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Application Notes

Using the Power-Up Sequencing Feature of the

PT3400 Series of DC/DC Converters

Introduction

Power-up sequencing is a term used to describe the

order and timing that supply voltages power up in a

multi-voltage power supply system. Multi-voltage power

supply architectures are a common place requirement in

electronic circuits that employ high-performance mi-

croprocessors or digital signal processors (DSPs). These

circuits require a tightly regulated low-voltage supply

for the processor core, and a higher voltage to power

the processor’s system interface or I/O circuitry. Power-

up sequencing is often required between two such voltages

in order to manage the voltage differential during the brief

period of power-up. This reduces stress and improves the

long term reliability of the dual-voltage devices and their

associated circuitry. The most popular solution is termed

“Simultaneous Startup,” whereby the two affected voltages

both start at the same time and then rise at the same rate.

Configuration for Power-up Sequencing

The PT3400 series converters have a feature that allows

individual modules to be easily configured for simulta-

neous startup. Using the SEQ control (pin 5), two PT3400

modules are simply interconnected with just a few passive

components. This eliminates much of the application

circuitry that would otherwise be required for this type of

setup. The schematic is given in Figure 4-1. The setup is

relatively simple but varies slightly with the combination

required when the modules selected are a mix between

a high-voltage module (3.3V through 1.8V), and a low-

voltage module (

≤1.5V). For all other configurations

indicates which modules are a high voltage type (Type A),

and which are a low voltage type (Type B). Table 4-2

provides guidance as to the one combination that requires

sequenced start-up between two PT3400 series modules

are provided in Figure 4-2, Figure 4-3, and Figure 4-4.

In each case the voltage difference during the synchronized

portion of the power up sequence is typically within 0.4V.

Both the timing and tracking of output voltages during

the power-up sequence will vary slightly with input voltage,

temperature, and with differences in the output capaci-

tance and load current between the two converter modules.

This power-up sequencing solution may not be suitable

for every application. To ensure compatibility the appli-

cation should be tested against all variances. For additional

support please contact a Plug-in Power applications

specialist.

PT3400 Series

Table 4-1; PT3400 Module Type Identification

PART No.

VOUT

TYPE A

TYPE B

PT3401

(3.3V)

×

PT3402

(2.5V)

×

PT3403

(1.8V)

×

PT3404

(1.5V)

×

PT3405

(1.4V)

×

PT3406

(1.2V)

×

PT3407

(1.0V)

×

MODULE #1 MODULE #2

COMMENTS

A

A

Wire link

Waveforms given in Figure 4-2

B

B

Wire link

Waveforms given in Figure 4-3

A

B

Waveforms given in Figure 4-4

Notes

1. The two converters configured for sequenced power up

must be located close together on the same printed circuit

board.

2. When configured for power-up sequencing, a minimum

of 1,000µF output capacitance is recommended at the

output of each converter.

3. The best results are obtained if a load of 1A or greater is

present at both converter outputs.

their associated converter, Module #1, and Module #2

equivalent value is not recommended.

Type B converter are connected together for sequenced

the SEQ control (pin 5) of the Type B module, or the

converter with the lowest output voltage. For all other

replaced by a copper trace or wire link.

good quality and have stable characteristics. Capacitors

with an X7R dielectric, and 5% tolerance are

recommended.

7. The enable controls, EN1 & EN2, are optional for a

sequenced pair of converters. If an enable signal is desired,

EN1 or EN2 of both converters units must be controlled

from a single transistor.