On/Off Inhibit

V

I

GND

PTH08T255W

6

7

8

9

12

13

GND

GND

GND

C

I

+

UDG-08148

14

15

V

I

V

I

V

I

GND

V

I

21

Inhibit/UVLO

Q1

BSS 138

1=Inhibit

t – Time – 10 ms/div

V

INH

(2 V/div)

V

O

(2 V/div)

I

I

(5 A/div)

Overcurrent Protection

Overtemperature Protection (OTP)

PTH08T255W

For applications requiring output voltage on/off control, the PTH08T255W incorporates an 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 pin is left open-circuit, providing a regulated output

Figure 12 shows the typical application of the inhibit function. Note the discrete transistor (Q1). The Inhibit input

has its own internal pull-up. An external pull-up resistor should never be used with the inhibit pin. The input is not

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

control.

When Q1 it turned on, low voltage is applied to the Inhibit control pin 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 20 ms. Figure 13 shows the typical rise in both the output voltage and input current, following the turn-off

25-A constant current load.

Figure 12. On/Off Inhibit Control Circuit

Figure 13. Power-Up Response from Inhibit Control

For protection against load faults, all modules incorporate output overcurrent protection. Applying a load that

exceeds the regulator's overcurrent threshold causes the regulated output to shut down. Following shutdown, the

module periodically attempts to recover by initiating a soft-start power-up. This is described as a hiccup mode of

operation, whereby the module continues in a cycle of successive shutdown and power up until the load fault is

removed. During this period, the average current flowing into the fault is significantly reduced. Once the fault is

removed, the module automatically recovers and returns to normal operation.

A thermal shutdown mechanism protects the module’s internal circuitry against excessively high temperatures. A

rise in the internal temperature may be the result of a drop in airflow, or a high ambient temperature. If the

internal temperature exceeds the OTP threshold, the module’s Inhibit control is internally pulled low. This turns

the output off. The output voltage drops as the external output capacitors are discharged by the load circuit. The

recovery is automatic, and begins with a soft-start power up. It occurs when the sensed temperature decreases

by about 10°C below the trip point.

18

Copyright © 2008, Texas Instruments Incorporated

Product Folder Link(s): PTH08T255W