Application Notes

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PT3400 Series

On/Off Enable Turn-On Time

The total turn-on time of the module is the combination

of a short delay period, followed by the time it takes the

output voltage to rise to full regulation. When the con-

verter is enabled from the EN1 or EN2 control inputs, the

turn-on delay time (measured from the transition of the

enable signal to the instance the outputs begin to rise)

is typically 50 milliseconds. By comparison, the rise time

of the output voltage is relatively short, and is between 1

and 2 milliseconds. The rise time varies with input voltage,

output load current, output capacitance, and the SEQ pin

function. Figure 2-3 shows the power-up response of a

PT3401 (3.3V), following the removal of the ground

signal at EN1 in Figure 2-1.

Using the On/Off Enable Controls on the

PT3400 Series of DC/DC Converters

The PT3400 series of DC/DC converters incorporate

two output enable controls. EN1 (pin 1) is the ‘positive

enable’ input, and EN2 (pin 2) is the ‘negative enable’

(pin 3), at the input or primary 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. The logic truth

table for EN1 and EN2 is given in Table 2-1, below.

Table 2-1; On/Off Enable Logic

EN1 (pin 1)

EN2 (pin 2)

Output Status

0×

Off

10

On

×

1

Off

Logic ‘0’

= –Vin (pin 3) potential

Logic ‘1’

= Open Circuit

Automatic (UVLO) Power-Up

configures the converter for automatic power up (see data

sheet ‘Standard Application’). The converter control

circuitry incorporates an ‘under-voltage lockout’ (UVLO),

which disables the converter until a minimum input

The UVLO ensures a clean transition during power up

and power down, allowing the converter to tolerate a

slowly 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 function,

signal to EN1. In this configuration, applying less than

outputs. Figure 2-1 is an example of this implemention.

DC/DC

Module

EN 1

EN 2

–Vin

–V

IN

1 =Outputs Off

1

2

3

BSS138

DC/DC

Module

EN 1

EN 2

–Vin

–V

IN

1 =Outputs On

1

2

3

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 is left open circuit, and the system On/Off control

signal is applied to EN2. Applying less than 0.8V (with

example of this configuration is provided in Figure 2-2.

Note: The converter will only produce an output voltage if a

Vo (2V/Div)

HORIZ SCALE: 5ms/DIV

Delay Time

Figure 2-3; PT3401 Enable Turn-On