PT8139 Datasheet, PDF (6/9 Pages) Texas Instruments – 30-A Programmable Integrated Switching Regulator

Application Notes
PT8120 Series & PT8139
Using the On/Off Standby Function of the
PT8120 Series of Programmable ISRs
The PT8120 series of programmable ISRs incorporates
an On/Off Standby function. This feature may be used
to turn the regulated output of the module off while input
voltage is applied. This places the module in “standby”
mode. The standby control may be used for power-up
sequencing, or wherever there is a requirement to control
the module’s output status from another circuit.
The Standby function is provided by the STBY* control,
pin 6. If pin 6 is left open-circuit the regulator operates
normally, providing a regulated output when a valid sup-
ply voltage is applied to V_in (pins 10-16) with respect to
GND (pins 17-22). Connecting pin 6 to ground ¹ places
the regulator in standby mode ², and reduces the input
current to typically 35mA. Applying a ground signal to
pin 6 prior to power-up, will inhibit the output during
the period that input power is applied. When the ground
signal to pin 6 is removed, the regulator initiates a soft-
start to re-establish the set output voltage. ³ To ensure
that the regulator output is properly enabled, the STBY*
control pin must be open circuit.
Table 1 Standby Control Requirements ²
Parameter
Min
Typ
Max
V_IH
VIL
ISTBY
—
–0.2V
Open Cct. 1
—
0.8V
–0.5mA
Notes:
1. The standby on a PT8120 series regulators must be
controlled with an open-collector (or open-drain)
transistor (See fig. 1). Do Not use a pull-up resistor.
Table 1 gives the STBY* pin parameters. The control
pin has an open-circuit voltage of 5Vdc. To shut the
regulator output off, the control pin must be “pulled” to
less than 0.8Vdc with a low-impedance sink to ground.
2. In the standby mode the output of the regulator is tri-
state, and the output voltage falls at the rate that the
load circuit discharges the output filter capacitors.
3. When the ground signal to the Standby pin is
removed, the regulator output initiates a soft-start
cycle by first asserting a low impedance to ground. If
an external voltage is applied to the output bus, it will
sink current and possibly over-stress the part.
Turn-On Time
Turning Q₁ in Figure 1 off, removes the low-voltage
signal at pin 6. After approximately 5-ms the regulator
output rises and reaches full regulation within 40ms.
Fig. 2 shows the typical waveforms of a PT8121 following
the prompt turn-off of Q₁. The turn-off of Q₁ corresponds
to the rise in V_stby. The output voltage was set to 3.3V,
and the waveforms were measured with a 12V input source,
and 18A resistive load.
Figure 2
Vo (1V/Div)
HORIZ SCALE: 5ms/Div
Iin (5A/Div)
Vstby (5V/Div)
Figure 1
V_O SENSE
V_IN
1 =Off
L_in
1µH
11
VID
Common
12–16
V _IN
5 4 3 2 1 31
VID4 – VID0 Sense
PT8121
23–30
V_OUT
V_O =3.3V
+
C_IN
STBY
6
Q1
BSS138
GND
17–22
+
L
C_OUT
O
A
D
GND
GND
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