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Application Notes

Using the Inhibit Control of the PT5800 & PT5810

Series of Step-Down ISRs

For applications requiring output voltage On/Off control,

the PT5800 & PT5810 series of ISRs incorporate an

inhibit function. This function can be used wherever there

is a requirement for the output voltage from the ISR to be

turned off. The On/Off function is provided by the Inhibit*

control (pin 2).

The ISR functions normally with pin 2 open-circuit,

providing a regulated output whenever a valid source

9–13). When a low-level ground signal is applied to

Figure 4-1 shows the typical application of the Inhibit*

open-collector or open-drain device is recommended to

Table 4-1.

Table 4-1; Inhibit Control Requirements

Parameter

Min

Typ

Max

Enable (VIH)Vin – 0.5 V

—

Open

Disable (VIL)

–0.2 V

—

+0.6 V

I IL

—

–0.2 mA

—

Notes:

1. Use an open-collector device with a breakdown voltage

of at least 10 V (preferably a discrete transistor) for the

Inhibit* input. A pull-up resistor is not necessary. To

disable the output voltage the control pin should be

pulled low to less than +0.8 VDC.

2. When a ground signal is applied to the Inhibit* control

(pin 2) the module output is turned off (tri-state). The

output voltage decays to zero as the load impedance

discharges the output capacitors.

Figure 4-1

Turn-On Time:

applies a low-voltage to the Inhibit* control (pin 2) and

off, the ISR executes a soft-start power up. Power up

consists of a short delay (approx. 2msec), followed by a

period in which the output voltage rises to its full regu-

lation voltage. The module produces a regulated output

voltage within 10msec. Figure 4-2 shows the typical rise

in both the output voltage and input current for a PT5812

waveforms were measured with a 3.3 VDC input voltage,

and 10-A load.

Figure 4-2

Vo (1V/Div)

Iin (5A/Div)

Q1 Vds (2V/Div)

HORIZ SCALE: 2ms/Div

PT5800 & PT5810 Series

V

IN

C

IN

1,000 µF

C

OUT

330 µF

1 =Inhibit

GND

GND

V

OUT

Output Sense (+)

Output Sense (–)

Q

1

BSS138

PT5812

9–13

4

118

14–17

5, 6, 7

8

2

3