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Application Notes

Using the Remote On/Off Function on the PT4120/

PT4140 Series of Isolated DC/DC Converters

For applications requiring output voltage on/off control,

the PT4120/4140 series of DC/DC converters incorporate

a remote on/off function. This function may be used in

applications that require battery conservation, power-up/

shutdown sequencing, and/or to co-ordinate the power-

up of the regulator for active in-rush current control. (See

the related application note, AN21).

This function is provided by the Remote On/Off control,

pin1. If pin 1 is left open-circuit, the converter provides a

Table 1 provides details of the threshold requirements for

the Remote On/Off pin. Figure 1 shows how a discrete

voltage rail and used with this control input.

Table 1 Inhibit Control Thresholds

Parameter

min

max

2.5V

–0.3V

0.8V

Notes:

the converter as its ground reference. All voltages specified

2. The on/off control internal circuitry is a high impedance

10

µA current source. The open-circuit voltage may be as

high as 8.3Vdc.

3. The PT4120/40 series incorporates an “Under Voltage

Lockout” (UVLO) function. This function automatically

inhibits the converter output until there is sufficient input

voltage for the converter to produce a regulated output.

Table 2 gives the applicable UVLO thresholds.

Table 2 UVLO Thresholds

Series

UVLO Threshold

V

in

Range

PT4120

31V Typical

36 – 75V

PT4140

15V Typical

18 – 40V

4. The Remote On/Off input of the PT4120/40 series regulators

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 8mA (16mA

maximum).

Figure 1

Turn-On Time:

The converter typically produces a fully

regulated output voltage within 50ms after the applica-

from the Remote On/Off 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 1, Figure 2 shows the output

voltage and input current waveforms of a PT4121 after

with a 48Vdc input voltage, and 2.75-A resistive load.

Figure 2

+V

in

–V

in

+V

o

–V

o

PT4121

+Vout

–V out

+V in

–Vin

Remote

On/Off

V

1

2

35

4

6

Q1

BSS138

Inh

Note 2

PT4120/4140 Series

Vout (2V/Div)

Iin (0.2A/Div)

Q1 Vgs (10V/Div)

HORIZ. SCALE: 5ms/Div

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.