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APPLICATION INFORMATION

MINIMUM PULSE WIDTH

SETTING THE SWITCHING FREQUENCY (PROGRAMMING THE CLOCK OSCILLATOR)

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* 23 kW

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1.5

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MINIMUM OUTPUT VOLTAGE

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FREQUENCY

SWITCHING FREQUENCY

vs

TIMING RESISTANCE

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TPS40070

TPS40071

SLUS582D–DECEMBER 2003–REVISED NOVEMBER 2004

The TPS40070 family of parts allows the user to construct synchronous voltage-mode buck converters with

inputs ranging from 4.5 V to 28 V and outputs as low as 700 mV. Predictive gate drive circuitry optimizes

switching delays for increased efficiency and improved converter output power capability. Voltage feed-forward is

employed to ease loop compensation and provide better line transient response.

A converter based on the TPS40070 operates as a single quadrant (source only) converter at all times. When

the rectifier FET is on and the controller senses that current is near zero in the inductor, the rectifier FET is

turned off, preventing the buildup of negative or reverse current in the inductor. This feature prevents the

converter from pulling energy from its output and forcing that energy onto its input.

Converters based on the TPS40071 operates as a two quadrant converter all the time (source and sink current).

This is the controller of choice for most applications.

The TPS4007x devices have limitations on the minimum pulse width that can be used to design a converter.

Reliable operation is guaranteed for nominal pulse widths of 250 ns and above. This places some restrictions on

the conversion ratio that can be achieved at a given switching frequency. Figure 1 shows minimum output

voltage for a given input voltage and frequency.

The TPS4007x has independent clock oscillator and PWM ramp generator circuits. The clock oscillator serves as

the master clock to the ramp generator circuit. Connecting a single resistor from RT to ground sets the switching

Figure 1.

Figure 2.

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