MIC2171 100kHz 2.5A Switching Regulator General Description

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MIC2171 100kHz 2.5A Switching Regulator General Description

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MIC2171
MIC2171
100kHz 2.5A Switching Regulator
General Description
MIC2171 complete 100kHz SMPS current-mode controller with internal 2.5A power switch. Although primarily intended voltage step-up applications, floating switch architecture MIC2171 makes practical step-down, inverting, configurations well isolated topologies. Operating from 40V, MIC2171 draws only quiescent current, making attractive battery operated supplies. MIC2171 available 5-pin TO-220 TO-263 -40°C +85°C operation.
Features
2.5A, internal switch rating input voltage range Current-mode operation, 2.5A peak Internal cycle-by-cycle current limit Thermal shutdown Twice frequency LM2577 external parts count Operates most switching topologies quiescent current (operating) Fits LT1171/LM2577 TO-220 TO-263 sockets
Applications
Laptop/palmtop computers Battery operated equipment Hand-held instruments Off-line converter (requires external power switch) Predriver higher power capability
Typical Applications
(4.75V min.)
15µH
47µF
MIC2171
COMP
1N5822
+12V, 0.25A
47µF
1N5818
0.5A
3.74k 470µF
10.7k
1.24k 470µF
MIC2171
1:1.25 12µH
COMP
Locate near MIC2171 when supply leads
1.24k
Optional voltage clipper (may req'd leakage inductance high)
Figure MIC2171 Boost Converter
Figure MIC2171 Flyback Converter
Micrel, Inc. 2180 Fortune Drive Jose, 95131 (408) 944-0800 (408) 474-1000 http://www.micrel.com
April 2005
M9999-041505
MIC2171
Micrel, Inc. Part Number Junction Temp. Range -40°C +85°C -40°C +85°C Package TO-220-5 TO-263-5
Ordering Information
Standard MIC2171BT MIC2171BU Pb-Free* MIC2171WT MIC2171WU
Pb-Free RoHS compliant with "hot-melting solder" exemption.
Configuration
COMP
TO-220-5 (BT)
TO-263-5 (BU)
COMP
Description
Number Name COMP Function 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. Ground: Connect directly input filter capacitor proper operation (see applications info). Power Switch Collector: Collector switch. Connect external inductor input voltage depending circuit topology. Supply Voltage: 3.0V
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MIC2171
Micrel, Inc. Junction Temperature -55°C 150°C Thermal Resistance 5-lead TO-220, Note .45°C/W 5-lead TO-263, Note .45°C/W Storage Temperature -65°C +150°C Soldering sec.) +300°C
Absolute Maximum Ratings
Input Voltage (VIN) Switch Voltage (VSW) Feedback Voltage (transient, 1ms) (VFB) ±15V Operating Temperature Range +85°C
Electrical Characteristics
25°C, bold values indicate -40°C +85°C; unless noted. Parameter Reference Section Feedback Voltage (VFB) Feedback Voltage Line Regulation Feedback Bias Current (IFB) Error Amplifier Section Transconductance (gm) Voltage Gain (AV) 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 80%, Note 0.37 0.50 0.55 ICOMP ±25µA 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 µA/mV µA/mV VCOMP 1.24V VCOMP 1.24V 1.24V 1.220 1.214 1.240 1100 1.264 1.274 Conditions Units
Breakdown Voltage (BV) Oscillator Section Frequency (fO) Duty Cycle [(max)] Input Supply Voltage Section Minimum Operating Voltage Quiescent Current (IQ) Supply Current Increase (IIN)
Note Note Note
40V, VCOMP 0.6V, VCOMP 1.5V, during swich on-time
General Note Devices sensitive. Handling precautions required. Mounted vertically, external heat sink, inch leads soldered board containing approximently inch squared copper area surrounding leads. ground leads soldered approximently inches squared horizontal board copper area. duty cycles between 95%, minimum guaranteed switch current 1.66 (2-) (Pk).
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MIC2171
Typical Performance Characteristics
Minimum Operating Voltage Feedback Voltage Line Regulation
125°C
-100 Temperature (°C)
Switch Current
Feedback Bias Current (nA)
-100 Temperature (°C)
Feedback Voltage Change (mV)
Minimum Operating Voltage
Feedback Bias Current
25°C
-40°C
Operating
Average Supply Current (mA)
Supply Current (mA)
Supply Current (mA)
Supply Current
Supply Current
-100
Supply Current
0.6V
Operating Voltage
Switch Current
Temperature(°C)
Switch Voltage
Switch On-Voltage
25°C
-40°C
Oscillator Frequency
-40°C
Current Limit
Switch Current
Frequency (kHz)
25°C
125°C
125°C
Switch Current
Temperature(°C)
Duty Cycle
Transconductance (µA/mV) -100
Error Amplifier Gain
7000 Transconductance(µS) 6000 5000
Error Amplifier Gain
Phase Shift 1000 Frequency (kHz) 10000
Error Amplifier Phase
4000 3000 2000 1000
Temperature(°C)
1000 Frequency (kHz)
10000
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MIC2171
Block Diagram MIC2171
Reg.
2.3V
Anti-Sat.
100kHz Osc.
Logic
Driver
parator
Error 1.24V Amp. Ref.
Current Amp.
COMP
Functional Description
Refer "Block Diagram MIC2171". Internal Power MIC2171 operates when 2.6V. internal 2.3V regulator supplies biasing internal circuitry including precision 1.24V band reference. 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 MIC2171 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 MIC2171 eliminating base collector stored charge which would delay Q1's turnoff. Compensation Loop stability compensation MIC2171 accomplished connecting appropriate network from either COMP circuit ground (see "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.
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MIC2171
Micrel, Inc. 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). MIC2171 supply current almost constant regardless supply voltage (see "Electrical Characteristics"). driver section losses (not including switch) function supply voltage, power switch current, duty cycle. P(bias+driver) (VIN (VIN(min) IIN) where: P(bias+driver) device operating losses VIN(min) supply voltage typical quiescent supply current power switch current limit typical supply current increase practical example refer Figure 5.0V 0.007A
Applications Information
Soft Start diode-coupled capacitor from COMP circuit ground slows output voltage rise turn (Figure
MIC2171
COMP
Figure Soft Start additional time takes error amplifier charge capacitor corresponds time takes output reach regulation. Diode discharges when removed. Current Limit
2.21A 66.2% (0.662) Then: VIN(min) (2.21 0.37) 4.18V P(bias+driver) 0.007) (4.18 2.21 .009) P(bias+driver) 0.1W 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 where: duty cycle VOUT VIN(min) VOUT (RSW) VOUT output voltage forward voltage drop IOUT From Typical performance Characteristics: Then: 0.37 (2.21)2 0.37 0.662 PSW) 1.2W P(total) P(total) 1.3W
MIC2171
COMP
0.6V/R2
Note: Input output returns common.
Figure Current Limit maximum current limit MIC2171 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 MIC2171 family contains thermal protection circuitry, best reliability, avoid prolonged operation with junction temperatures near rated maximum. junction temperature determined first calculating power dissipation device. MIC2171, total power dissipation device operating losses power switch losses.
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MIC2171 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 45°C/W (TO-220)
Micrel, Inc. mode preferred because feedback control converter simpler. When discharges current completely during MIC2171 off-time, operating discontinuous mode. operating continuous mode does discharge completely before MIC2171 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. VIN(min) VOUT VOUT
Then:
(1.24 126°C This junction temperature below rated maximum 150°C. Grounding Refer Figure Heavy lines indicate high current paths.
MIC2171
(1a)
IOUT
VOUT VIN(min)
Where: internal switch current limit 2.5A when 1.67 when (Refer Electrical Characteristics.) IOUT maximum output current VIN(min) minimum input voltage duty cycle VOUT required output voltage forward voltage drop example Figure IOUT 0.25A 1.67 (2-0.662) 2.24A VIN(min) 4.18V 0.662 VOUT 12.0V 0.36V .26A, 70°C) 2.235 4.178 0.662
Single point ground
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 MIC2171 switching regulator from functioning properly. Boost Conversion Refer Figure typical boost conversion application where logic supply available +12V 0.25A required. first step designing boost converter determining whether inductor will cause converter operate either continuous discontinuous mode. Discontinuous
Then:
IOUT
IOUT 0.258A This value greater than 0.25A output current requirement, proceed find minimum inductance value discontinuous operation POUT. POUT Where: POUT 0.25 (100kHz) (VIN
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MIC2171 practical example: 12.4µH (use 15µH) Equation solves L1's maximum current value. Where: IL1(peak) (4.178 0.662)2 (failure) MIC2171's internal power switch. 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. POUT
IL1(peak)
4.178 6.62 10-6 IL1(peak) 1.84A 15µH inductor with peak current rating least 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 MIC2171. 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 MIC2171 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 Frequency Compensation simple frequency compensation network consisting prevents output oscillations. High impedance output stages (transconductance type) MIC2171 often permit simplified loop-stability solutions connected circuit ground, although more conventional technique connecting components from error amplifier output inverting input also possible. Voltage Clipper Care must taken minimize T1's leakage inductance, otherwise necessary incorporate voltage clipper consisting avoid second breakdown
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(VIN(min) VSW) practical example let: (see Figure POUT 5.0V 0.5A 2.5W 4.0V 6.0V 2.5A when 1.67 when Then: VIN(min) (ICL RSW) VIN(min) 0.78V VIN(min) 3.22V 0.74 (74%), less than discontinous permitted. iterations equation required duty cycle found greater than 50%. Calculate maximum transformer turns ratio NPRI/NSEC, that will guarantee safe operation MIC2171 power switch. VIN(max)
VSEC 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. MIC2171 VSEC 5.6V
Then:
(NPRI/NSEC) Next, calculate maximum primary inductance required store needed output energy with power switch duty cycle 55%.
April 2005
MIC2171 LPRI VIN(min)2 TON2 POUT (12) Then:
(10) Where:
LSEC
11.4 1.20
LPRI
Then:
LPRI maximum primary inductance device switching frequency (100kHz) VIN(min) minimum input voltage power switch time LPRI (3.22)2 (7.4 10-6)2
This ratio less than ratio calculated equation (9). When specifying transformer necessary know primary peak current which must withstood without saturating transformer core. IPEAK(pri) VIN(min) LPRI 3.22 10-6 12µH
LPRI 11.4µH 12µH 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. LSEC (11) Where: VSEC2 TOFF2 POUT
(13)
IPEAK(pri)
IPEAK(pri) 2.1A find minimum reverse voltage requirement output rectifier. This rectifier must have average current rating greater than maximum output current 0.5A. VIN(max) (VOUT
(14) Where:
Then:
LSEC maximum secondary inductance TOFF power switch time (5.41)2 (2.6 10-6)2 Then:
output rectifier maximum peak reverse voltage rating transformer turns ratio (1.2) reverse voltage safety derating factor (0.8) (5.0 1.2)
LSEC
LSEC 7.9µH Finally, recalculate transformer turns ratio insure that less than value earlier found equation (9).
12.5V 1N5817 will safely handle voltage current requirements this example.
April 2005
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MIC2171 Forward Converters Micrel's MIC2171 used several circuit configurations generate output voltage which less than input voltage (buck step-down topology). Figure shows MIC2171 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 MIC457x family buck switchers). However, step-down conversion requiring transformer (forward), MIC2171 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
Micrel, Inc. 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 50%, MIC2172 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 MIC2171 exceeds duty cycle 50%, T1's reset winding current turns This action reduces duty cycle MIC2171 until able reset during each cycle.
1:1:1
1N5819
100µH
1N5819
3.74k 470µF
22µF
MIC2171
COMP
1N5819
1.24k
Voltage clipper Duty cycle limiter
Figure MIC2171 Forward Converter
M9999-041505
April 2005
MIC2171
Package Information
5-Lead TO-220
5-Lead TO-263
April 2005
M9999-041505
MIC2171
MICREL INC.
(408) 944-0800 (408) 474-1000 http://www.micrel.com
2180 FORTUNE DRIVE
JOSE, 95131
This information furnished Micrel this data sheet believed accurate reliable. However responsibility assumed Micrel use. Micrel reserves right change circuitry specifications time without notification customer. Micrel Products designed authorized components life support appliances, devices systems where malfunction product reasonably expected result personal injury. Life support devices systems devices systems that intended surgical implant into body support sustain life, whose failure perform reasonably expected result significant injury user. Purchaser's sale Micrel Products life support appliances, devices systems Purchaser's risk Purchaser agrees fully indemnify Micrel damages resulting from such sale. 1997 Micrel Incorporated M9999-041505
April 2005

MIC2171 100kHz 2.5A Switching Regulator General Description

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