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Current Limit Protection
Thermal Shutdown
Output On/Off Inhibit
V =12V
I
V =3.3V
O
Inhibit
GND GND
1.87k
0.05W,1%
W
PTH08000W
1
2
5
6
3
4
V Adj
O
Track
GNDInhibit
V
I
V
O
Q1
BSS138
L
O
A
D
C
100 F
(Optional)
O
m
C
100 F
(Required)
I
m
PTH08000W
SLTS248B – JUNE 2005 – REVISED FEBRUARY 2008
less than 5 ms) before allowing the output voltage to rise. The output then progressively rises to the module
set-point voltage. Figure 11 shows the soft-start power-up characteristic of the PTH08000W, operating from a
12-V input bus and configured for a 3.3-V output. The waveforms were measured with a 2-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.
The PTH08000W 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 PTH08000 power module incorporates an output
on/off Inhibit control (pin 4). 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
whenever a valid source voltage is connected to V
I
with respect to GND.
Figure 12 shows the typical application of the inhibit function. Note the discrete transistor (Q1). The Inhibit control
has its own internal pull-up to 3 V. An open-collector or open-drain device is recommended to control this input.
Do not place an external pull-up on this pin.
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
corresponds to the rise in the waveform, Q1 V
DS
. The waveforms were measured with a 2-A resistive load.
Figure 12. On/Off Inhibit Control Circuit
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