AIC1578 high performance step-down DC/DC converter, designed drive ext

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AIC1578 high performance step-down DC/DC converter, designed drive ext

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AN007 High Efficiency Monitor Power Design Using AIC1578
AIC1578 high performance step-down DC/DC converter, designed drive external P-channel MOSFET generate programmable output voltages. main schematics Pulse-Skipping Pulse-Frequency Modulation employed maintain quiescent current high conversion efficiency under wide ranges input voltage loading condition. current sense comparator with both inverting non-inverting input uncommitted included provide crucial function either current limit protection constant output current control. When AIC1578 used high-side current source step-down constant current source, efficiency typically greater than 90%. Duty cycle adjusted greater than connecting resistor from DUTY VIN. Switching frequency being around 90KHZ 280KHZ range small size switching components
ideal portable equipment. order maintain good conversion efficiency form light loads full loads, AIC1578 uses intermittent switch control method (Pulse-Frequency Modulation) rather than conventional control method, Fig1. shows basic structure. When feedback voltage greater than reference voltage (1.22V),the Amp. output Low, Pin6 Level, turn outside drive device(P MOSFET), Whereas when feedback voltage lower than reference voltage, DRI( Pin6 Level, turn outside drive device. kind control method works similar full load, with stable switch waveform, Whereas when light load uses intermittent switching efficiently sustain output loading requirements.
March 1999
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Current Limit Comparator 60mV
DUTY LATCH
SHDN
Error Comparator 1.22V Reference Voltage Output Driver
Fig1. AIC1578 Function Block
addition, AIC1578 converter following feature: operate under input voltage 20V. Output voltage adjusted externally. design automatically adjusted switching frequency duty cycle, which makes possible obtain highly efficient conversion over wide input output voltage range. shutdown mode control works high frequency range 90KHZ 280KHZ,and only requires small size
inductors. complementary push-pull output,and drive external P-channel MOSFET transistor. cost.
Buck Switching Regulator Topology
Basic operation: Fig. shows basic structure Buck DC/DC converter (switching regulator).
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Drive Signal Current Signal
Waveform
VOUT IOUT
LOAD
VOUT Output Voltage
CONTROL CIRCUIT
IN-V
TOFF
Fig2. Typical Buck Converter Topology
basic operation principle feedback control ON-and-OFF power switch obtain specified output voltage, power applications, conventional control schemes ideal, because first, conversion efficiency high switching losses compared output power, second, fact that controller requires minimum load maintain stability. most efficient reliable control method then Pulse-Skipping-Modulation switching control with control waveforms shown Fig. This switching control method DC/DC converter into quasi-sleeping mode under load light load condition, which reduces switching
losses
while
maintaining
high
conversion
efficiency good stability. order choose appropriate switching converter electronic product, therefore, factors need considered: current capacity regulation output current should meet what product demands. High conversion efficiency. power consumption. Small size light weight.
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TOFF
Oscillator Output Error Comparator Input Error Comparator Output Driver Output
Sensed Output
VREF
Time Sequence Waveform
TYPICAL APPLICATION
circuit shown Fig.4 output power MONITOR when high efficiency obtained full load. Output Voltage VOUT 4.75 Output Current IOUT 0.2A Output Ripple VRIPPLE Voltage 5.25
Power Specification Item Input Voltage Symbol Min. Typ. Max. Unit
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220µF 0.01µF DUTY SHDN 4435 AIC1578 470µF 0.1µF 470µF :10V~14V VRIPPLE 100mV
300PF 47KF VOUT2: 3.3V/1A 1N5820 47µH 470µF 470µF 0.1µF VOUT1: 5V/2A VRIPPLE <50mV
AIC1085 VOUT 10µF 750RF 1.2K
MOMITOR POWER SOLUTION Switching
10µF
Fig4. AIC1578 MONITOR Power solution
Fig5. Frequency Duty Cycle
27°C Duty Frequency
Fig6. Duty Cycle RDUTY
VIN=5V
Frequency (KHz)
Duty Cycle
Duty
VIN=10V VIN=15V
VIN=20V
RDUTY refer Typ. App. Circuit.
RDUTY
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(ii) Design note Component selection:
Switching losses: These losses encountered during MOSFET states. They depend nature load well switching speed MOSFET.
Design note
DC-DC Converter efficiency Efficiency
POUT IOUT VOUT IOUT VOUT IOUT VOUT PLOSS
VDSIDdt
DSIDdt
feedback component R1,R2 following Datasheet equation
VOUT =1.22 47KF 15KF.
(R1+R2) must bigger than 50KR,for high efficiency request. noise filter depend device' switching frequency. Duty range MOSFET CEM4435 RDS-ON ,1N5820 :VF=0.475V)
VDSID(ts1 ts2)fs
switching frequency turn-on time turn-off time :supply voltage drain current
0.475 37.9% 0.04 0.475 0.475 0.04 0.475
Duty range 35.5% ,When ,FSW range 180KHZ 230KHZ Duty range 78%. Duty directly connect need larger Duty cycle than typical applications ,can reference Fig6 RDUTY adjust output inductor
Select MOSFET factors: RDS-ON CISS Short Reverse recovery time
SCHOTTKY BARRIER RECTIFIER SELECTION Conduction losses:Diode losses recovery time conduction strongly related circuit topology load impedance. IOUT (1-D) VF:Forward Conduction Voltage Select SCHOTTKY factors forward conduction voltage(
(VDC (VDC VO)TON 0.2ION
Component selection
Sitching MOSFET Selection power dissipation MOSFET divide into parts :Conduction losses Switching losses. Conduction losses :On-state losses related load current MOSFET
Short Reverse recovery time large Reverse Breakdown Voltage ID-PEAK IL-PEAK
Output Capacitors Selection
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bulk filter capacitor values generally determined ESR(effective series resistance) (effective series inductance) parameters rather than actual capacitance. High frequency decoupling capacitors Should placed close power pins load physically possible. careful inductance circuit board wiring that could cancel usefulness these inductance component, capacitors only specialized switching low-ESR regulator intended with lower available larger case sizes. Output Inductor Selection output inductor selected meet output voltage ripple requirements sets converter' response time load transient. inductor value determines converter' ripple current ripple voltage function ripple current. ripple voltage current approximate following equation
VOUT VOUT VOUT=I FSLO
applications bulk capacitors. bulk capacitor' determines output ripple voltage initial voltage drop after high slew-rate transient. aluminum electrolytic capacitor' value related case size
Increasing value inductance reduces ripple current converter' response time load transient.
Efficiency Test:
Input Voltage Input Current 1149 1754 1491 1271 Output Voltage 5.06V 5.06V 5.05V 5.05V 5.06V 5.06V 5.05V 5.06V 5.09V 5.09V 5.08V 5.07V Output Current 503mA 1003 2001 3001 503mA 1003 2001 3001 503mA 1003 2001 3001 Output Load 500mA 500mA 500mA Efficiency 87.8 89.0 87.9 86.4 84.2 86.5 86.1 84.9 84.3 87.0 86.9% 85.5
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2.Temperature Test
LOAD Unit: 1578 34.1 36.7 39.3 40.8 42.4 LOAD 35.5 37.9 38.3 40.2 40.6 32.8 34.5 34.5 34.8 35.1 35.9 36.4 36.6 37.5 39.3 1578 38.1 41.5 43.7 44.3 45.5 LOAD 42.2 48.8 51.2 56.6 58.9 35.1 35.6 36.1 38.7 39.5 44.1 49.9 50.7 62.1 69.4 1578 40.2 45.5 49.7 50.3 53.9 LOAD 51.3 53.3 65.8 66.4 67.2 37.3 38.9 37.4 39.2 44.3 61.1 66.5 70.7 74.6 80.1
3.TEST WAVEFORM:
Switching Signal Switching Signal
CH1: VG-GND DIV) CH2: VS-GND DIV) Status: VIN= 10VDC VOUT= 5.06VDC Output Load
CH1: VG-GND DIV) CH2: VS-GND DIV) Status: VIN= 12VDC VOUT= 5.05VDC Output Load
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FIG3: Switching Signal
FIG4: Switching Signal
CH1: VG-GND DIV) CH2: VS-GND DIV) STATUS: VIN= 10Vdc VOUT= 5.05Vdc Output Load FIG5: Switching Signal
CH1: VG-GND div) CH2: VS-GND div) STATUS: VIN= 10Vdc VOUT= 5.05Vdc Output Load Switching Signal
CH1: VG-GND DIV) CH2: VS-GND DIV) Status: VIN= 12VDC VOUT= 5.05Vdc Output Load
CH1: VG-GND DIV) CH2: VS-GND DIV) Status: VIN= 12VDC VOUT= 5.06Vdc Output Load
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FIG7: output ripple voltage
Switching Signal
CH1: Output (Ripple Voltage) Status: Input Voltage: Output Load: output ripple
CH1: Output (Ripple Voltage) Status: Input Voltage: Output Load: output ripple
CH1: Output (Ripple Voltage) Status: Input Voltage: Output Load:
CH1: Output (Ripple Voltage) Status: Input Voltage: Output Load:
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MONITOR LIST
Reference C1,C2,C3,C4 C7,C8 C9,10 Part Number AIC1578CS CEM4435 AIC1085CM 47µH 1N5820 470µF 0.1µF 0.01µF 47K/ 15K/ 12K/ 750/ SO-8 SO-8 TO-263 Manufacturer Cailcraft Cailcraft Remark N-MOSFET
Schottky

AIC1578 high performance step-down DC/DC converter, designed drive ext

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