PT6345 datasheet(4/9 Pages) TI

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Application Notes

Using the Inhibit Function on the PT6340

12V Bus Excalibur™ Series Converters

The PT6340 series are high efficiency regulators that

are designed to operate off a 12V input bus. These devices

incorporate an inhibit function, which may be used in

applications that require a power-up/shutdown feature.

The inhibit function is provided by the Inhibit* control,

pin 1. If pin 1 is left open-circuit the regulator operates

normally, and provides a regulated output whenever a valid

supply voltage is applied to Vin (pins 2– 4) with respect

to GND (pins 5–8). If a low voltage 2 is then applied to

pin 1 the regulator output will be disabled and the input

current drawn by the ISR will typically drop to 0.5mA 4.

The standby control may also be used to hold-off the

regulator output during the period that input power is

applied.

The Inhibit* input can be controlled with an open-collector

(or open-drain) discrete transistor (See Figure 1). The

input is internally pulled-up to the input voltage, Vin 1.

Table 1 gives the control voltage requirements.

Table 1 Inhibit Control Requirements 3

Parameter

Min

Typ

Max

VIL

-0.1V

0.6V

VIH

2.0V

Vin

IIL

0.5mA

Notes:

1. The inhibit control input requires no external pull-up

resistor. The open-circuit voltage of the Inhibit* input is

typically the input voltage, Vin.

2. The inhibit control input is Not compatible with TTL

devices. An open-collector device, preferably a discrete

bipolar transistor (or MOSFET) is recommended. To

ensure the regulator output is disabled, the control pin

must be pulled to less than 0.6Vdc with a low-level

0.5mA sink to ground.

3. An external source voltage can be used to control the

Inhibit* pin. To guarantee the inhibit and enable status

of the regulator, the source must be capable of meeting

the voltage requirements in Table 1 .

4. When the regulator output is disabled the current drawn

from the input source is typically reduced to 0.5mA.

Figure 1

Turn-On Time:

In the circuit of Figure 1, turning Q1 on

applies a low voltage to the Inhibit* control (pin 1) and

disables the regulator ouput. Correspondingly, turning

Q1 off removes the low-voltage signal and enables the

output. Once enabled, the output will typically experience

a 10–15ms delay followed by a predictable ramp-up of

voltage. The regulator should provide a fully regulated

output voltage within 30ms. The waveform of Figure 2

shows the output voltage response of a PT6342 (3.3V)

following the turn-off of Q1. The turn off of Q1 corre-

sponds to the rise in Vinh. The waveforms were measured

with a 12Vdc input voltage, and 2 ½ Adc load.

Figure 2

out

Inhibit

+12V

+3.3V

COM

BSS138

PT6342

2,3,4

5–8

1 2

9,10,11

Vin

V o

V o(adj)

GND

Inhibit*

PT6340 Series

Vo (2V/Div)

Iin (0.5A/Div)

Vinh (10V/Div)

HORIZ SCALE: 5ms/Div

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