Datasheet
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Output On/Off Inhibit
V
I
L
O
A
D
1=Inhibit
GND
GND
V
O
V Sense
O
Q1
BSS138
R
SET
5.49kW
C1 C3C2
PTV03020W
V
I
V
O
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
whenever a valid source voltage is connected to V
I
with respect to GND.
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.
The turnoff of Q1 corresponds to the rise in the waveform, Q1 V
DS
. The waveforms were measured with a 9-A
constant current load.
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