MIC2172/3172 100kHz 1.25A Switching Regulators Preliminary Inform
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MIC2172/3172
MIC2172/3172
100kHz 1.25A Switching Regulators Preliminary Information
General Description
MIC2172 MIC3172 complete 100kHz SMPS current-mode controllers with internal 1.25A power switches. MIC2172 features external frequency synchronization frequency adjustment, while MIC3172 features enable/shutdown control input. Although primarily intended voltage step-up applications, floating switch architecture MIC2172/3172 makes practical step-down, inverting, configurations well isolated topologies. Operating from 40V, MIC2172/3172 draws only quiescent current making attractive battery operated supplies. MIC3172 applications that require on/off control regulator. MIC3172 externally shutdown applying signal (enable). When disabled, MIC3172 draws only leakage current (typically less than 1µA). must high normal operation. applications requiring control, must tied high. MIC2172 applications requiring more SMPS regulators that operate from same input supply. MIC2172 features SYNC input which allows locking internal oscillator external reference. This makes possible avoid audible beat frequencies that result from unequal oscillator frequencies independent SMPS regulators. reference signal supplied MIC2172 designated master. insure locking slave's oscillators, reference oscillator frequency must higher than
slave's. master MIC2172's oscillator frequency increased 135kHz connecting resistor from SYNC ground (see applications information). MIC2172/3172 available 8-pin plastic SOIC -40°C +85°C operation.
Features
1.25A, internal switch rating input voltage range Current-mode operation Internal cycle-by-cycle current limit Thermal shutdown external parts count Operates most switching topologies quiescent current (operating) <1µA quiescent current, shutdown mode (MIC3172) shutdown compatibility (MIC3172) External frequency synchronization (MIC2172) External frequency trim (MIC2172) Fits most LT1172 sockets (see applications info)
Applications
Laptop/palmtop computers Toys Hand-held instruments Off-line converter (requires external power switch) Predriver higher power capability Master/slave configurations (MIC2172)
Typical Applications
(4.75V min.) 27µH SYNC MIC2172 COMP 1N5822
1N5818 470µF VOUT 0.25A 3.74k
22µF VOUT +12V, 0.14A 10.7k 1.24k 470µF
Enable Shutdown
22µF MIC3172
1:1.25 LPRI 100µH
COMP
1.24k
Locate near MIC2172 when supply leads
Optional voltage clipper (may req'd leakage inductance high)
Figure MIC2172 Boost Converter
Figure MIC3172 Flyback Converter
1997
4-13
MIC2172/3172
Ordering Information
Part Number MIC2172BN MIC2172BM MIC3172BN MIC3172BM Temperature Range -40°C +85°C -40°C +85°C -40°C +85°C -40°C +85°C Package 8-pin plastic 8-lead SOIC 8-pin plastic 8-lead SOIC
Configuration
MIC2172*/3172
COMP *SYNC/EN
MIC2172*/3172
COMP *SYNC/EN
8-lead
8-lead SOIC
Description
Number Name Function Signal Ground: Internal analog circuit ground. Connect directly input filter capacitor proper operation (see applications info). Keep separate from power grounds. Frequency Compensation: Output transconductance type error amplifier. Primary function loop stabilization. also used output voltage soft-start current limit tailoring. Feedback: Inverting input error amplifier. Connect external resistive divider power supply output voltage. Synchronization/Frequency Adjust: Capacitively coupled input signal greater than device's free running frequency 135kHz) will lock device's oscillator falling edge. Oscillator frequency trimmed 135kHz adding resistor ground. unused, must float connection). Enable: Apply high connect enable regulator. Apply connect ground disable regulator. Device draws only leakage current (<1µA) when disabled. Supply Voltage: 3.0V Power Ground power switch emitters with current sense resistor series. Required. Connect external inductor input voltage ground depending circuit topology. Power Switch Collector: Collector switch. Connect external inductor input voltage depending circuit topology. Power Ground power switch emitters with current sense resistor series. Optional. maximum power capability connect Floating reduces current limit factor two.
COMP
(MIC2172)
SYNC
(MIC3172)
4-14
1997
MIC2172/3172
Absolute Maximum Ratings MIC2172
Input Voltage Switch Voltage Sync Current 50mA Feedback Voltage (Transient, 1ms) ±15V Operating Temperature Range 8-pin PDIP +85°C 8-pin SOIC +85°C Junction Temperature -55°C +150°C Thermal Resistance 8-pin PDIP .130°C/W 8-pin SOIC .120°C/W Storage Temperature -65°C +150°C Soldering sec.) +300°C
Electrical Characteristics MIC2172
Parameter Reference Section Feedback Voltage (VFB) Feedback Voltage Line Regulation Feedback Bias Current (IFB) Error Amplifier Section Transconductance (ICOMP/VFB) ICOMP ±25µA Voltage Gain (VCOMP/VFB) Output Current Output Swing Compensation Threshold Output Switch Section Resistance Current Limit Conditions tied
Note Unless otherwise specified, Units
1.220 1.214
1.240
1.264 1.274 0.03
1100
0.25
0.35
2000 0.52 1.08 1.25
µA/mV µA/mV
0.9V VCOMP 1.4V VCOMP 1.5V High Clamp, Clamp, 1.5V Duty Cycle
0.8V Duty Cycle 50%, 25°C Duty Cycle 50%, 25°C Duty Cycle Note 1.25 1.25
0.76
Breakdown Voltage (BV)
1997
4-15
MIC2172/3172
Parameter Oscillator Section Frequency (fO) Duty Cycle [(max)] Sync Coupling Capacitor Required Frequency Lock Peak-to-Peak Voltage Required Frequency Lock Input Supply Voltage Section Minimum Operating Voltage Quiescent Current (IQ) Supply Current Increase (IIN)
Note Note
Conditions Units
3.0V CCOUPLING 12pF
40V, VCOMP 0.6V, VCOMP 1.5V
Bold type denotes specifications applicable full operating temperature range. Devices sensitive. Handling precautions required. duty cycles between 95%, minimum guaranteed switch current given 0.833 (2-) MIC3172.
Absolute Maximum Ratings MIC3172
Input Voltage Switch Voltage Enable Voltage Feedback Voltage (Transient, 1ms) ±15V Operating Temperature Range 8-pin PDIP +85°C 8-pin SOIC +85°C 8-pin CerDIP +125°C Junction Temperature -55°C 150°C Thermal Resistance 8-pin PDIP .130°C/W 8-pin SOIC .120°C/W 8-pin CerDIP 100°C/W Storage Temperature -65°C 150°C Soldering sec.) 300°C
Electrical Characteristics MIC3172
Parameter Reference Section Feedback Voltage (VFB) Feedback Voltage Line Regulation Feedback Bias Current (IFB) Conditions tied
Note Unless otherwise specified, Units
1.224 1.214
1.240 0.07
1.264 1.274
1100
4-16
1997
MIC2172/3172
Parameter Error Amplifier Section Transconductance (ICOMP/VFB) ICOMP ±25µA Voltage Gain (VCOMP/VFB) Output Current Output Swing Compensation Threshold Output Switch Section Resistance Current Limit 0.8V Duty Cycle 50%, 25°C Duty Cycle 50%, 25°C Duty Cycle Note 1.25 1.25 0.76 0.9V VCOMP 1.4V VCOMP 1.5V High Clamp, Clamp, 1.5V Duty Cycle 0.25 0.35 2000 0.52 1.08 1.25 Conditions
Units
µA/mV µA/mV
Breakdown Voltage (BV) Oscillator Section Frequency (fO) Duty Cycle [(max)]
Input Supply Voltage Section Enable Section Minimum Operating Voltage Quiescent Current (IQ) Quiescent Current Increase (IIN) Enable Input Threshold Enable Input Current 2.4V 40V, VCOMP 0.6V, Shutdown, VCOMP 1.5V
Bold type denotes specifications applicable full operating temperature range. Note Note Devices sensitive. Handling precautions required. duty cycles between 95%, minimum guaranteed switch current given 0.833 (2-) MIC3172.
1997
4-17
MIC2172/3172
Typical Performance Characteristics
MIC2172 Minimum Operating Voltage
Minimum Operating Voltage
Feedback Bias Current
Feedback Voltage Change (mV)
Feedback Voltage Line Regulation
-40°C Operating 125°C
Feedback Bias Current (nA) -100 Temperature (°C)
Switch Current -100 Temperature (°C)
25°C
Supply Current
Supply Current (mA) D.C. Supply Current (µA) Operating Voltage -100
Supply Current (Shutdown Mode)
Enable Voltage (mV) MIC3172
Enable Thresholds
MIC3172 -100 Temperature (°C)
D.C. D.C.
Temperature (°C)
Supply Current
Average Supply Current (mA) Switch Voltage
Switch Voltage
Current Limit
-40°C 25°C
Switch Current
-40°C 25°C 125°C
125°C
Switch Current
Switch Current
Duty Cycle
Supply Current
Supply Current (mA) -100 Temperature (°C) VCOMP 0.6V Frequency (kHz)
Oscillator Frequency
Oscillator Frequency
MIC2172
fOSC (kHz) Temperature (°C)
RADJ
1000
4-18
1997
MIC2172/3172
Typical Performance Characteristics
Error Amplifier Gain
Transconductance (µA/mV) -100 Temperature (°C) Transconductance (µS) 7000 6000
Error Amplifier Gain
Phase Shift 1000 Frequency (kHz) 10000
Error Amplifier Phase
5000 4000 3000 2000 1000
1000 Frequency (kHz)
10000
Block Diagram MIC2172
SYNC 2.3V Anti-Sat. 100kHz Osc. Logic Driver
Reg.
Comparator 1.24V Ref. Error Amp. Current Amp.
COMP
1997
4-19
MIC2172/3172
Block Diagram MIC3172
2.3V Anti-Sat. 100kHz Osc. Logic Driver
Reg.
Comparator 1.24V Ref. Error Amp. Current Amp.
COMP
Functional Description
Refer "Block Diagram MIC2172" "Block Diagram MIC3172." Internal Power MIC2172/3172 operates when 2.6V (and 2.0V MIC3172). internal 2.3V regulator supplies biasing internal circuitry including precision 1.24V band reference. enable control (MIC3172 only) enables disables internal regulator which supplies power other internal circuitry. Operation 100kHz oscillator generates signal with duty cycle approximately 90%. current-mode comparator output used reduce duty cycle when current amplifier output voltage exceeds error amplifier output voltage. resulting signal controls driver which supplies base current output transistor Current Mode Advantages MIC2172/3172 operates current mode rather than voltage mode. There three distinct advantages this
technique. Feedback loop compensation greatly simplified because inductor current sensing removes pole from closed loop response. Inherent cycle-by-cycle current limiting greatly improves power switch reliability provides automatic output current limiting. Finally, current-mode operation provides automatic input voltage feed forward which prevents instantaneous input voltage changes from disturbing output voltage setting. Anti-Saturation anti-saturation diode (D1) increases usable duty cycle range MIC2172/3172 eliminating base collector stored charge which would delay Q1's turnoff. Compensation Loop stability compensation MIC2172/3172 accomplished connecting appropriate network from either COMP circuit ground (Typical Applications) COMP error amplifier output (COMP) also useful soft start current limiting. Because error amplifier output transconductance type, output impedance relatively high which means output voltage easily clamped adjusted externally.
4-20
1997
MIC2172/3172
using MIC3172, shown figure eliminated, reducing total components count. Synchronizing MIC2172 Using several unsynchronized switching regulators same circuit will cause beat frequencies appear inputs outputs. These beat frequencies very making them difficult filter. Micrel's MIC2172 synchronized single master frequency avoiding possibility undesirable beat frequencies multiple regulator circuits. master frequency external oscillator designated master MIC2172. master frequency should 1.05 1.20 times slave's 100kHz nominal frequency guarantee synchronization.
SYNC SYNC MIC2172 Slave Master SYNC MIC2172 Additional Slaves Slave MIC2172
Applications Information
Using MIC3172 Enable Control (New Designs) designs requiring enable/shutdown control, connect CMOS control signal (figure very driver current requirement ensures compatibility regardless driver gate used.
Enable Shutdown
Logic Gate
MIC3172
Figure MIC3172 Enable/Shutdown Using MIC3172 LT1172 Applications MIC3172 used most original LT1172 applications adapting MIC3172's enable/shutdown feature existing LT1172 circuit. Unlike LT1172 which shutdown reducing voltage (VC) below 0.15V, MIC3172 dedicated enable/shutdown pin. replace LT1172 with MIC3172, determine LT1172's shutdown feature used.
Circuits without Shutdown
shutdown feature being used, connect continuously enable MIC3172 MIC2172 with SYNC open (figure
SYNC MIC2172 MIC3172
Figure Master/Slave Synchronization
Figure MIC2172/3172 Always Enabled
Figure shows typical application where several MIC2172s operate from same supply voltage. U1's oscillator frequency increased above U2's U3's connecting resistor from SYNC ground. U2-SYNC U3-SYNC capacitively coupled master's output (VSW). slaves lock negative (falling edge) U1's output waveform.
SYNC MIC2172 External Signal Slave SYNC MIC2172
Circuits with Shutdown
shutdown used original LT1172 application, connect logic gate that produces logic-level output signal that matches shutdown signal. MIC3172 will enabled logic-high input shutdown with logic-low input (figure actual components performing functions vary according original application.
MIC3172 COMP
connection
Additional Slaves Slave
Enable Shutdown
Figure External Synchronization
Existing VN2222 equiv.
Existing Logic Gate
Care must exercised insure that master MIC2172 always operating continuous mode.
Figure Adapting LT1172 Socket 1997 4-21
MIC2172/3172
Figure shows more MIC2172s locked external reference frequency. slaves lock negative (falling edge) external reference waveform. Soft Start diode-coupled capacitor from COMP circuit ground slows output voltage rise turn (figure
MIC2172/3172 COMP
total power dissipation device operating losses power switch losses. device operating losses losses associated with biasing internal functions plus losses power switch driver circuitry. losses calculated from supply voltage (VIN) device supply current (IQ). MIC2172/3172 supply current almost constant regardless supply voltage (see "Electrical Characteristics"). driver section losses (not including switch) function supply voltage, power switch current, duty cycle. 0.004 P(bias+driver)
where: P(bias+driver) device operating losses supply voltage quiescent supply current power switch current (see Design Hints: Switch Current Calculations") duty cycle
Figure Soft Start additional time takes error amplifier charge capacitor corresponds time takes output reach regulation. Diode discharges when removed. Current Limit designs demanding less output current than MIC2172/ 3172 capable delivering, left open reducing current capability one-half.
MIC2172/3172 COMP VOUT
VOUT output voltage forward voltage drop practical example refer figure 5.0V 0.006A 0.625A (0.6) Then: P(bias+driver) 0.006) 0.625 P(bias+driver) 0.068W Power switch dissipation calculations greatly simplified making assumptions which usually fairly accurate. First, majority losses power switch on-losses. find these losses, assign resistance value collector/emitter terminals device using saturation voltage versus collector current curves (see Typical Performance Characteristics). Power switch losses calculated modeling switch resistor with switch duty cycle modifying average power dissipation. (ISW)2 From Typical performance Characteristics: 0.004
0.6V/R2 Note: Input output returns common.
Figure Current Limit Alternatively, maximum current limit MIC2172/3172 reduced adding voltage clamp COMP output (figure This feature useful applications requiring either complete shutdown Q1's switching action form current fold-back limiting. This COMP output does disable oscillator, amplifiers other circuitry, therefore supply current never less than approximately 5mA. Thermal Management Although MIC2172/3172 family contains thermal protection circuitry, best reliability, avoid prolonged operation with junction temperatures near rated maximum. junction temperature determined first calculating power dissipation device. MIC2172/3172,
4-22
1997
MIC2172/3172
Then: (0.625)2 P(SW) 0.234W P(total) 0.068 0.234 P(total) 0.302W junction temperature semiconductor calculated using following: P(total) Where: junction temperature ambient temperature (maximum) P(total) total power dissipation junction ambient thermal resistance practical example: 70°C 130°C/W (for plastic DIP) Then: 0.30 109°C This junction temperature below rated maximum 150°C. Grounding Refer figure Heavy lines indicate high current paths.
MIC2172/3172
Applications Design Hints
Access both collector emitter(s) power switch makes MIC2172/3172 extremely versatile suitable most power supply topologies. Boost Conversion Refer figure typical boost conversion application where logic supply available +12V 0.14A required.
(4.75V min.) 27µH SYNC MIC2172 COMP 1N5822 22µF VOUT +12V, 0.14A 10.7k 1.24k 470µF
Locate near MIC2172 when supply leads
Figure Boost Converter first step designing boost converter determining whether inductor will cause converter operate either continuous discontinuous mode. Discontinuous mode preferred because feedback control converter simpler. When discharges current completely during MIC2172/3172's off-time, operating discontinuous mode. operating continuous mode does discharge completely before MIC2172/3172 power switch turned again.
Discontinuous Mode Design
Given maximum output current, solve equation determine whether device operate discontinuous mode without initiating internal device current limit.
Single point ground MIC3172 only
Figure Single Point Ground single point ground strongly recommended proper operation. signal ground, compensation network ground, feedback network connections sensitive minor voltage variations. input output capacitor grounds power ground conductors will exhibit voltage drop when carrying large currents. Keep sensitive circuit ground traces separate from power ground traces. Small voltage variations applied sensitive circuits prevent MIC2172/3172 switching regulator from functioning properly. IOUT
(1a)
Where: internal switch current limit 1.25A when 0.833 when (Refer Electrical Characteristics.) IOUT maximum output current minimum input voltage duty cycle
1997
4-23
MIC2172/3172
VOUT required output voltage forward voltage drop example figure IOUT 0.14A 1.147A 4.75V (minimum) 0.623 VOUT 12.0V 0.6V Then: 1.147 4.75 0.623
Micrel Switch Operation
During Q1's time internal transistor-see block diagrams), energy stored T1's primary inductance. During Q1's time, stored energy partially discharged into (output filter capacitor). Careful selection capacitor provide satisfactory output ripple voltage making additional filter stages unnecessary. (input capacitor) reduced eliminated MIC3172 located near impedance voltage source.
Output Diode
output diode allows store energy primary inductance nonconducting) release energy into conducting). forward voltage drop Schottky diode minimizes power loss
IOUT
IOUT 0.141A This value greater than 0.14A output current requirement proceed find inductance value
Frequency Compensation
simple frequency compensation network consisting prevents output oscillations. High impedance output stages (transconductance type) MIC2172/3172 often permit simplified loop-stability solutions connected circuit ground, although more conventional technique connecting components from error amplifier output inverting input also possible.
(VIN
POUT
Where: POUT 0.14 1.68W (100kHz) practical example:
Voltage Clipper
Care must taken minimize T1's leakage inductance, otherwise necessary incorporate voltage clipper consisting avoid second breakdown (failure) MIC3172's power
0.623) 1.68
4.75
Enable/Shutdown
MIC3172 includes enable/shutdown feature. When device shutdown, total supply current less than 1µA. This ideal battery applications where portions system powered only when needed. this feature required, simply connect high voltage.
26.062µH (use 27µH) Equation solves L1's maximum current value.
IL1(peak)
Where: IL1(peak) 4.75 6.23 10-6 10-6
Discontinuous Mode Design
When designing discontinuous flyback converter, first determine whether device safely handle peak primary current demand placed output power. Equation finds maximum duty cycle required given input voltage output power. duty cycle greater than 0.8, discontinuous operation cannot used.
IL1(peak) 1.096A 27µH inductor with peak current rating least 1.4A. Flyback Conversion Flyback converter topology used power applications where voltage isolation required whenever input voltage less than greater than output voltage. with step-up converter inductor (transformer primary) current continuous discontinuous. Discontinuous operation recommended. Figure shows practical flyback converter design using MIC3172.
POUT VIN(min)
practical example let: POUT 5.0V 0.25A 1.25W 4.0V 6.0V 1.25A when 0.833 when
4-24
1997
MIC2172/3172
Then: (10)
LPRI VIN(min)2 POUT
1.25 1.25
(50%) 0.55. slightly higher duty cycle value used overcome circuit inefficiencies. iterations equation required duty cycle found greater than 50%. Calculate maximum transformer turns ratio NPRI/NSEC, that will guarantee safe operation MIC2172/ 3172 power switch.
VIN(max)
Where: LPRI maximum primary inductance device switching frequency (100kHz) VIN(min) minimum input voltage power switch time Then: LPRI 4.02 10-6 1.25
Where: transformer maximum turns ratio power switch collector emitter maximum voltage safety derating factor (0.8 most commercial industrial applications) VIN(max) maximum input voltage VSEC transformer secondary voltage (VOUT practical example: max. MIC2172/3172 VSEC 5.6V Then:
LPRI 19.23µH 18µH primary inductance overcome circuit inefficiencies. complete design inductance value secondary found which will guarantee that energy stored transformer during power switch time will completed discharged into output during off-time. This necessary when operating discontinuous-mode. (11) POUT
Where: LSEC maximum secondary inductance TOFF power switch time Then: 10-6 1.25
8.2143 Next, calculate maximum primary inductance required store needed output energy with power switch duty cycle 55%.
LSEC 25.4µH
1N5818 470µF
VOUT 0.25A 3.74k
22µF MIC3172
1:1.25 LPRI 100µH
Enable Shutdown
COMP
1.24k
Optional voltage clipper (may req'd leakage inductance high)
Figure MIC3172 0.25A Flyback Converter
1997
4-25
MIC2172/3172
Finally, recalculate transformer turns ratio insure that less than value earlier found equation (9). (12) Then: 10-5 2.54 10-5 LPRI Then:
transformer turns ratio (0.8) reverse voltage safety derating factor (0.8)
15.625V 1N5817 will safely handle voltage current requirements this example. Forward Converters Micrel's MIC2172/3172 used several circuit configurations generate output voltage which less than input voltage (buck step-down topology). Figure shows MIC3172 voltage step-down application. Because internal architecture these devices, more external components required implement step-down regulator than with other devices offered Micrel (refer LM257x LM457x family buck switchers). However, step-down conversion requiring transformer (forward), MIC2172/3172 good choice. step-down converter using transformer isolation (forward) shown figure Unlike isolated flyback converter which stores energy primary inductance during controller's on-time releases load during off-time, forward converter transfers energy output during on-time, using off-time reset transformer core. application shown, transformer core reset tertiary winding discharging T1's peak magnetizing current through most forward converters duty cycle limited 50%, allowing transformer flux reset with only times input voltage appearing across power switch. Although during normal operation this circuit's duty cycle well below
0.84 (same 1:1.25). This ratio less than ratio calculated equation (9). When specifying transformer necessary know primary peak current which must withstood without saturating transformer core. (13)
IPEAK(pri) VIN(min) LPRI
IPEAK(pri)
10-6
IPEAK(pri) 1.22A find minimum reverse voltage requirement output rectifier. This rectifier must have average current rating greater than maximum output current 0.25A. (14)
VIN(max)
Where: output rectifier maximum peak reverse voltage rating
1N4148
2.2µF 100µF MIC3172 COMP 3.7k 1.2k 100µH 330µF 1N4148
0.1A (ILOAD 100mA)
Locate near MIC2172/3172 when supply leads R3/R2 sets output voltage
Figure Step-Down Buck Converter
4-26
1997
MIC2172/3172
50%, MIC2172 (and MIC3172) maximum duty cycle capability 90%. required during operation (start-up high load currents), complete reset transformer during off-time would require voltage across power switch times input voltage. This would limit input voltage less forward converter applications. prevent core saturation, application given here uses duty cycle limiter consisting Whenever MIC3172 exceeds duty cycle 50%, T1's reset winding current turns This action reduces duty cycle MIC3172 until able reset during each cycle. Fluorescent Lamp Supply extremely useful application MIC3172 generating voltage fluorescent lamps used liquid crystal display back lighting portable computers. Figure shows complete power supply lighting fluorescent lamp. Transistors together with capacitor form Royer oscillator. Royer oscillator generates sine wave whose frequency determined series circuit comprised Assuming that MIC3172 absent, transistors' emitters grounded, circuit operation described "Oscillator Operation."
into saturation period determined 1/C2 time constant. Once voltage across reached maximum circuit value, Q1's collector current will longer increase. Since series with this drop primary current causes flux change because mutual coupling feedback winding further reduces primary current eventually turning off. primary windings change state with feedback winding forcing repeating alternate half cycle exactly with This action produces sinusoidal voltage wave form; whose amplitude proportional input voltage, across T1's primary winding which stepped capacitively coupled lamp.
Lamp Current Regulation
Initial ionization (lighting) fluorescent lamp requires several times voltage across than required sustain current through device. current through lamp sampled regulated MIC3172 achieve given intensity. MIC3172 uses maintain constant average current through transistor emitters. This current controls voltage amplitude Royer oscillator maintains lamp current. During negative half cycle, lamp current rectified During positive half cycle, lamp current rectified through filter voltage dropped across MIC3172's feedback pin. MIC3172 maintains constant lamp current adjusting duty cycle keep feedback voltage 1.24V. intensity lamp adjusted using potentiometer MIC3172 adjusts duty cycle accordingly bring average voltage across back 1.24V.
Oscillator Operation
Resistor provides initial base current that turns transistor impresses input voltage across half T1's primary winding (Pri T1's feedback winding provides additional base drive (positive feedback) forcing well
1:1:1 MIC3172 22µF COMP 1N5819
1N5819
100µH
1N5819
3.74k 470µF
VOUT
Enable Shutdown
1.24k
Voltage clipper Duty cycle limiter
Figure Forward Converter
1997
4-27
MIC2172/3172 On/Off Control
Especially important battery powered applications, lamp remotely automatically turned using MIC3172's pin. entire circuit draws less than while shutdown.
Micrel Efficiency
obtain maximum circuit efficiency careful selection collector emitter saturation voltage must. Inductor should chosen minimal core copper losses switching frequency MIC3172, should carefully constructed from magnetic materials optimized output power required Royer oscillator frequency. Suitable inductors obtained from Coiltronics, Inc., tel: (407) 241-7876.
Cold Cathode Fluorescent Lamp
4.5V
Enable (On) Shutdown (Off)
MIC3172 COMP 300µH
1N4148
1N4148
Intensity Control
Coiltronics CTX300-4P Coiltronics CTX110602 Polyfilm, WIMA FKP2 0.1µF 0.68µF 15pF 30pF, min.
Figure Backlight Fluorescent Lamp Supply
4-28
1997
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