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Output On/Off Inhibit

L

O

A

D

1=Inhibit

GND

GND

V Sense

O

Q1

BSS138

5.49kW

C1

C3

C2

PTV03020W

GND

GND

Inhibit

V Adj

O

Track

Sense

PTV03020W

SLTS243A – FEBRUARY 2005 – REVISED OCTOBER 2007

From the moment a valid input voltage is applied, the soft-start control introduces a short time delay (typically

less than 5 ms) before allowing the output voltage to rise. The output then progressively rises to the module

set-point voltage. Figure 8 shows the soft-start power-up characteristic of the PTV03020W, operating from a

3.3-V input bus and configured for a 1.8-V output. The waveforms were measured with a 5-A resistive load and

the Auto-Track feature disabled. The initial rise in input current when the input voltage first starts to rise is the

charge current drawn by the input capacitors. Power up is complete within 25 ms.

For applications requiring output voltage on/off control, the modules incorporate an output Inhibit control pin. The

inhibit feature can be used wherever there is a requirement for the output voltage from the regulator to be turned

off.

The power modules function normally when the Inhibit input is left open-circuit, providing a regulated output

Figure 9 shows the typical application of the inhibit function. Note the discrete transistor (Q1). The Inhibit input

has its own internal pullup (see footnotes to electrical characteristics table). The input is not compatible with TTL

logic devices. An open-collector (or open-drain) discrete transistor is recommended for control.

Figure 9. On/Off Inhibit Application Circuit

Turning Q1 on applies a low voltage to the Inhibit control and disables the output of the module. If Q1 is then

turned off, the module executes a soft-start power-up sequence. A regulated output voltage is produced within

25ms. Figure 10 shows the typical rise in both the output voltage and input current, following the turnoff of Q1.

constant current load.

Copyright © 2005–2007, Texas Instruments Incorporated

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