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SS23 Folha de dados(PDF) 7 Page - Monolithic Power Systems |
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SS23 Folha de dados(HTML) 7 Page - Monolithic Power Systems |
7 / 11 page MP1591 – 2A, 32V, 330KHz STEP-DOWN CONVERTER MP1591 Rev. 2.3 www.MonolithicPower.com 7 9/27/2006 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2006 MPS. All Rights Reserved. Compensation The system stability is controlled through the COMP pin. COMP 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. The DC loop gain is: LOAD CS VEA OUT REF VDC R G A V V A × × × = Where VREF is the feedback threshold voltage, 1.230V, AVEA is the transconductance error amplifier voltage gain, 400 V/V, and GCS is the current sense gain (roughly the output current divided by the voltage at COMP), 3.5 A/V. The system has 2 poles of importance; one is due to the compensation capacitor (C4) and the other is due to the output capacitor (C5). These are: ) 4 C A 2 ( G f VEA MEA 1 P × × π = Where fP1 is the first pole, and GMEA is the error amplifier transconductance (770µS) and ) 5 C R 2 ( 1 f LOAD 2 P × × π = The system has one zero of importance due to the compensation capacitor (C4) and the compensation resistor (R3) which is ) 4 C 3 R 2 ( 1 f 1 Z × × π = If large value capacitors with relatively high equivalent-series-resistance (ESR) are used, the zero due to the capacitance and ESR of the output capacitor can be compensated by a third pole set by R3 and C3 ) 3 C 3 R 2 ( 1 f 3 P × × π = The system crossover frequency fC, (the frequency where the loop gain drops to 1, or 0dB) is important. A good rule of thumb is to set the crossover frequency to approximately one tenth of the switching frequency. In this case, the switching frequency is 330KHz, so use a crossover frequency of 33KHz. Lower crossover frequencies result in slower response and worse transient load recovery. Higher crossover frequencies can result in instability. Choosing the Compensation Components The values of the compensation components given in Table 4 yield a stable control loop for the output voltage and given capacitor. Table 4—Compensation Values for Typical Output Voltage/Capacitor Combinations VOUT C5 R3 C3 C4 2.5V 22µF Ceramic 3.9kΩ None 4.7nF 3.3V 22µF Ceramic 5.1kΩ None 3.9nF 5V 22µF Ceramic 7.5kΩ None 2.7nF 12V 22µF Ceramic 18kΩ None 1.2nF 2.5V 47µF SP-Cap 8.2kΩ None 2.2nF 3.3V 47µF SP-Cap 10kΩ None 2.2nF 5V 47µF SP-Cap 16kΩ None 1.5nF 12V 47µF SP-Cap 36kΩ None 1nF 2.5V 560µF/6.3V, AL 30mΩ ESR 100kΩ 150pF 1nF 3.3V 560µF/6.3V, AL 30mΩ ESR 120kΩ 120pF 1nF 5V 470µF/10V, AL 30mΩ ESR 150kΩ 82pF 1nF 12V 220µF/25V, AL 30mΩ ESR 180kΩ 33pF 1nF Note: “AL” = Electrolytic |
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