AN3442| Application Note
Maxim > App Notes > POWER-SUPPLY CIRCUITS Keywords: PSPICE, SPICE, bode plot, open loop gain, loop reponse
Dec 30, 2004
APPLICATION NOTE 3442
Simple PSPICE Model Predicts MAX8546 Stability and Transient Response
Abstract: The MAX8546 is a low-cost, wide-input-voltage range, step-down controller with foldback current limit. A simple PSPICE model that can generate open-loop-gain Bode plots and predict load transient response for MAX8546 designs is presented. The MAX8546 is a low-cost, wide-input-voltage range, step-down controller with foldback current limit. Its 300kHz switching frequency is well suited for use with low-cost external components such as powdered iron inductors and aluminum electrolytic capacitors, and still provides the necessary control bandwidth for good transient response. Low-cost, lossless overcurrent protection is achieved by utilizing the low-side MOSFET Rds
(on).
A detailed design procedure for the MAX8546 is presented in the MAX8546 data sheet.
Presented here is an additional design tool, a simple PSPICE model that can generate open-loop-gain Bode plots and predict load transient response for MAX8546 designs. The evaluation kit application circuit which is used for modeling is available for download. The model is simple enough to run on the evaluation version of PSPICE that can be downloaded. Figure 1 shows the AC model for a step-down converter with a constant DC input voltage. The AC voltage driving the output L-C filter and load is the product of the DC input voltage, Vin, and the duty cycle perturbation, d, the output of the compensated controller.
Figure 1. Model of step-down converter for AC analysis. Figure 2 shows the PSPICE implementation of the AC model. The output voltage is divided down to a feedback threshold voltage, Vfb, of 0.8V by resistors R1 and R2. The transconductance error amplifier of the MAX8546 is modeled by a voltage-controlled current source, G, with a gain of 108uS, as specified in the data sheet. R4 and
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