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Power-Up Characteristics

t - Time = 5 ms/div

I (0.5 A/div)

I

V (1 V/div)

O

V (5 V/div)

I

Current Limit Protection

Thermal Shutdown

Output On/Off Inhibit

PTH08080W

SLTS235A – FEBRUARY 2005 – REVISED MARCH 2005

When configured per the standard application, the PTH08080 power module produces a regulated output voltage

following the application of a valid input source voltage. During power up, internal soft-start circuitry slows the

rate that the output voltage rises, thereby limiting the amount of in-rush current that can be drawn from the input

source. Figure 11 shows the power-up waveforms for a PTH08080W, operating from a 12-V input and with the

output voltage adjusted to 1.8 V. The waveforms were measured with a 2-A resistive load.

Figure 11. Power-Up Waveforms

The PTH08080 modules protect against load faults with an output overcurrent trip. Under a load fault condition,

the output current cannot exceed the current limit value. Attempting to draw current that exceeds the current limit

value causes the output voltage to enter into a hiccup mode of operation, whereby the module continues in a

cycle of successive shutdown and power up until the load fault is removed. On removal of the fault, the output

voltage promptly recovers.

Thermal shutdown protects the module internal circuitry against excessively high temperatures. A rise in

temperature may be the result of a drop in airflow, a high ambient temperature, or a higher than normal output

current. If the junction temperature of the internal components exceeds 165

°C, the module shuts down. This

reduces the output voltage to zero. The module starts up automatically, by initiating a soft-start power up when

the sensed temperature decreases 10

°C below the thermal shutdown trip-point.

For applications requiring output voltage on/off control, the PTH08080 power module incorporates an output

on/off Inhibit control (pin 5). The inhibit feature can be used wherever there is a requirement for the output

voltage from the regulator to be turned off.

The power module functions 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 control

has its own internal pullup to 3 volts. An open-collector or open-drain device is recommended to control this

input.

Turning Q1 on applies a low voltage 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 the output voltage, following the turn off of Q1. The turn off of Q1

11