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PT5400 Series

Operating Features of the PT5400

SWIFT™

Series of Power Modules

Under-Voltage Lockout (UVLO)

The PT5400 SWIFT series of power modules incor-

porate an under-voltage lockout (UVLO) function. The

UVLO function provides a clean transition during power-

up and power-down, allowing the regulator to tolerate a

slowly rising input voltage. The UVLO prevents opera-

tion of the module until the input voltage has risen above

2.95 V. Below this threshold the status of the inhibit

control pin is overriden, and the module will not produce

an output. When the input voltage rises above this thresh-

old, the output status of the module is determined by the

inhibit control pin. If the inhibit control is open-circuit

(not grounded), the module will automatically power up.

The UVLO circuit has approximately 0.16 V of hysteresis,

and will completely turn off the module when a falling

input voltage drops below about 2.8 V. (Note: Even though

the applied input voltage may be above the UVLO threshold,

operation to the published specifications requires that the input

voltage be at or above the minimum specified for each model in

the series. This ensures that the output voltage of the module is in

regulation.)

Soft-Start Power Up

Following either the application of a valid input source

voltage, or the removal of a ground signal to the inhibit

control pin (with input power applied), the module will

initiate a soft-start power up. The soft start has two

effects on the start-up characteristic. It introduces a short

time delay prior to the start-up of the output voltage,

and also slows the rate at which the output voltage rises.

Figure 1-1 shows the power-up characteristic of a PT5404

from the point at which the input voltage rises above

2.95 V (the UVLO threshold), to the point that the output

are are approximately 10 ms and 7.5 ms respectively.

Vin (1V/Div)

Vout (1V/Div)

d

t

Vin =2.95V

HORIZ SCALE: 5ms/Div

Figure 1-1; Soft-Start Characteristic and Timing

If desired, both time periods can be lengthened with the

addition of a low value capacitor between the Inhibit

control (pin 1) and the COM (pin 3). For a given value

5 µA

5 µA

Note: The capacitor should be placed as close to the regulator as

possible. Adding 0.047 µF of external capacitance to the Inhibit

Current Limit Protection

The output current limit feature is one of two fault

protection mechanisms built into the PT5400 modules.

Its purpose is to protect both the module and input source

against the occurance of a load fault, thereby isolating the

fault and preventing it from propagating to other parts

of the power system. The PT5400 regulators sense the

current switched by the series (high-side) power MOSFET.

The circuit implements a continuous current limit char-

acteristic. Upon the removal of the fault the output voltage

will promptly recover, and the module will return to nor-

mal operation.

A current limit condition will also increase the module’s

power dissipation, which may cause the temperature of

the internal components to significantly rise. If the con-

dition persists, the module may begin to cycle in and out

of thermal shutdown.

Thermal Shutdown

Thermal shutdown is the second fault protection mecha-

nism and protects the module’s internal circuitry against

excessively high temperatures. A rise in the temperature

of the internal circuitry may be the result of a drop in

airflow, a high ambient temperature, or a sustained over-

current load fault. If the junction temperature of the

internal components exceed 150 °C, the module will

shutdown. Once in thermal shutdown, the regulator is

disabled and the output voltage is reduced to zero. The

recovery is automatic and begins with a soft-start power

up. Recovery occurs when the the sensed temperature

decreases 10 °C below the trip point.

Application Notes