APPLICATION INFORMATION

MINIMUM PULSE WIDTH

SLEW RATE LIMIT ON VDD

15

9

16

13

12

10

ILIM

HDRV

SW

LDRV

VDD

PGND

TPS40070

C

R

VIN

UDG−05058

+

_

TPS40070

TPS40071

www.ti.com .................................................................................................................................................. SLUS582J – DECEMBER 2003 – REVISED APRIL 2009

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 2 shows minimum output

voltage for a given input voltage and frequency.

The regulator that supplies power for the drivers on the TPS40070/1 requires a limited rising slew rate on VDD

for proper operation if the input voltage is above 10 V. If the slew rate is too great, this regulator can over shoot

and damage to the part can occur. To ensure that the part operates properly, limit the slew rate to no more than

0.12 V/

µs as the voltage at VDD crosses 8 V. If necessary, an R-C filter can be used on the VDD pin of the

device. Connect the resistor from the VDD pin to the input supply of the converter. Connect the capacitor from

the VDD pin to PGND. There should not be excessive (more than a 200-mV) voltage drop across the resistor in

normal operation. This places some constraints on

↔ the R-C values that can be used. Figure 1 is a schematic

fragment that shows the connection of the R-C slew rate limit circuit. Equation 1 and Equation 2 give values for R

and C that limits the slew rate in the worst case condition.

Figure 1. Limiting the Slew Rate

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Product Folder Link(s): TPS40070 TPS40071