TPS73701DCQ datasheet(13/22 Pages) TI

13 / 22 page

OUT

80kW ø RLOAD

(4)

OUT

80kW ø (R1 ) R2) ø RLOAD

(5)

POWER DISSIPATION

REVERSE CURRENT

+ VIN * VOUT

OUT

(6)

THERMAL PROTECTION

PACKAGE MOUNTING

TPS737xx

www.ti.com.................................................................................................................................................... SBVS067I – JANUARY 2006 – REVISED MARCH 2009

The TPS737xx does not have active pull-down when

Any tendency to activate the thermal protection circuit

the output is over-voltage. This architecture allows

indicates

excessive

power

dissipation

applications that connect higher voltage sources,

inadequate heatsink. For reliable operation, junction

such as alternate power supplies, to the output. This

temperature should be limited to +125°C maximum.

architecture also results in an output overshoot of

To estimate the margin of safety in a complete design

several percent if the load current quickly drops to

(including

heatsink),

increase

the

ambient

zero when a capacitor is connected to the output. The

temperature until the thermal protection is triggered;

duration of overshoot can be reduced by adding a

use worst-case loads and signal conditions. For good

load resistor. The overshoot decays at a rate

reliability, thermal protection should trigger at least

determined

output

capacitor

COUT and the

+35°C

above

the

maximum

expected

ambient

internal/external load resistance. The rate of decay is

condition

your

application.

This

produces

given by:

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

highest

expected

ambient

temperature

and

(Fixed voltage version)

worst-case load.

The internal protection circuitry of the TPS737xx has

been designed to protect against overload conditions.

It was not intended to replace proper heatsinking.

(Adjustable voltage version)

Continuously running the TPS737xx into thermal

shutdown degrades device reliability.

The ability to remove heat from the die is different for

each

package

type,

presenting

different

The NMOS pass element of the TPS737xx provides

considerations in the PCB layout. The PCB area

inherent protection against current flow from the

around the device that is free of other components

output of the regulator to the input when the gate of

moves the heat from the device to the ambient air.

the pass device is pulled low. To ensure that all

Performance data for JEDEC low- and high-K boards

charge is removed from the gate of the pass element,

are shown in the Power Dissipation Ratings table.

the EN pin must be driven low before the input

Using heavier copper will increase the effectiveness

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

in removing heat from the device. The addition of

element may be left on because of stored charge on

plated through-holes to heat-dissipating layers also

the gate.

improves the heatsink effectiveness.

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

Power dissipation depends on input voltage and load

needed on any pin for reverse current blocking. Note

conditions. Power dissipation (PD) is equal to the

that reverse current is specified as the current flowing

product of the output current times the voltage drop

out of the IN pin because of voltage applied on the

across the output pass element (VIN to VOUT):

OUT pin. There will be additional current flowing into

the OUT pin as a result of the 80k

Ω internal resistor

divider to ground (see Figure 1 and Figure 2).

Power dissipation can be minimized by using the

For the TPS73701, reverse current may flow when

lowest possible input voltage necessary to assure the

VFB is more than 1.0V above VIN.

required output voltage.

Thermal protection disables the output when the

Solder

pad

footprint

recommendations

for

the

junction temperature rises to approximately +160°C,

TPS737xx are presented in Application Bulletin

allowing the device to cool. When the junction

Solder Pad Recommendations for Surface-Mount

temperature cools to approximately +140°C, the

Devices

(SBFA015),

available

from

the

Texas

output circuitry is again enabled. Depending on power

Instruments web site at www.ti.com.

dissipation,

thermal

resistance,

and

ambient

temperature, the thermal protection circuit may cycle

on and off. This cycling limits the dissipation of the

regulator,

protecting

from

damage

due

overheating.

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