AN007 High Efficiency LCD Monitor Power Design Using AIC1578 DESCRIPTION are ideal for portable equipment. The AIC1578 is a high performance step-down In order to maintain good conversion efficiency DC/DC converter, designed to drive an external form light loads to full loads, the AIC1578 uses the P-channel MOSFET to generate programmable intermittent switch control method of PFM output of (Pulse-Frequency Modulation) rather than the Pulse-Skipping and Pulse-Frequency Modulation conventional PWM control method, Fig1. shows are employed to maintain low quiescent current its basic structure. voltages. Two main schematics and high conversion efficiency under wide ranges of input voltage and loading condition. A current sense comparator with both inverting and non-inverting input uncommitted is included to provide the crucial function of either current limit protection or constant output current control. When the AIC1578 is used in a high-side current source step-down constant current source, the efficiency is typically greater than 90%. Duty cycle can be adjusted to greater than 90% by connecting a resistor from DUTY pin to VIN. Switching frequency being in around 90KHZ to When the feedback voltage is greater than the reference voltage (1.22V),the Err Amp. output is Low, and DRI ( Pin6 ) is Hi Level, turn off outside drive device(P MOSFET), Whereas when the feedback voltage is lower than the reference voltage, the Err Amp is Hi, and DRI( Pin6 ) is Low Level, turn off outside drive device. The kind of control method works similar to PWM at full load, with a stable switch waveform, Whereas when at light load it uses intermittent switching to efficiently sustain output loading requirements. 280KHZ range small size switching components March 1999 1 AN007 Current Limit Comparator VIN 60mV 1 DUTY SHDN 2 PFM OSC 3 CS+ VIN 7 LATCH 6 + FB 8 + CS- DRI Error Comparator 4 1.22V Reference Voltage Output Driver 5 GND Fig1. AIC1578 Function Block In addition, the AIC1578 converter has the following feature: 1. It can operate under an input voltage of 4V to 20V. 2. Output voltage can be adjusted externally. inductors. 6. It has complementary push-pull output,and can drive external P-channel MOSFET or PNP transistor. 7. Low cost. 3. It has a PFM design and automatically adjusted switching frequency and duty cycle, Buck Switching Regulator Topology which makes it possible to obtain highly Basic operation: Fig. 2 shows the basic structure of an Buck DC/DC converter (switching regulator). efficient conversion over a wide input and output voltage range. 4. It has a shutdown mode control 5. It works in the high frequency range of 90KHZ to 280KHZ,and only requires small size 2 AN007 VG t SW Drive Signal SW ISW VL + t VOUT - L SW Current Signal IOUT VG IL VIN + D IL LOAD C t Waveform VOUT Output Voltage CONTROL CIRCUIT t VL V IN-V OUT t -V OUT TOFF TON T Fig2. Typical Buck Converter Topology The basic operation principle is to use feedback to control the ON-and-OFF of the power switch to losses while maintaining high conversion efficiency and good stability. obtain the specified output voltage, for low power In order to choose the appropriate switching applications, conventional PWM control schemes converter for an electronic product, therefore, 4 key are not ideal, because of, first, the low conversion factors need to be considered: efficiency due to high switching losses as compared to low output power, and second, the fact that the PWM controller requires a minimum (1) The current capacity and regulation of the output current should meet what the product demands. load to maintain its stability. The most efficient and reliable control method is then to use a (2) High conversion efficiency. Pulse-Skipping-Modulation switching control with (3) Low power consumption. the control waveforms shown in Fig. 3. This switching control method can put the DC/DC (4) Small size and light weight. converter into quasi-sleeping mode under no load or light load condition, which reduces switching 3 AN007 TOFF TON Oscillator Output Error Comparator Input Sensed Output VREF Error Comparator Output Driver Output Fig 3. PSM Time Sequence Waveform TYPICAL APPLICATION The circuit shown in Fig.4 is an output power for LCD MONITOR , when VIN is 10V ~ 14V ,a high efficiency of 86% can be obtained at full load. Output Voltage VOUT 4.75 Output Current IOUT 0.2A Output Ripple VRIPPLE Voltage 5 100 5.25 3A V A mV (1) Power Specification : Item Input Voltage Symbol Min. Typ. Max. Unit VIN 10 14 V 4 AN007 L1 1µH Q1 C5 220µF 1 VIN CS+ 8 2 DUTY CS- 7 C11 3 SHDN 0.01µF 4 FB VIN :10V~14V VRIPPLE < 100mV C1 470µF Q2 DRI 6 C2 470µF GND 5 4435 C9 0.1µF AIC1578 L2 C6 VOUT1: 5V/2A 47µH 300PF VRIPPLE <50mV D1 1N5820 R1 C3 470µF C10 0.1µF C4 470µF 47KF R2 15K AIC1085 2 VOUT2: 3.3V/1A VIN VOUT 3 ADJ R4 C8 LCD MOMITOR POWER SOLUTION (1) Q3 1 C7 10µF 10µF 750RF Switching + LDO R3 1.2K F Fig4. AIC1578 for LCD MONITOR Power solution Fig6. Duty Cycle vs RDUTY Fig5. Frequency & Duty Cycle vs VIN 90 350 100 Ta = 27°C Duty 250 Frequency 75 200 70 150 65 100 60 Duty Cycle (%) 80 Duty (%) VIN=5V 300 Frequency (KHz) 85 90 VIN=10V 80 VIN=15V 70 VIN=20V 50 RDUTY refer to Typ. App. Circuit. 55 4 6 8 10 12 VIN ( V) 14 16 18 0 20 60 0 1 2 RDUTY (MΩ) 3 4 5 AN007 2 (ii) Design note and Component selection: Design note 1. DC-DC Converter efficiency Efficiency = POUT IOUT × VOUT IOUT × VOUT = = PIN IIN × VIN IOUT × VOUT + PLOSS PC = I out RDS –ON D Switching losses: These losses are encountered during the MOSFET on and off states. They depend on the nature of the load as well ws the switching speed of the MOSFET. ts1 PS = fS [ 0 2. Set feedback component ( R1,R2 ) following the Datasheet equation : R1 VOUT =1.22 (1+ )⇒ R1= 47KF , R2 = 15KF. R2 (R1+R2) must be bigger than 50KR,for high efficiency request. C6 is noise filter depend on device’s switching frequency. 3. Set Duty range :( if MOSFET CEM4435 : RDS-ON =20 mR ,1N5820 :VF=0.475V) 5 + 0.475 = 37.9% D min = 14 − 0.04 + 0.475 5 + 0.475 D max = = 65% 8 − 0.04 + 0.475 ∫ VDSIDdt + ≒ ts2 ∫V DSIDdt ] 0 VDSID(ts1 + ts2)fs 6 fs : switching frequency ts1 : turn-on time ts2 : turn-off time VDS :supply voltage ID : drain current Select MOSFET key factors: 1. Low RDS-ON 2. Low CISS 3. Short Reverse recovery time Duty range is : 35.5% ~ 65% See Fig 5 ,When VIN = 10V ~ 14V ,FSW range is 180KHZ ~ 230KHZ and Duty range is 74% ~ (2) SCHOTTKY BARRIER RECTIFIER SELECTION : Conduction losses:Diode losses due to recovery 78%. So, Duty pin can directly connect to VIN time and conduction are strongly related to circuit pin .If you need larger Duty cycle than typical topology and load impedance. applications ,can reference Fig6 add RDUTY to PCR = VF IOUT (1-D) adjust it . VF:Forward Conduction Voltage 4. Set output inductor L= (VDC − V0) (VDC − VO)TON = dI 0.2ION Select SCHOTTKY Key factors : 1. Low forward conduction voltage( VF ) 2. Low ESR Component selection : 3. Short Reverse recovery time 4. large Reverse Breakdown Voltage (1) Sitching MOSFET Selection The power dissipation of MOSFET is divide into two parts :Conduction losses and Switching losses. Conduction losses :On-state losses are related to the load current and MOSFET RDS –ON . 5. ID-PEAK > IL-PEAK (3) PWM Output Capacitors Selection 6 AN007 The bulk filter capacitor values are generally with lower ESR available in larger case sizes. determined by the ESR(effective series resistance) (4) PWM Output Inductor Selection and ESL (effective series inductance) parameters The output inductor is selected to meet the output rather than actual capacitance. High frequency voltage decoupling capacitors converter’s response time to a load transient. The Should be placed as close to the power pins of the load as physically possible. Be careful not to add inductance in the circuit board wiring that could cancel the usefulness of these low inductance component, capacitors use only intended for specialized low-ESR switching regulator applications for the bulk capacitors. The bulk capacitor’s ESR determines the output ripple voltage and the initial voltage drop after a high slew-rate transient. An aluminum ripple requirements and sets the inductor value determines the converter’s ripple current and the ripple voltage is a function of the ripple current. The ripple voltage and current are approximate by the following equation : ΔI= VIN − VOUT VOUT × , ΔVOUT=ΔI ×ESR FSLO VIN Increasing the value of inductance reduces the ripple current and converter’s response time to a load transient. electrolytic capacitor’s ESR value is related to the case size 1. Efficiency Test: Input Voltage 10V 10V 10V 10V 12V 12V 12V 12V 14V 14V 14V 14V Input Current 290 mA 570 mA 1149 mA 1754 mA 252 mA 489 mA 979 mA 1491 mA 217 mA 419 mA 836 mA 1271 mA 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 mA 2001 mA 3001 mA 503mA 1003 mA 2001 mA 3001 mA 503mA 1003 mA 2001 mA 3001 mA Output Load 500mA 1A 2A 3A 500mA 1A 2A 3A 500mA 1A 2A 3A 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 % 7 AN007 2.Temperature Test LOAD = 1A LOAD VIN 8V 10V 12V 14V 15V LOAD = 2A LOAD = 3A 1578 MOS L1 L2 1578 MOS L1 L2 1578 MOS L1 L2 34.1 36.7 39.3 40.8 42.4 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 38.1 41.5 43.7 44.3 45.5 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 40.2 45.5 49.7 50.3 53.9 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 Unit: °C 3.TEST WAVEFORM: FIG 1: Switching Signal CH1: VG-GND (5V / DIV) CH2: VS-GND (5V / DIV) Status: VIN= 10VDC VOUT= 5.06VDC Output Load = 1A FIG 2: Switching Signal CH1: VG-GND (5V / DIV) CH2: VS-GND (5V / DIV) Status: VIN= 12VDC VOUT= 5.05VDC Output Load = 2A 8 AN007 FIG3: Switching Signal CH1: VG-GND (5V / DIV) CH2: VS-GND (5V / DIV) STATUS: VIN= 10Vdc VOUT= 5.05Vdc Output Load = 3A FIG5: Switching Signal CH1: VG-GND (5V / DIV) CH2: VS-GND (5V / DIV) Status: VIN= 12VDC VOUT= 5.05Vdc Output Load = 2A FIG4: Switching Signal CH1: VG-GND (5V / div) CH2: VS-GND (5V / div) STATUS: VIN= 10Vdc VOUT= 5.05Vdc Output Load = 2A FIG 6: Switching Signal CH1: VG-GND (5V / DIV) CH2: VS-GND (5V / DIV) Status: VIN= 12VDC VOUT= 5.06Vdc Output Load = 3A 9 AN007 FIG7: 5V output ripple voltage CH1: 5V Output (Ripple Voltage) Status: Input Voltage: 10V Output Load: 1A FIG 9: 5V output ripple CH1: 5V Output (Ripple Voltage) Status: Input Voltage: 12V Output Load: 1A FIG 8: Switching Signal CH1: 5V Output (Ripple Voltage) Status: Input Voltage: 10V Output Load: 3A FIG 10: 5V output ripple CH1: 5V Output (Ripple Voltage) Status: Input Voltage: 12V Output Load: 3A 10 AN007 4. LCD MONITOR BOM LIST Reference Q1 Q2 Q3 L1 L2 D1 C1,C2,C3,C4 C5 C6 C7,C8 C9,10 C11 R1 R2 R3 R4 Part Number AIC1578CS CEM4435 AIC1085CM 1µH / 2A 47µH / 3A 1N5820 470µF / 16V 220 µF / 16V 330 PF 10 µF / 16V 0.1µF 0.01µF 47KΩ/ 1% 15KΩ/ 1% 12KΩ/ 1% 750Ω/ 1% QTY 1 1 1 1 1 1 4 1 1 2 2 1 1 1 1 1 PKG SO-8 SO-8 TO-263 SMD SMD DIP DIP DIP SMD DIP SMD SMD SMD SMD SMD SMD Manufacturer AIC CET AIC H&D / Cailcraft H&D / Cailcraft Remark N-MOSFET Schottky 11