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MP1472 Datasheet(PDF) 9 Page - Monolithic Power Systems |
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MP1472 Datasheet(HTML) 9 Page - Monolithic Power Systems |
9 / 13 page MP1472 – 2A, 18V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER MP1472 Rev. 1.0 www.MonolithicPower.com 9 9/2/2011 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2011 MPS. All Rights Reserved. The DC gain of the voltage feedback loop is given by: OUT FB EA CS LOAD VDC V V A G R A Where AVEA is the error amplifier voltage gain; GCS is the current sense transconductance and RLOAD is the load resistor value. The system has two poles of importance. One is due to the compensation capacitor (C3) and the output resistor of the error amplifier, and the other is due to the output capacitor and the load resistor. These poles are located at: VEA EA 1 P A 3 C 2 G f LOAD 2 P R 2 C 2 1 f Where GEA is the error amplifier transconductance. The system has one zero of importance, due to the compensation capacitor (C3) and the compensation resistor (R3). This zero is located at: 3 R 3 C 2 1 f 1 Z The system may have another zero of importance, if the output capacitor has a large capacitance and/or a high ESR value. The zero, due to the ESR and capacitance of the output capacitor, is located at: ESR ESR R 2 C 2 1 f In this case, a third pole set by the compensation capacitor (C6) and the compensation resistor (R3) is used to compensate the effect of the ESR zero on the loop gain. This pole is located at: 3 R 6 C 2 1 f 3 P The goal of compensation design is to shape the converter transfer function to get a desired loop gain. The system crossover frequency where the feedback loop has the unity gain is important. Lower crossover frequencies result in slower line and load transient responses, while higher crossover frequencies could cause system instability. A good rule of thumb is to set the crossover frequency below one-tenth of the switching frequency. Table 3 lists the typical values of compensation components for some standard output voltages with various output capacitors and inductors. The values of the compensation components have been optimized for fast transient responses and good stability at given conditions. Table 3—Compensation Values for Typical Output Voltage/Capacitor Combinations VOUT L1 C2 R3 C3 C6 1.8V 6.8uH 22μF/6.3V Ceramic 3.3kΩ 5.6nF None 3.3V 10μH 22μF/6.3V Ceramic 5.6kΩ 3.3nF None 5.0V 15μH 22μF/6.3V Ceramic 10kΩ 2.2nF None 12.0V 22μH 22μF/16V Ceramic 15kΩ 1.0nF None To optimize the compensation components, the following procedure can be used. 1. Choose the compensation resistor (R3) to set the desired crossover frequency. Determine the R3 value by the following equation: FB OUT CS EA S FB OUT CS EA C V V G G f 1 . 0 2 C 2 V V G G f 2 C 2 3 R Where fC is the desired crossover frequency which is typically below one tenth of the switching frequency. 2. Choose the compensation capacitor (C3) to achieve the desired phase margin. For applications with typical inductor values, setting the compensation zero, fZ1, below one-forth of the crossover frequency provides sufficient phase margin. Determine the C3 value by the following equation: C f 3 R 2 4 3 C where R3 is the compensation resistor. |
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