t

=160 s+(540xC

)

START

m

s

m

ms

nF

TPS717

www.ti.com

SBVS068H – FEBRUARY 2006 – REVISED JANUARY 2015

Feature Description (continued)

For output voltages below 1.6 V, a voltage divider on the band-gap reference voltage is employed to optimize

output regulation performance for lower output voltages. This configuration results in an additional resistor in the

voltages below 1.6 V.

(1)

7.3.4 Undervoltage Lockout (UVLO)

The TPS717xx uses an undervoltage lockout circuit to keep the output shut off until the internal circuitry is

operating properly. The UVLO circuit has a limited glitch immunity so undershoot transients are typically ignored

on the input if these transients are less than 5

μs in duration.

7.3.5 Minimum Load

The TPS717xx is stable with no output load. Traditional PMOS LDO regulators suffer from lower loop gain at

very light output loads. The TPS717xx employs an innovative low-current mode circuit to increase loop gain

under very light or no-load conditions, resulting in improved output voltage regulation performance down to zero

output current.

7.3.6 Thermal Protection

Thermal protection disables the output when the junction temperature rises to approximately 160°C, allowing the

device to cool. When the junction temperature cools to approximately 140°C the output circuitry is again enabled.

Depending on power dissipation, thermal resistance, and ambient temperature, the thermal protection circuit can

cycle on and off. This cycling limits the dissipation of the regulator, protecting it from damage because of

overheating.

Any tendency to activate the thermal protection circuit indicates excessive power dissipation or an inadequate

heatsink. For reliable operation, limit junction temperature to 125°C maximum. To estimate the margin of safety

in a complete design (including heatsink), increase the ambient temperature until the thermal protection is

triggered; use worst-case loads and signal conditions. For good reliability, trigger thermal protection at least 35°C

above the maximum expected ambient condition of a particular application. This configuration produces a worst-

case junction temperature of 125°C at the highest expected ambient temperature and worst-case load.

The internal protection circuitry of the TPS717xx is designed to protect against overload conditions. This circuitry

is not intended to replace proper heatsinking. Continuously running the TPS717xx into thermal shutdown

degrades device reliability.

7.4 Device Functional Modes

7.4.1 Normal Operation

The device regulates to the nominal output voltage under the following conditions:

•

The input voltage has previously exceeded the UVLO rising voltage and has not decreased below the UVLO

falling threshold.

•

The input voltage is greater than the nominal output voltage added to the dropout voltage.

•

The enable voltage has previously exceeded the enable rising threshold voltage and has not decreased

below the enable falling threshold.

•

The output current is less than the current limit.

•

The device junction temperature is less than the maximum specified junction temperature.

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