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Application Notes

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-

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-

that the regulator output is properly enabled, the STBY*

control pin must be open circuit.

Parameter

Min

Typ

Max

—

–0.2V

—

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.

Figure 1

Figure 2

PT8120 Series & PT8139

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

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

and the waveforms were measured with a 12V input source,

and 18A resistive load.

L

O

A

D

C

OUT

+

C

IN

+

V

IN

V

GND

V

1 =Off

L

in

1µH

GND

PT8121

4321

31

23–30

617–22

12–16

5

11

VID4 – VID0

Sense

V

IN

V

OUT

STBY

GND

VID

Common

Q1

BSS138

Vo (1V/Div)

Iin (5A/Div)

Vstby (5V/Div)

HORIZ SCALE: 5ms/Div