PT4821 datasheet(28/32 Pages) TI

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Application Notes

PT4820 Series

On/Off Enable Turn-On Time

When the On/Off enable inputs, EN1 or EN2 are used

to enable the PT4820's output voltages, the turn-on

delay time (measured from the transition of the enable

signal to the instance the outputs begin to rise) will vary

with the input voltage and the module’s internal timing.

At an input voltage of 48V, the total turn-on time is

between 20 and 60 milliseconds. This turn-on time

reduces as the input voltage is increased. The rise time

of the output voltages is between 10 and 15 milliseconds.

Using the On/Off Enable Controls on the PT4820

Series of Triple Output DC/DC Converters

The PT4820 (48V input) series of triple-output DC/DC

converters incorporate two output enable controls. EN1

(pin 1) is the Positive Enable input, and EN2 (pin 2) is the

Negative Enable input. Both inputs are electrically refer-

enced to -Vin (pin 4) on the primary or input side of the

converter. The Enable pins are ideally controlled with an

open-collector (or open-drain) discrete transistor. A pull-

up resistor is not required. If a pull-up resistor is added,

the pull-up voltage must be limited to 15V.

Automatic (UVLO) Power-Up

Connecting EN2 (pin 2) to -Vin (pin 4) and leaving EN1

(pin 1) open-circuit configures the converter for auto-

matic power up. (See data sheet “Typical Application”).

The converter control circuitry incorporates an “Under

Voltage Lockout” (UVLO) function, which disables the

converter until the minimum specified input voltage is

present at ±Vin. (See data sheet Specifications). The UVLO

circuitry ensures a clean transition during power-up and

power-down, allowing the converter to tolerate a slow-

rising input voltage. For most applications EN1 and

EN2, can be configured for automatic power-up.

Positive Output Enable (Negative Inhibit)

To configure the converter for a positive enable func-

tion, connect EN2 (pin 2) to -Vin (pin 4), and apply the

system On/Off control signal to EN1 (pin 1). In this

configuration, applying less than 0.8V (with respect to

-Vin potential) to pin 1 disables the converter outputs.

Figure 2-1 is an example of this implemention using a

buffer transistor.

DC/DC

Module

EN 1

EN 2

–Vin

–V

1 =Outputs Off

BSS138

DC/DC

Module

EN 1

EN 2

–Vin

–V

1 =Outputs On

BSS138

Figure 2-2; Negative Enable Configuration

Figure 2-1; Positive Enable Configuration

Negative Output Enable (Positive Inhibit)

To configure the converter for a negative enable function,

EN1 (pin 1) is left open circuit, and the system On/Off

control signal is applied to EN2 (pin 2). Applying less

than 0.8V (with respect to -Vin potential) to pin 2, enables

the converter outputs. An example using a buffer transistor

is again detailed in Figure 2-2. Note: The converter will

only produce and output voltage if a valid input voltage is

applied to ±Vin.

PT4821 Datasheet(PDF) 28 Page - Texas Instruments

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