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Overcurrent Protection

Overtemperature Protection (OTP)

On/Off Inhibit

11

Inhibit/

UVLO

GND

2, 6

GND

3,4

1 = Inhibit

Q1

BSS 138

PTH08T240W

PTH08T240W

SLTS264A–NOVEMBER 2005–REVISED MARCH 2006

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.

The overtemperature protection is a last resort mechanism to prevent thermal stress to the regulator.

Operation at or close to the thermal shutdown temperature is not recommended and reduces the long-term

reliability of the module. Always operate the regulator within the specified safe operating area (SOA) limits for

the worst-case conditions of ambient temperature and airflow.

For applications requiring output voltage on/off control, the PTH08T240W 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

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.

Figure 20. On/Off Inhibit Control Circuit

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