TPS731 datasheet(13/24 Pages) TI

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dV dt

OUT

80kW ø RLOAD

dV dt

OUT

80kW ø (R1 ) R2) ø RLOAD

TPS731xx

www.ti.com .......................................................................................................................................... SBVS034M – SEPTEMBER 2003 – REVISED AUGUST 2009

TRANSIENT RESPONSE

After the EN pin is driven low, no bias voltage is

needed on any pin for reverse current blocking. Note

The low open-loop output impedance provided by the

that reverse current is specified as the current flowing

NMOS

pass

element

voltage

follower

out of the IN pin due to voltage applied on the OUT

configuration allows operation without an output

pin. There will be additional current flowing into the

capacitor

for

many

applications.

with

any

OUT pin due to the 80k

Ω internal resistor divider to

regulator, the addition of a capacitor (nominal value

ground (see Figure 1 and Figure 2).

μF) from the output pin (OUT) to ground will reduce

undershoot magnitude but increase its duration. In

For the TPS73101, reverse current may flow when

the adjustable version, the addition of a capacitor,

VFB is more than 1.0V above VIN.

CFB, from the OUT pin to the FB pin will also improve

the transient response.

THERMAL PROTECTION

The TPS731xx does not have active pull-down when

Thermal protection disables the output when the

the output is over-voltage. This allows applications

junction temperature rises to approximately +160°C,

that

connect

higher

voltage

sources,

such

allowing the device to cool. When the junction

alternate power supplies, to the output. This also

temperature cools to approximately +140°C, the

results in an output overshoot of several percent if the

output circuitry is again enabled. Depending on power

load current quickly drops to zero when a capacitor is

dissipation,

thermal

resistance,

and

ambient

connected to the output. The duration of overshoot

temperature, the thermal protection circuit may cycle

can be reduced by adding a load resistor. The

on and off. This limits the dissipation of the regulator,

overshoot decays at a rate determined by output

protecting it from damage due to overheating.

capacitor

COUT

and

the

internal/external

load

Any tendency to activate the thermal protection circuit

resistance. The rate of decay is given by:

indicates

excessive

power

dissipation

(Fixed voltage version)

inadequate heatsink. For reliable operation, junction

temperature should be limited to +125°C maximum.

To estimate the margin of safety in a complete design

(4)

(including

heatsink),

increase

the

ambient

temperature until the thermal protection is triggered;

(Adjustable voltage version)

use worst-case loads and signal conditions. For good

reliability, thermal protection should trigger at least

+35°C

above

the

maximum

expected

ambient

(5)

condition

your

application.

This

produces

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

REVERSE CURRENT

highest

expected

ambient

temperature

and

worst-case load.

The NMOS pass element of the TPS731xx provides

inherent protection against current flow from the

The internal protection circuitry of the TPS731xx has

output of the regulator to the input when the gate of

been designed to protect against overload conditions.

the pass device is pulled low. To ensure that all

It was not intended to replace proper heatsinking.

charge is removed from the gate of the pass element,

Continuously running the TPS731xx into thermal

the EN pin must be driven low before the input

shutdown degrades device reliability.

voltage is removed. If this is not done, the pass

element may be left on due to stored charge on the

gate.

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