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Application Notes

Using the Inhibit Function on the PT4470 &

PT4480 Series of Isolated DC/DC Converters

The PT4470/44480 series of DC/DC converters incor-

porate an On/Off Inhibit function. This function may be

used in applications that demand battery conservation,

power-up/shutdown sequencing, and/or to coordinate

power-up for active in-rush current control.

The On/Off feature is provided by the Inhibit control,

pin 3. The Inhibit pin of the PT4470/4480 series of

converters is an active low enable. The pin must be

to enable the converter output (see standard applica-

tion schematic). When pins 2 & 3 are connected, the

converter provides a regulated output whenever a valid

(pin 2). If pin 3 is disconnected, or allowed to become

Table 2-1 provides details of the interface requirements

for the Inhibit pin. Figure 2-1 shows how a discrete

control the input.

Table 2-1 Inhibit Control Requirements

Parameter

Min

Max

–0.5V

0.8V

2.5V

(Open Circuit)

Notes:

of the converter, as its ground reference. All voltages

2. The internal circuitry is simple pull-up resistor. The

open-circuit voltage may be as high as 6.5Vdc.

3. These converters incorporate an “Under-Voltage

Lockout” (UVLO) function. This function automatically

disables the converter output until there is sufficient input

voltage to produce a regulated output. Table 2 gives the

applicable UVLO thresholds.

Series

UVLO Threshold

PT4470

17.0V Typical

18 – 36V

PT4480

34.5V Typical

36 – 75V

4. The Inhibit input must be controlled with an open-

collector (or open-drain) discrete transistor or

MOSFET.

5. When the converter output is disabled, the current

drawn from the input supply is typically reduced to

4mA (10mA maximum).

Figure 2-1

Turn-On Time:

With input power applied, the converter

typically produces a fully regulated output voltage within

25ms after applying a low-voltage signal to the Inhibit

control pin. The actual turn-on time will vary with

the input voltage, output load, and the total amount of

capacitance connected to the output. Using the circuit of

Figure 2-1, Figure 2-2 shows the typical output voltage

was measured with a 48-Vdc input voltage, and 15-A load

current.

Figure 2-2

PT4470, PT4480 Series

6. Keep the on/off transition to less than 1ms. This

prevents erratic operation of the ISR, whereby the

output voltage may drift un-regulated between 0V

and the rated output during power-up.

Vout (1V/Div)

Iin (2A/Div)

Vinh (5V/Div)

HORIZ SCALE: 5ms/Div

PT4472

+V

sns

+V

out

-V

out

-V

sns

7

8-13

14-19

20

21

25

22

24

23

VID0 - VID4

+V

in

– V

in

1

2

3

Inhibit

C

1

+

PROGRAMMING PINS

+2.5V

+V

IN

–V

IN

0V

1 =Enable

Q

1

BSS138