PT5821 datasheet(6/10 Pages) TI

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Application Notes

Using the Inhibit Control of the PT5820 Series

of Step-Down Regulators

For applications requiring output voltage On/Off control,

the PT5820 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

voltage is applied between Vin (pins 5–7) and GND (pins

11–13). When a low-level ground signal is applied to

pin 2, the regulator output is turned off.

Figure 2-1 shows the typical application of the Inhibit

function. Note the discrete transistor (Q1). The Inhibit

control has its own internal pull-up to +Vin potential. An

open-collector or open-drain device is required to control

this input 1. The voltage thresholds are given in Table 2-1.

Table 2-1; Inhibit Control Requirements

Parameter

Min

Typ

Max

Enable (VIH)Vin – 0.5V

—

Open

Disable (VIL)

–0.2V

—

+0.8V

I IL

—

–0.5 mA

—

Notes:

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

of at least 15V (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 +0.8VDC.

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

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

voltage decays to zero as the load impedance discharges

the output capacitors.

3. During the period ‘td’ in Figure 2-2 the module will

assert a low impedance to ground. This will discharge any

residual voltage across the output capacitors, and will

crowbar any voltage supplied from external sources.

Figure 2-1

Turn-On Time:

In the circuit of Figure 2-1, turning Q1 on

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

disables the output of the regulator 2. If Q1 is then turned

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

consists of a short delay period, td (approx. 10-15msec) 3,

followed by a period in which the output voltage rises

to its full regulation voltage. The module produces a

regulated output voltage within 20msec. Figure 2-2

shows the typical rise in both the output voltage and

input current for a PT5821 (3.3V), following the turn-

off of Q1. The turn off of Q1 corresponds to the rise in the

waveform, Q1 Vds. The waveforms were measured with a

12VDC input voltage, and 10-A load.

Figure 2-2

Vo (2V/Div)

Iin (2A/Div)

Q1 Vds (10V/Div)

HORIZ SCALE: 5ms/Div

PT5820 Series

PT5820

9–13

118

14–17

5, 6, 7

560µF

OUT

330µF

1 =Inhibit

GND

OUT

Output Sense (+)

Output Sense (–)

BSS138

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