Datasheet MP2307 (Monolithic Power Systems) - 9

MP2307 – 3A, 23V, 340KHz SYNCHRONOUS RECTIFIED STEP-DOWN CONVERTER
The system has one zero of importance, due to the 2. Choose the compensation capacitor (C3) to compensation capacitor (C3) and the compensation achieve the desired phase margin. For resistor (R3). This zero is located at: applications with typical inductor values, setting the compensation zero (fZ1) below one-forth of 1 f = the crossover frequency provides sufficient 1 Z 2π × 3 C × 3 R phase margin. The system may have another zero of Determine C3 by the following equation: importance, if the output capacitor has a large capacitance and/or a high ESR value. The zero, 4 3 C > due to the ESR and capacitance of the output 2π × 3 R × fC capacitor, is located at: Where R3 is the compensation resistor. 1 f = 3. Determine if the second compensation ESR 2π × 2 C × RESR capacitor (C6) is required. It is required if the In this case, a third pole set by the ESR zero of the output capacitor is located at compensation capacitor (C6) and the less than half of the switching frequency, or the compensation resistor (R3) is used to following relationship is valid: compensate the effect of the ESR zero on the 1 fS loop gain. This pole is located at: < 2π × 2 C × R 2 ESR 1 f = If this is the case, then add the second P3 2π × 6 C × 3 R compensation capacitor (C6) to set the pole fP3 The goal of compensation design is to shape at the location of the ESR zero. Determine C6 the converter transfer function to get a desired by the equation: loop gain. The system crossover frequency 2 C × R where the feedback loop has the unity gain is 6 C ESR = 3 R MP2393 important. Lower crossover frequencies result in slower line and load transient responses,
PCB Layout Guide
while higher crossover frequencies could cause PCB layout is very important to achieve stable system instability. A good standard is to set the O operation. It is highly recommended to duplicate crossover frequency below one-tenth of the EVB layout for optimum performance. switching frequency. If change is necessary, please follow these To optimize the compensation components, the guidelines and take Figure2 for reference. following procedure can be used. RECOMMENDED FOR 1) Keep the path of switching current short 1. Choose the compensation resistor (R3) to set and minimize the loop area formed by Input NEW DESIGNS the desired crossover frequency. cap., high-side MOSFET and low-side Determine R3 by the following equation: MOSFET. 2π × 2 C × π × × × 2) Bypass ceramic capacitors are suggested C f OUT V 2 2 C 0.1 S f OUT V 3 R = × < × NOT× × to be put close to the Vin Pin. EA G CS G FB V EA G CS G FB V Where fC is the desired crossover frequency 3) Ensure all feedback connections are short REFER T which is typically below one tenth of the and direct. Place the feedback resistors switching frequency. and compensation components as close to the chip as possible. 4) ROUT SW away from sensitive analog areas such as FB. MP2307 Rev. 1.9 www.MonolithicPower.com
9
5/28/2008 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2008 MPS. All Rights Reserved.