Datasheet

EN
SS/TR
V
SENSE
VOUT
Tss(ms) Iss( A)
Css(nF) =
Vref (V) 0.8
m´
´
ENA
START ENA
ENA
1
V
R2
V V
V
I
R1 R3
=
-
+ -
START STOP
OUT
HYS
V V
R1
V
I
R3
-
=
+
TPS54240
www.ti.com SLVSAA6A –APRIL 2010–REVISED DECEMBER 2012
DETAILED DESCRIPTION (continued)
(4)
(5)
Slow Start/Tracking Pin (SS/TR)
The TPS54240 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 TPS54240 has an internal pull-up current source of 2μA 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 2μA. The slow start capacitor should
remain lower than 0.47μF and greater than 0.47nF.
(6)
At power up, the TPS54240 will not start switching until the slow start pin is discharged to less than 40 mV to
ensure a proper power up, see Figure 29.
Also, during normal operation, the TPS54240 will stop switching and the SS/TR must be discharged to 40 mV,
when the VIN UVLO is exceeded, EN pin pulled below 1.25V, or a thermal shutdown event occurs.
The VSENSE voltage will follow the SS/TR pin voltage with a 45mV 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.
spacer
Figure 29. Operation of SS/TR Pin when Starting
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