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MP1472 Arkusz danych(PDF) 8 Page - Monolithic Power Systems |
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MP1472 Arkusz danych(HTML) 8 Page - Monolithic Power Systems |
8 / 13 page MP1472 – 2A, 18V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER MP1472 Rev. 1.0 www.MonolithicPower.com 8 9/2/2011 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2011 MPS. All Rights Reserved. Optional Schottky Diode During the transition between high-side switch and low-side switch, the body diode of the low- side power MOSFET conducts the inductor current. The forward voltage of this body diode is high. An optional Schottky diode may be paralleled between the SW pin and GND pin to improve overall efficiency. Table 2 lists example Schottky diodes and their Manufacturers. Table 2—Diode Selection Guide Part Number Voltage/Current Rating Vendor B230 30V, 2A Diodes, Inc. SL23 30V, 2A Vishay, Inc. MBRS230 30V, 2A International Rectifier Input Capacitor The input current to the step-down converter is discontinuous, therefore a capacitor is required to supply the AC current to the step-down converter while maintaining the DC input voltage. Use low ESR capacitors for the best performance. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic capacitors may also suffice. Choose X5R or X7R dielectrics when using ceramic capacitors. Since the input capacitor (C1) absorbs the input switching current it requires an adequate ripple current rating. The RMS current in the input capacitor can be estimated by: IN OUT IN OUT LOAD 1 C V V 1 V V I I The worst-case condition occurs at VIN = 2VOUT, where IC1 = ILOAD/2. For simplification, choose the input capacitor whose RMS current rating greater than half of the maximum load current. The input capacitor can be electrolytic, tantalum or ceramic. When using electrolytic or tantalum capacitors, a small, high quality ceramic capacitor, i.e. 0.1μF, should be placed as close to the IC as possible. When using ceramic capacitors, make sure that they have enough capacitance to provide sufficient charge to prevent excessive voltage ripple at input. The input voltage ripple for low ESR capacitors can be estimated by: IN OUT IN OUT S LOAD IN V V 1 V V f 1 C I V Where C1 is the input capacitance value. Output Capacitor The output capacitor is required to maintain the DC output voltage. Ceramic, tantalum, or low ESR electrolytic capacitors are recommended. Low ESR capacitors are preferred to keep the output voltage ripple low. The output voltage ripple can be estimated by: 2 C f 8 1 R V V 1 L f V V S ESR IN OUT S OUT OUT Where C2 is the output capacitance value and RESR is the equivalent series resistance (ESR) value of the output capacitor. In the case of ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance. The output voltage ripple is mainly caused by the capacitance. For simplification, the output voltage ripple can be estimated by: IN OUT 2 S OUT OUT V V 1 2 C L f 8 V ΔV In the case of tantalum or electrolytic capacitors, the ESR dominates the impedance at the switching frequency. For simplification, the output ripple can be approximated to: ESR IN OUT S OUT OUT R V V 1 L f V ΔV The characteristics of the output capacitor also affect the stability of the regulation system. The MP1472 can be optimized for a wide range of capacitance and ESR values. Compensation Components MP1472 employs current mode control for easy compensation and fast transient response. The system stability and transient response are controlled through the COMP pin. COMP pin is the output of the internal transconductance error amplifier. A series capacitor-resistor combination sets a pole-zero combination to control the characteristics of the control system. |
Podobny numer części - MP1472 |
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Podobny opis - MP1472 |
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