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MP2235 Datasheet(PDF) 13 Page - Monolithic Power Systems |
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MP2235 Datasheet(HTML) 13 Page - Monolithic Power Systems |
13 / 21 page MP2235 – SYNCHRONOUS STEP-DOWN CONVERTER MP2235 Rev. 1.0 www.MonolithicPower.com 13 5/16/2013 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2013 MPS. All Rights Reserved. APPLICATION INFORMATION Setting the Output Voltage The external resistor divider sets the output voltage (see Typical Application on page 1). Choose R1 around 40kΩ for V OUT>1.2V then R2 is then given by: OUT R1 R2 V 1 0.807V The T-Type resistor R5 is used to control the bandwidth of control loop which will be introduced below. Control Loop Compensation MP2235 employs peak current mode control for easy compensation and fast transient response. To simplify the compensation design and minimize external components, MP2235 integrates internal compensation. Figure 6 shows an equivalent model for the device control loop. VOUT VOUT VC FB COUT RESR RL R1 R2 RT CF RZ CZ CP 300k Ω 50pF 1pF Figure 6: Equivalent Control Loop Model The device power stage can be approximated to a voltage controlled current source (duty cycle modulator) supplying current to the output capacitor and load resistor. The control (VC) to output (VOUT) transfer function is shown as below: OUT Z1 DC C P1 s 1 V 2f A s V 1 2f L DC i R A R Z1 ESR OUT 1 f 2 R C P1 L OUT 1 f 2 R C Where ADC is the DC gain of power stage, RL is the load resistance, Ri is the current sense resistance (Ri=0.22Ω). RESR is the equivalent series resistance of output capacitor. COUT is the output capacitance. MP2235 uses voltage type amplifier for the feedback error amplifier and integrates compensation to ease the system design. The output to control transfer function is given by: C Z2 Z3 EA OUT P2 P3 ss (1 )(1 ) V 2 f 2 f A ss V s(1 )(1 ) 2 f 2 f 2 EA Z P 1 T 1 2 2 T R A (C C )(R R R R R R ) Z2 ZZ 1 f 2 R C Z3 1F 1 f 2 R C P2 ZP 1 f 2 R C P3 1 2 T Z 1 f 2 (R // R // R )C Where R1, R2 are the feedback resistors, RT is the T-type resistor between feedback resistor divider and FB pin. CF is the type III compensation feed forward capacitor. RZ, CZ and CP are internal compensation resistor and capacitors. 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 frequency results in slower line and load transient responses, while higher crossover frequency could cause system instability. A good rule of thumb is to set the |
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