AN-8023

www.fairchildsemi.com AN-8023 Negative Voltage Management Using a FAN8303 Buck Regulator Abstract FAN8303 is a 2A, 370kHz monolithic integrated buck regulator with internal power MOSFETs. It is simple to use and needs minimal external components. This application note describes how to generate negative voltage using FAN8303. It introduces application examples and discusses optimized designs for a buck-boost circuit. for linear regulators in PC monitor and TV applications. In some cases, a non-synchronous buck regulator also can be utilized for buck-boost circuit to generate negative voltage with respect to ground. These applications include audio amplifier, timing control circuit for LCD panel, and so on. Figure 1 shows a practical application of an LCD panel; it needs negative voltage for contrast control. In this block diagram, a charge pump is usually adopted due to the simple design and low cost. However, it has an amount of power dissipation and poor output voltage regulation relative to input voltage variation. FAN8303 with negative output would be a solution to overcome these problems. Introduction Buck regulators are widely used for higher voltage to lower voltage DC conversion. Likewise, FAN8303 was originally designed for application needing regulated DC voltage, such as set-top box microcontrollers and efficient pre-regulators Charge pump AV Board 12V -5V , 15V LDO DC/DC (buck) DC/DC (boost) 2.5V, 3.3V SoC Row Drivers TFT LCD Panel 2.5V DDR2 EEPROM Column Drivers 16V P-gamma IC Figure 1. Example of Timing Control Block © 2009 Fairchild Semiconductor Corporation Rev. 1.0.0 • 7/30/09 www.fairchildsemi.com AN-8023 APPLICATION NOTE Principle of Operation To understand buck-boost topology, buck topolgy is briefly compared below. When the MOSFET switch (Q1 in Figure 3) is turned on, the voltage across inductor (VL) is VIN -VOUT. During Q1 off-time, VL is equal to -VOUT in buck topology. So the inductor current (IL) ramps up with (VIN-VOUT)/L and ramps down with VOUT/L slope. Thus, the energy can be transferred to the load with positive output voltage. Meanwhile, in buck-boost topology, the inductor and freewheeling diode switch positions. When the MOSFET Q1 D1 Vo switch Q1 (Figure 2) is turned on, VL is same as VIN, so IL ramps up with VIN/L. During the Q1 off-time, VL has reverse polarity to maintain continuous inductor current with -VOUT. Therefore, it can generate negative output voltage. Buck-boost circuit with buck regulators require several design considerations. Table 1 summarizes the design parameter comparison between buck and buck-boost circuit. LOUT Q1 Vo PWM VIN CIN LOUT COUT Load VIN CIN PWM D1 COUT Load Q1 ON Q1 OFF Q1 ON Q1 OFF I Q1 I Q1 t I D1 t I D1 IL ∆I L VL VIN IL ∆I L VL VIN − VOU T VOUT −VOUT VOUT −VOUT Figure 2. Buck-Boost Topology Figure 3. Buck Topology Table 1. Buck and Buck-Boost Design Parameters IL (Average) IOUT 1− D Topology Buck-Boost Buck Maximum VSW VIN + VOUT Duty Cycles VOUT VIN + VOUT I OUT VIN VOUT VIN First of all, inductor current is limited by (1–D); so attention is needed to see that the maximum output current of buck regulator is be always lower than the maximum current in buck-boost circuit. Second, the switch node is a sum of input voltage and output voltage in buck-boost. It also needs to be © 2009 Fairchild Semiconductor Corporation Rev. 1.0.0 • 7/30/09 2 limited to the maximum switch node voltage of buck regulator. Since buck-boost is very noisy on input and output compared to buck circuit, it requires good-quality MLCC as input and output filters. www.fairchildsemi.com AN-8023 APPLICATION NOTE Design Considerations Inductor Selection When choosing inductor, the main concerns are inductance value, RMS current rating, and DCR. Inductance value is usually adopted higher than the minimum inductance to operate Continuous Current Mode (CCM). RMS current should be higher than the inductor current to prevent inductor saturation without core loss. A low-DCR inductor is usually adopted when a power system needs high efficiency. To operate in continuous current mode, critical minimum inductance is calculated by: L= VIN × D fSW × ΔIL VOUT VOUT + VIN Input Capacitor The input capacitor should handle the maximum input RMS current, so use the equations below for calculation. Good estimation is given by 10µF or 22µF per amp with MLCC. Maximum RMS input current: IRMS _ MAX = IOUTMAX × (D × (1 − D)) (4) Required minimum capacitance: CMIN = (IRMS × D) / (fSW × ΔVIN ) (5) (1) where ΔVIN is desired input voltage ripple. where: D= Freewheeling Diode = Duty cycle; = Switching frequency; and = Ripple current to maintain continuous current mode (typically 20%~30% of IL). The freewheeling diode acts as a inductor current path when the switch is turned off. Breakdown voltage, lower forward drop voltage, and the maximum current rating are considered for low power dissipation. A Schottky diode is preferred, which has low forward voltage drop. Required diode current rating: > ILMAX (6) fSW ΔIL Output Capacitor An output capacitor is needed to satisfy the output voltage ripple requirement and to maintain constant output voltage during dynamic load condition. Ripple voltage depends on ESR, output capacitance, and ESL. To obtain the desired output ripple, the below equation for required minimum capacitance is useful: CMIN = IOUTMAX × DMAX fSW × ΔVOUT (2) where ILMAX is maximum inductor current. Required breakdown voltage: > VIN + VOUT (7) where: DMAX ΔVOUT = Maximum Duty Cycle; Maximum Output Current; and Desired Output Voltage Ripple. IOUTMAX = = ΔVOUT ILMAX The equation for required ESR is: ESR = (3) © 2009 Fairchild Semiconductor Corporation Rev. 1.0.0 • 7/30/09 www.fairchildsemi.com 3 AN-8023 APPLICATION NOTE Design Example A design example with test conditions VIN =12V, VOUT = -5V, IOUT = 1A, and fSW =370 kHz (fixed) is shown below. The first step is to set the critical design parameters, such as inductor ripple current (∆IL) and desired output ripple voltage (∆VOUT). The second step is calculation of duty cycle. To achieve accurate value, consider the forward voltage drop of diode and MOSFET switch on drop voltage. Fairchild FAN8303, non-synchronous buck regulator has integrated 0.22Ω N-channel MOSFET, so on drop voltage is about 0.4V. Forward voltage of the Schottky diode (40VRRM/ 2A IOUT) is 0.45V. When it comes to the inductor, a higher value than calculated is recommended and a low DCR inductor is preferred: Table 2. Design Example Calculations = (|VOUT|+ VF) / (VIN +|VOUT|+VF-VQ1) = (VIN × D)/ (fSW × ∆IL) = (IOUT × D) / (fSW × ∆VOUT) IRMS = IOUT × D × (1 − D) CIN = IRMS × D/ (∆VIN ×fSW ) 0.33 35.6µH (desired ∆IL = 20%) 86.8µF (desired ∆VOUT = 10mV) 0.47A 4.05µF 1.77A Duty Cycle: Inductance: Output Capacitance: Input Capacitance: Diode Current Rating: IDIODE_MAX = IAVG + ∆IL/2 where IAVG = Average Inductor Current 10nF CBS INPUT 12V BOOT VIN Q1 VSW FB 39µH L1 GND 2.45k R2 PWM GND 10µF COMP SS 1nF 10nF CIN CIN2 10µF CC D1 CSS 18k R3 COUT 22µF x 4EA Load 40k RC OUTPUT -5V / 1A Figure 4. Buck-Boost Schematic Using FAN8303 © 2009 Fairchild Semiconductor Corporation Rev. 1.0.0 • 7/30/09 www.fairchildsemi.com 4 AN-8023 APPLICATION NOTE Typical Waveforms & Graphs Figure 5 and Figure 6 show the typical waveforms of the FAN8303 output ripple voltage. To achieve low ripple voltage, lower than 10mΩ MLCC is used. Figure 7 shows FAN8303 efficiency and power-loss graph. It indicates a maximum of 87% efficiency with 0.31W at 400mA load condition. VOUT, 50mV/div VOUT, 50mV/div IOUT, 500mA/div IOUT, 500mA/div VSW, 5V/div VSW, 5V/div Figure 5. VOUT Ripple (1µs/div), 33mV at 100mA Figure 6. VOUT Ripple (1µs/div), 89mV at 1A Note: 1. Test conditions: VIN =12V, VOUT = -5V, fSW = fixed 370 kHz, and IOUT = 0~1A. Efficiency and Power Loss 88 86 84 Efficiency (%) 82 80 0.5 78 0.4 76 0.3 0.2 0.1 0.0 0 200 400 600 800 1000 Load (mA) 1.0 0.9 0.8 0.7 0.6 Power Loss (W) Efficiency 74 72 70 Power loss Figure 7. Efficiency and Power Loss © 2009 Fairchild Semiconductor Corporation Rev. 1.0.0 • 7/30/09 www.fairchildsemi.com 5 AN-8023 APPLICATION NOTE Conclusion Fairchild 2A monolithic and non-synchronous buck regulator, FAN8303, has wide input range (~23V) with excellent load and line regulation. In spite of buck regulator, FAN8303 also can be utilized for buck-boost circuit to generate negative output voltage with simple changes of passive element. Author DSEOM Application Engineer, SGYOON Application Engineer Related Datasheets FAN8303 — 2A 23V Non-Synchronous Step-Down DC/DC Regulator DISCLAIMER FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION, OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS. LIFE SUPPORT POLICY FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, or (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. © 2009 Fairchild Semiconductor Corporation Rev. 1.0.0 • 7/30/09 www.fairchildsemi.com 6