AN024 For LCD Motherboard Power Supply Using AIC1563 Introduction The newest technology of monitor is Flat-Panel Display Fig. 1 shows the structure of LCD monitor (LCDM), (FPD). In FPD field, the widest and most grow up there are three main parts in it: motherboard, panel application is Liquid Crystal Display (LCD). It has many and inverter. For normal working, LCD monitor needs a advantages, such as: light weight, less space, low good power management to get in. This application voltage operating and power saving. They are very note focus on the power design for the motherboard of attractive. Certainly, on the brightness, contrast and the LCD. It needs 5V; 3.3V and 2.5V output voltage, which activity transient despondence of display, they can’t depend on vender’s desire. This application note will compare with Cathode Ray Tube (CRT) monitor. discuss the steps of design. ADC TMDS Deccoder Scaler Key Pads ‘ Adaptor MCU EEPROM Source Driver OSD LCD Panel CCFL Video Gate Driver DVI Timing Controller RGB PLL LCD MB Power LCD Panel Power Inverter Fig.1 LCD structure February, 2002 1 AN024 Application Fig. 2 shows the application circuit for LCD power voltage stores energy on inductor through Q2 and the management. This application circuit provides 5V, output voltage starts rise until the feedback voltage 3.3V and 2.5V output voltage. It uses AIC1563 to get between R3 and R4 get in 1.25V. When the feedback in the 12V-5V1A outputs. AIC1563 is a versatile voltage gets bigger than the reference voltage, the DC/DC converter, designed to drive an external comparator in IC turns the bootstrapped driver off. At P-MOSFET to generate a programmable output this time PIN1 is floating, the voltages at collector and voltage. It consists of an internal temperature base terminal of transistor Q1 are the same. It turns compensated Q1 on and Q2 off, and then the inductor supplies the reference, comparator and a bootstrapped driver, which can drive the NPN output energy to output terminal. switch to saturation for higher efficiency and less heat During Q2 turns on, the voltage across the input dissipation. Fig. 3 shows the block diagram, and it has parasitism capacitor of Q2 is approximately zero. If following features: there is no charge path for the parasitism capacitor, 1. 3V to 30V input voltage operation. Q2 will get over temperature. Therefore during IC 2. 5A continuous output current. turns off, the transistor Q1 turns on and provides a 3. Bootstrapped driver. charge path for the input parasitism capacitor of Q2 4. High side current sense capability. during Q2 turns off. 5. Frequency operation from 100Hz to 100kHz. Concerning 6. High efficiency (up to 90%). AIC1084-33CT to achieve easily. The function of D3 7. PSM (Pulse Skipping Modulation) control. and D4 are to reduce the dropout voltage from 5V The operate principle is when the power turn on; the P output of AIC1563. Due to there is no LDO has such channel MOSFET Q2 doesn’t turn on yet and there is low dropout voltage can be used, we use AIC431 with no feedback voltage. The comparator in AIC1563 an N channel MOSFET to construct a LDO structure delivers a high signal to turn on the bootstrapped with 2.5V output voltage from 3.3V output. the 3.3V/1A output, we can use driver. At this time, PIN 1 connects to ground through a NPN transistor and makes the transistor Q1 turns off and the P channel MOSFET Q2 turns on. The input 2 AN024 5VOUT/1A VOUT L1 Q2 GF4435 8 R1 0.1 VIN 8~12VIN + 7 6 BOOST DC IS DE VCC CF R2 2.2K 1 2 4 5 C1 FB GND 470µF AIC1563 3 47µH Q1 MMBT2222A D2 1N5820 + C3 R3 470µF 3K D1 LL4148 3 3.3VOUT/1A N1 3.3VOUT/1A AIC1084-33CM D3 1N5401 VOUT GND 1 + D4 1N5401 C4 470µF/6.3V N11 GND R4 1k GND C2 VIN 2 Q3 GF4410 N14 2.5VOUT/1A + R5 1.2k N12 GND R6 C5 470µF/6.3V U3 3.9k AIC431 360pF Fig. 2 The application circuit for the motherboard of LCD 1 DC 8 Q2 QS Q1 R 80 2 DE BOOST 7 IS Is CT Oscillator CF 6 3 Comparator 1.25V Reference Voltage GND 4 VCC + - 5 FB Fig. 3 The block diagram of AIC1563. Components Selection should be (VOUT+VD2)*TOFF, where VD2 (0.75V) is (1). Current Limit Resistor (R1) Selection The current limit resistor can be calculated by following formula: R1=0.3/ILIMIT (1) Since we need a 3A loading at least, the current limit resistor should be R1=0.3/3A=0.1Ω. the forward voltage of the free-wheel diode D2. According to the law of the conservation of magnetic flux, the energy stored in inductor should be the same at MOSFET turns on and off. Therefore the relationship can be written as (VIN- VOUT-VQ1,ON)*TON= (VOUT +VD2)*TOFF (2) (2). Oscillator Timing Capacitor (CT) Selection During the MOSFET turns on, the energy stores and the duty cycle is in inductor should be (VIN-VOUT-VQ2,ON)*Ton, DUTY = where VQ2,on (0.475V) is the dropout voltage at MOSFET (Q2) turns on. Similarly, during the TON VOUT + VD2 = = 69.49% T VIN − VQ2,ON + VD2 (3) MOSFET turns off, the energy stores in inductor 3 AN024 If we set the switching frequency been 80kHz, Consider the output current capacity, we select a then the turn-on time of Q1 is 47µH inductor with 5A in this case. It is TON = DUTY × T = 69.49% × 1 = 8.69µS 80K (4) For the oscillator of AIC1563, the typical charging current is 25µA and the voltage amplitude is 0.6V. From the formula ICT recommended to use the inductor with magnetic shield to reduce the magnetic loop. (4). Output Capacitor (C3) Selection If we set the maximum ripple voltage is 100mA, then the minimum capacitor at output terminal dVCT = CT dT (5) can be obtained by C OUT and it can be written as C T = ICT dt 8.69 × 10 −6 = 25 × 10 − 6 = 361.91pF dVCT 0. 6 is 80KHz. 8 VRIPPLEF (3 + 0.5) 8 × 100 × 10 −3 × 80 × 10 3 = 54.69µF (10) where F is the switching frequency, which we set before. However, the output ripple voltage (3). Inductor (L1) Selection Owing IP(MAX ) = (6) so, we select CT=360pF for switching frequency = to this depends on ESR (equivalent series resistance) of application circuit supply output capacitor. The output ripple voltage can be three-output voltage, which concludes 5V/1A, maintained by 3.3V/1A and 2.5V/1A. The total loading current is Vripple = ∆IP × ESR 3A. For working at CCM (Continuous Conduction and it can be written as Mode), assuming the minimum loading is 500mA. Then the peak-to peak current amplitude of ESR = VRIPPLE 100 × 10 −3 = = 0.1(Ω ) ∆IP 1 (11) (12) inductor is Thus, we select HERMEI LT series 470µF/6.3V ∆IP = 2IOUT(MIN) = 2 × 500mA = 1A .................. (7) capacitor. To maintain the minimum loading at the continuous condition mode, the minimum value of inductor can be obtained form VL = L dIL ...................................................... (8) dT it can be written as L (MIN) = VL dT Ton = ( VIN − VOUT − VQ2 ) dIL ∆IP = (8 − 5 − 0.475 ) 8.69 × 10 −6 1 (9) = 21.94µH 4 AN024 Layout Consideration 2. Each ground should be placed at the same point to effectively reduce the loop. And the There are several notes needs to be considered ground plane shouldn’t be cut apart, or it may at layout PCB board. occur disturbed signals and noise. 1. The components, which follow large current, should be placed as close as possibly for 3. The traces, which following large current, should be layout as width and shorten as reducing the loop. And each component possibly for reducing the parasitism resistor. should be placed along the direction of output current following in the circuit for reducing the Following shows the recommend layout of this application circuit. loss. Fig. 4. Top Layer Fig. 5. Bottom Layer Fig. 6. Top Over Layer Test Result Efficiency Test VIN (V) IIN (mA) VOUT (V) IOUT (mA) EFF (%) 8 343 4.952 500 90.23324 8 679 4.949 1000 91.10825 8 1374 4.9465 2000 90.00182 8 2103 4.943 3000 88.14194 12 246 4.983 500 84.40041 12 476 4.979 1000 87.16737 12 942 4.9585 2000 87.73001 12 1436 4.9585 3000 86.32486 14 215 4.9795 500 82.71595 14 421 4.976 1000 84.42484 14 829 4.9725 2000 85.68844 14 1257 4.97 3000 84.72554 5 AN024 Frequency Test VIN=12V, CT=360pF. Ripple Voltage Test VIN=12V, VOUT=5.0V, IOUT=500mA VIN=12V, VOUT=5.0V, IOUT=1A VIN=12V, VOUT=5.0V, IOUT=2A VIN=12V, VOUT=5.0V, IOUT=3A 6 AN024 Conclusion step-down and step-up DC/DC converter and inverting converter. This application note is using In this application note, the main part focuses on the step-down function of AIC1563 and cooperates AIC1563 power design. It can supply a stable 5V AIC1084-33CT and AIC431 to achieve the power output voltage for the main power of motherboard management for LCD motherboard. There are and has high efficiency at light loading (82% many application on AIC1563 needs to be upward). It can be used at many fields, such as: discovered. constant current source for battery charger, 7