02
100ns/DIV
AN90 FB03
Figure Response 100µA Input Clean. Displayed Amplitude Agrees with Input Stimulus, Indicating Calibrated Measurement
Figure 10µA Noise Floor Determined Removing Current Loop from Transformer. Remaining Noise Primarily x100 Amplifier
PROBE SHIELDED CABLE
10µA/DIV
TERMINATION OUTPUT
CONNECTION TERMINATION J.W. MILLER #100267
100ns/DIV
AN90 FB04
DAMPING ADJUST 4700pF
AN90 FB05
Figure Verifying Gain Near Noise Floor. 10µA Input Pulse Produces Calibrated, Readily Discernable Output
Figure Simple Trigger Probe Eliminates Board Level Ground Loops. Termination Components Damp L1's Ringing Response
Note Veterans application notes, hardened crew, will recognize this probe's description from Application Note (Reference directly applies this topic reproduced here reader convenience.
Note previous comments beginning this appendix.
AN90-19
Application Note
Trigger Probe Amplifier field around switching magnetics small adequate reliably trigger some oscilloscopes. such cases, Figure B9's trigger probe amplifier useful. uses adaptive triggering scheme compensate variations probe output amplitude. stable trigger output maintained over 50:1 probe output range. operating gain 100, provides wideband gain. output this stage biases 2-way peak detector through Q4). maximum peak stored Q2's emitter capacitor, while minimum excursion retained Q4's emitter capacitor. value midpoint A1's output signal appears junction 500pF capacitor units. This point always sits midway between signal's excursions, regardless absolute amplitude. This signal-adaptive voltage buffered trigger voltage LT1394's positive input. LT1394's negative input biased directly from A1's output. LT1394's output, circuit's trigger output, unaffected >50:1 signal amplitude variations. x100 analog output available Figure shows circuit's digital output (trace responding amplified probe signal (trace
Figure Trigger Probe Termination Box. Clip Lead Facilitates Positioning Probe, Electrically Neutral
AN90-20
Application Note
10mV/DIV
10mV/DIV
10µs/DIV
AN90 FB07
10µs/DIV
AN90 FB08
Figure Misadjusted Termination Causes Inadequate Damping. Unstable Oscilloscope Triggering Result
Figure Properly Adjusted Termination Minimizes Ringing with Small Amplitude Penalty
ANALOG OUTPUT
0.005µF 0.005µF 500pF
LT1227
10µF
0.1µF
100µF
0.1µF
0.1µF
CA3096 ARRAY: SUBSTRATE (PIN GROUND 1N4148 TRIGGER PROBE TERMINATION (SEE FIGURE DETAILS)
Figure Trigger Probe Amplifier Analog Digital Outputs. Adaptive Threshold Maintains Digital Output over 50:1 Probe Signal Variations
1V/DIV COUPLED
5V/DIV
10µs/DIV (UNCALIB)
AN90 FB10
Figure B10. Trigger Probe Amplifier Analog (Trace Digital (Trace Outputs
LT1006
DIGITAL TRIGGER 'SCOPE
AN90 FB09
LT1394
AN90-21
Application Note
APPENDIX NOTES CURRENT PROBES NOISE MEASUREMENT Appendix explained current probes advantages switching regulator related current noise measurement. Their minimally invasive nature eases connection parasitics, enhancing measurement fidelity. Different combinations current probes amplifiers provide varying degrees performance convenience. Figure summarizes characteristics probes applicable amplifiers. general, noise floor uncertainties convenient split core types compromised their construction. closed core probes less noisy some types have inherently higher gain, distinct advantage. laboratory based comparison revealing. Figure shows CT-1 (closed core)-HP461A combination responding 100µA pulsed input. waveform clearly outlined, with pulse bottom trace thickening deriving from noise floor.1 Figure taken with same input, degraded. split core P6022-Preamble 1855 combination used much greater noise. decreased performance almost entirely split core probe's construction. closing, worthwhile noting that Hall element stabilized current probes (e.g., Tektronix AM503, P6042) suitable level measurement. Hall device based flux nulling loop extends probe response introduces 300µA noise.
CURRENT PROBE Tektronix P6022 (1mV/mA)
AMPLIFIER Preamble 1855 (1M)
NOISE FLOOR (100 COMMENTS 100µA Split Core Convenient Sensitivity Low, Resulting Relatively High Overall Noise Floor Probe's Higher Gain Accounts Most Noise Floor Reduction- Input Amplifier Provides Some Additional Benefit. Closed Core Probe Requires Breaking Conductor Make Measurement
Tektronix Hewlett-Packard CT-1 461A (5mV/mA) (50)
15µA
Figure Recommended Instrumentation Current Noise Measurement. Split Core "Current Probe" Convenient; Closed Core Provides Higher Gain Lower Noise
Note Diehard curmudgeons still using high quality analog oscillscopes routinely discern noise presence trace thickening. Those stuck with modern instruments routinely view thick, noisy traces.
AN90-22
Application Note
100µA/DIV
500ns/DIV
Figure CT-1/HP-461A Combination Clearly Displays 100µA Pulse Train. Noise Floor Causes Slight Pulse Bottom Trace Thickening
100µA/DIV
500ns/DIV
Figure P6022/Preamble 1855 Presentation Previous Figure's Waveform Degraded Signal-to-Noise Performance. Split Core "Current Probe" Convenience Necessitates Measurement Fidelity Compromise
Information furnished Linear Technology Corporation believed accurate reliable. However, responsibility assumed use. Linear Technology Corporation makes representation that interconnection circuits described herein will infringe existing patent rights.
AN90-23
Application Note
AN90-24
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, 95035-7417
(408) 432-1900 FAX: (408) 434-0507
an90f LT/TP 0402 PRINTED
www.linear.com
LINEAR TECHNOLOGY CORPORATION 2002

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