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APPLICATION INFORMATION

Functional Description

Voltage Reference

Voltage Error Amplifier

Loop Compensation

Oscillator

UVLO

TPS40040, TPS40041

SLUS700B – MARCH 2006 – REVISED MARCH 2006

The TPS40040 (300 kHz) and TPS40041 (600 kHz) are fixed-frequency voltage-mode synchronous buck

controllers. In operation, the synchronous rectifier (SR) is allowed to conduct current in both directions, allowing a

converter to operate in continuous mode, even under no load conditions, simplifying feedback loop compensation

requirements. During startup, internal circuitry modulates the switching of the synchronous rectifier to prevent

discharging of the output if a pre-biased condition exists.

The 600-mV bandgap reference voltage cell is internally connected to the non-inverting input of the error

amplifier. The voltage reference is trimmed with the error amplifier in a unity gain configuration to remove

amplifier offset from the final regulation voltage.

The error amplifier has a bandwidth of greater than 5 MHz, and open loop gain of at least 55 dB. The output

voltage swing is limited to just above and below the oscillator ramp levels to improve transient response.

Voltage mode buck type converters are typically compensated using Type III networks. Please refer to the

Design Example for detailed methodology in designing feedback loops for voltage mode converters.

The oscillator frequency is internally fixed. The TPS40040/1 operating frequencies are 300 kHz/600 kHz,

respectively.

When the input voltage is below the UVLO threshold, the TPS40040/1 turns off the internal oscillator and holds

all gate drive outputs in the low (OFF) state. When the input rises above the UVLO threshold, and the EN pin is

below the turn ON threshold, the start-up sequence is allowed to begin.

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