P

D

I

OUT

TPS71202, TPS71219

TPS71229, TPS71247

TPS71256, TPS71257

www.ti.com

SBVS049D – MAY 2004 – REVISED AUGUST 2010

TRANSIENT RESPONSE

enabled. Depending on power dissipation, thermal

resistance, and ambient temperature, the thermal

As with any regulator, increasing the size of the

protection circuit may cycle on and off. This limits the

output

capacitor

will

reduce

over/undershoot

dissipation of the regulator, protecting it from damage

magnitude but increase duration of the transient

due to overheating.

response. In the adjustable version, the addition of a

Any tendency to activate the thermal protection circuit

will also improve stability and transient response. The

indicates

excessive

power

dissipation

or

an

transient response of the TPS712xx is enhanced with

inadequate heatsink. For reliable operation, junction

an active pull-down that engages when the output is

temperature should be limited to +125°C maximum.

over-voltaged. The active pull-down decreases the

To estimate the margin of safety in a complete design

output recovery time when the load is removed.

(including

heatsink),

increase

the

ambient

Figure 13 in the Typical Characteristics section shows

temperature until the thermal protection is triggered;

the output transient response.

use worst-case loads and signal conditions. For good

reliability, thermal protection should trigger at least

+35°C

above

the

maximum

expected

ambient

SHUTDOWN

condition

of

your

application.

This

produces

a

Both enable pins are active high and are compatible

worst-case junction temperature of +125°C at the

with standard TTL-CMOS levels. The device is only

highest

expected

ambient

temperature

and

completely disabled when both EN1 and EN2 are

worst-case load.

logic low. In this state, the LDO is completely off and

The internal protection circuitry of the TPS712xx was

the ground pin current drops to approximately 100

designed to protect against overload conditions. It

nA. With one output disabled, the ground pin current

was not intended to replace proper heatsinking.

is slightly greater than half the nominal value. When

Continuously running the TPS712xx into thermal

shutdown capability is not required, the enable pins

shutdown will degrade device reliability.

should be connected to the input supply.

POWER DISSIPATION

INTERNAL CURRENT LIMIT

The ability to remove heat from the die is different for

The TPS712xx internal current limit helps protect the

each

package

type,

presenting

different

regulator during fault conditions. During current limit,

considerations in the PCB layout. The PCB area

the output will source a fixed amount of current that is

around the device that is free of other components

largely independent of the output voltage.

moves the heat from the device to the ambient air.

The TPS712xx PMOS-pass transistors have a built-in

Performance data for a JEDEC high-K board is

back diode that conducts reverse current when the

shown in the Dissipation Ratings table. Using heavier

input voltage drops below the output voltage (that is,

copper will increase the effectiveness in removing

during power-down). Current is conducted from the

heat

from

the

device.

The

addition

of

plated

output to the input and is not internally limited. If

through-holes to heat-dissipating layers will also

extended reverse voltage operation is anticipated,

improve the heat-sink effectiveness.

external limiting may be appropriate.

Power dissipation depends on input voltage and load

THERMAL PROTECTION

product of the output current times the voltage drop

Thermal protection disables both outputs when the

junction temperature of either channel rises to

(4)

approximately +160°C, allowing the device to cool.

When

the

junction

temperature

cools

to

Power dissipation can be minimized by using the

approximately +140°C, the output circuitry is again

lowest possible input voltage necessary to assure the

required output voltage.

Copyright © 2004–2010, Texas Instruments Incorporated

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