Datasheet

VFB [V]
VSS/TR
[V]
0.2 0.4 0.6 0.8
0.4
1.2
0.8
TRSSFB
VV
/
64.0 ×»
[ ]
F
V
A
tC
SSSS
251
52
.
. m
×=
TPS62140, TPS62140A
TPS62141, TPS62142, TPS62143
SLVSAJ0B –NOVEMBER 2011–REVISED JUNE 2013 www.ti.com
Soft-Start Capacitor
A capacitance connected between the SS/TR pin and AGND allows a user-programmable start-up slope of the
output voltage. A constant-current source provides 2.5 µA to charge the external capacitance. The capacitor
required for a given soft-start ramp time for the output voltage is given by:
(10)
where
C
SS
is the capacitance (F) required at the SS/TR pin and
t
SS
is the desired soft-start ramp time (s).
spacing
NOTE
DC bias effect: High-capacitance ceramic capacitors have a dc bias effect, which has a
strong influence on the final effective capacitance. Therefore, the right capacitor value
must be chosen carefully. Package size and voltage rating in combination with dielectric
material are responsible for differences between the rated capacitor value and the
effective capacitance.
spacing
Tracking Function
If a tracking function is desired, the SS/TR pin can be used for this purpose by connecting it to an external
tracking voltage. The output voltage tracks that voltage. If the tracking voltage is between 50 mV and 1.2 V, the
FB pin tracks the SS/TR pin voltage as described in Equation 11 and shown in Figure 39.
spacing
(11)
Figure 39. Voltage Tracking Relationship
Once the SS/TR pin voltage reaches about 1.2V, the internal voltage is clamped to the internal feedback voltage
and device goes to normal regulation. This works for rising and falling tracking voltages with the same behavior,
as long as the input voltage is inside the recommended operating conditions. For decreasing SS/TR pin voltage,
the device doesn't sink current from the output. So, the resulting decrease of the output voltage may be slower
than the SS/TR pin voltage if the load is light. When driving the SS/TR pin with an external voltage, do not
exceed the voltage rating of the SS/TR pin which is V
IN
+0.3V.
If the input voltage drops into undervoltage lockout or even down to zero, the output voltage goes to zero,
independent of the tracking voltage. Figure 40 shows how to connect devices to get ratiometric and simultaneous
sequencing by using the tracking function.
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