buck型DCDC原理.pptVIP

Modulator The modulator which can be seen as a transconductance amplifier controls the inductor current as a function of the amplified error signal from the error amplifier. gm(MOD )is the transconductance of the modulator and determined by the change of inductor current divided by the corresponding output voltage swing of the error amplifier. z1?p1 Compensator The error amplifier provides most of the loop DC gain, which determines the load regulation of the converter. For the output resistance of error amplifier RO(EA) is much larger than R3 gm(EA) is the transconductance of error amplifier. z2p2 Feedback Network Feedback network is normally composed of external resistive divider, which sets the adjustable output voltage. No poles and zeros. Bode Plot Phase Margin Crossover Frequency Worst Case: No Load z3p30.1fs Independent of Load Output Voltage Dependent Checking Loop Stability Load transient response Soft-Start Soft-Start Limit Rush Current Avoid Voltage Overshoot Peak current linearly Peak current in steps Duty Cycle Peak Current Control Voltage Feedback Voltage Reference Voltage Linear Ramp Voltage Linear Ramp Voltage Design Considerations: Reduce the loop gain when FB is small Otherwise maybe unstable for normally frequency is reduced when FB is small Pay attention to soft start with heavy load In many applications peak current is reduced to half when FB is small Introduction Datasheet Key Technique and Design Considerations Design Instructions Good Source for Latest Developments Outline: Introduction to step-down DC-DC converter design Design Instructions Introduction to step-down DC-DC converter design Don’t short supply. Ensure no overlapping switches Limit the peak inductor current. Be stable under all conditions. Temperature, Vin, Vout, Iload, Load Step Take care of fault conditions. Short circuit, Under Voltage Lockout… Calculate layout parastics. Metal and bond wires are resistors Make it testable.