Datasheet

ManualsBrandsTEXAS INSTRUMENTS ManualsPMICsPMIC - LDO voltage regulator Positivbe, adjustable SON 8 (3x3)

t (s) = 170,000 x C

STR NR

(F)

TPS7A8101

SBVS179A –DECEMBER 2011–REVISED APRIL 2012

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STARTUP

Through a lower resistance, the bandgap reference can quickly charge the noise reduction capacitor (C

). The

TPS7A8101 has a quick-start circuit to quickly charge C

, if present; see the Functional Block Diagrams. At

startup, this quick-start switch is closed, with only 33 kΩ of resistance between the bandgap reference and the

NR pin. The quick-start switch opens approximately 100 ms after any device enabling event, and the resistance

between the bandgap reference and the NR pin becomes higher in value (approximately 250 kΩ) to form a very

good low-pass (RC) filter. This low-pass filter achieves very good noise reduction for the reference voltage.

Inrush current can be a problem in many applications. The 33-kΩ resistance during the startup period is

intentionally put there to slow down the reference voltage ramp up, thus reducing the inrush current. For

example, the capacitance of connecting the recommended C

value of 0.47 μF along with the 33-kΩ resistance

causes approximately 80-ms RC delay. Startup time with the other C

values can be calculated as:

(2)

Although the noise reduction effect is nearly saturated at 0.47 μF, connecting a C

value greater than 0.47 μF

can help reduce noise slightly more; however, startup time will be extremely long because the quick-start switch

opens after approximately 100 ms. That is, if C

is not fully charged during this 100-ms period, C

finishes

charging through a higher resistance of 250 kΩ, and takes much longer to fully charge.

Note that a low leakage C

should be used; most ceramic capacitors are suitable.

TRANSIENT RESPONSE

As with any regulator, increasing the size of the output capacitor reduces over/undershoot magnitude but

increases duration of the transient response. Line transient performance can be improved by using a larger noise

reduction capacitor (C

) and/or bypass capacitor (C

BYPASS

UNDERVOLTAGE LOCK-OUT (UVLO)

The TPS7A8101 uses an undervoltage lock-out circuit to keep the output shut off until the internal circuitry is

operating properly. The UVLO circuit has a de-glitch feature so that it typically ignores undershoot transients on

the input if they are less than 50-μs duration.

MINIMUM LOAD

The TPS7A8101 is stable and well-behaved with no output load. Traditional PMOS LDO regulators suffer from

lower loop gain at very light output loads. The TPS7A8101 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.

THERMAL INFORMATION

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 may 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, junction temperature should be limited 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, thermal protection should

trigger at least +35°C above the maximum expected ambient condition of your 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 TPS7A8101 has been designed to protect against overload conditions. It

was not intended to replace proper heatsinking. Continuously running the TPS7A8101 into thermal shutdown

degrades device reliability.