Datasheet

EN
SS/TR
V
SENSE
VOUT
Tss(ms) Iss( A)
Css(nF) =
Vref (V) 0.8
m´
´
TPS57040-Q1
www.ti.com SLVSAP4C –DECEMBER 2010–REVISED OCTOBER 2012
DETAILED DESCRIPTION (continued)
Slow Start/Tracking Pin (SS/TR)
The TPS57040-Q1 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 TPS57040-Q1 has an internal pullup 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
SS
) is 2 μA. The slow start capacitor
should remain lower than 0.47 μF and greater than 0.47 nF.
(6)
At power up, the TPS57040-Q1 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 TPS57040-Q1 will stop switching and the SS/TR must be discharged to 40
mV, when the V
IN
UVLO is exceeded, EN pin pulled below 1.25 V, or a thermal shutdown event occurs.
The V
SENSE
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 1.7 V.
Figure 30. Operation of SS/TR Pin When Starting
Overload Recovery Circuit
The TPS57040-Q1 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 V
SENSE
pin voltage using an internal pull down of
100 μA 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.
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