AN-8023 FAIRCHILD | Alldatasheet
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transferred to the load with positive output voltage. polarity to maintain continuous inductor current with -VOUT. Therefore, it can generate negative output voltage. parameter comparison between buck and buck-boost circuit. Figure 2. Buck-Boost Topology Figure 3. Buck Topology Table 1. Buck and Buck-Boost Design Parameters MLCC as input and output filters.
© 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com 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: LSW IN If DVL Δ× ×= (1) where: INOUT OUT VV V D = = Duty cycle; fSW = Switching frequency; and ΔIL = Ripple current to maintain continuous current mode (typically 20%~30% of IL). 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: OUTSW MAXOUTMAX MIN Vf DIC Δ× ×= (2) where: DMAX = Maximum Duty Cycle; IOUTMAX = Maximum Output Current; and ΔVOUT = Desired Output Voltage Ripple. The equation for required ESR is: LMAX OUT I VESR Δ= (3) 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: ()( )D1DII OUTMAXMAX_RMS −××= (4) Required minimum capacitance: ( ) ( )INSWRMSMIN Vf/DIC Δ××= (5) where ΔVIN is desired input voltage ripple. Freewheeling Diode 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: LMAXI> (6) where ILMAX is maximum inductor current. Required breakdown voltage: OUTIN VV +> (7)
-5V, IOUT = 1A, and fSW =370 kHz (fixed) is shown below. voltage drop of diode and MOSFET switch on drop voltage. Table 2. Design Example Calculations Figure 4. Buck-Boost Schematic Using FAN8303
© 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com 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 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.