Datasheet

SS/TR

SENSE

VOUT

( )

( ) ( )

( )

´ m

SS SS

REF

t ms I A

C nF

V V 0.8

TPS54140

SLVS889B –OCTOBER 2008 –REVISED SEPTEMBER 2013

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DETAILED DESCRIPTION (continued)

Slow Start and Tracking Pin (SS/TR)

The TPS54140 effectively uses the lower voltage of the internal voltage reference or the SS/TR pin voltage as

the power-supply's reference voltage and regulates the output accordingly. A capacitor on the SS/TR pin to

ground implements a slow start time. The TPS54140 has an internal pull-up current source of 2 μA that charges

the external slow start capacitor. The calculations for the slow start time (10% to 90%) are shown in Equation 6.

The voltage reference (V

REF

) is 0.8 V and the slow start current (I

) is 2 μA. The slow start capacitor should

remain lower than 0.47μF and greater than 0.47nF.

(6)

At power up, the TPS54140 will not start switching until the slow start pin is discharged to less than 40 mV to

ensure a proper power up, see Figure 30.

Also, during normal operation, the TPS54140 stops switching and the SS/TR must be discharged to 40 mV when

the voltage at the VIN pin is below the VIN UVLO, EN pin pulled below 1.25 V, or a thermal shutdown event

occurs.

The VSENSE voltage will follow the SS/TR pin voltage with a 45 mV offset up to 85% of the internal voltage

reference. When the SS/TR voltage is greater than 85% on the internal reference voltage the offset increases as

the effective system reference transitions from the SS/TR voltage to the internal voltage reference (see

Figure 23). The SS/TR voltage will ramp linearly until clamped at 1.7V.

Figure 30. Operation of SS/TR Pin when Starting

Overload Recovery Circuit

The TPS54140 has an overload recovery (OLR) circuit. The OLR circuit will slow start the output from the

overload voltage to the nominal regulation voltage once the fault condition is removed. The OLR circuit will

discharge the SS/TR pin to a voltage slightly greater than the VSENSE pin voltage using an internal pull down of

100μA when the error amplifier is changed to a high voltage from a fault condition. When the fault condition is

removed, the output will slow start from the fault voltage to nominal output voltage.

Product Folder Links: TPS54140