Application Notes

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PT6440 Series

Using the Inhibit Function on the PT6440

Excalibur

 5V/3.3V Bus Step-Down ISRs

For applications requiring output voltage On/Off control,

the 12-pin PT6440 series products incorporate an Inhibit

function. This function may be used wherever there is a

requirement for the module to be switched off. The

function is provided by the Inhibit control (pin 1) input.

providing a regulated output whenever a valid source

(pins 5–8). When a low-level ground signal is applied

to pin 1, the regulator output is disabled.

Figure 1 shows an application schematic, which details

the typical use of the Inhibit function. Note the discrete

The Inhibit pin control thresholds are given in Table 1.

Equation 1 may be used to determine the approximate

regulator is placed in the inhibit state.

Parameter

Min

Max

Enable (VIH)Vin – 0.5

Vin

Disable (VIL)

-0.2V

0.5V

Transition Time

Equation 1; Off Input Current

± 20%

Notes:

1. Use an open-collector device (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 +0.5VDC.

2. Do not control the Inhibit input with an external DC

voltage. This will lead to erratic operation of the ISR and

may over-stress the regulator.

3. Avoid capacitance greater than 500pF at the Inhibit

control pin. Excessive capacitance at this pin will cause the

ISR to produce a pulse on the output voltage bus at turn-

on.

4. Keep the On/Off transition to less than 10µs. This

prevents erratic operation of the ISR, which could cause

the output voltage to be momentarily higher than normal.

Figure 1

Turn-On Time:

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

disables the regulator output. Correspondingly, turning

its internal pull-up resistor. The ISR produces a fully

regulated output voltage within 10 milliseconds of the

release of the Inhibit control pin. The actual turn-on

time will vary with input voltage, output load, and the

total amount of load capacitance. Figure 2 shows the

typical rise in both output voltage and input current for a

The waveform was measured with a 5Vdc input voltage,

and 6 Adc resistive load.

Figure 2

C

in

+

C

out

+

Inhibit

+ 5 V

+2.5V

COM

COM

Q1

BSS138

PT6442

2,3,4

5 – 81 2

9,10,11

Vin

V o

V o(adj)

GND

1

Inhibit*

-1

01

23

45

678

9

10

t (milliseconds)