Datasheet

t (s) =
RAMPUP
C (F) + 110 10 (F)
SSx
´
-12
0.5 10 (A)´
-6
´ 1.2 (V)
TPS75005
www.ti.com SBVS144C –NOVEMBER 2011–REVISED APRIL 2012
Soft-Start (SS1 and SS2)
The TPS75005 has a soft-start (or slow-start) function for LDO1 and LDO2 that work independently of one
another. The ramp-up time for LDO1 and LDO2 is by default 260 µs due to the internal 110-pF capacitors. By
connecting an external capacitor(s) at the SS1 and/or SS2 pins, the ramp-up time for the LDOs increases
proportionately by following this equation:
(1)
where:
t
RAMPUP
= ramp up time
C
SSx
= external capacitor value at SS1 or SS2 pin
See Figure 23 for an actual measurement curve.
To ensure that these circuits are discharged during power down, the capacitors used can have a maximum value
of 10 nF approximately 24 ms of ramping time. When an application circuit must control a much larger timing
period, use the supervisor delay setting in addition to the soft-start mechanism. See and find T
SSx
and T
d(SVSx)
in
Figure 32. See the Delay Setting (CT1 and CT2) section for details.
All supported C2000 controllers work well with the TPS75005 default setting (without external soft-start
capacitors). In case a large number of output capacitors are connected for a specific application reason, it is
recommended to connect capacitors at SS1 and/or SS2 so that inrush current (into the TPS75005) does not
cause a large input voltage droop. This can be mitigated by increasing the bulk capacitance at V
IN
.
NOTE
SS1 and SS2 are very high impedance nodes with very low values of constant current
source. These two terminals cannot be monitored by regular oscilloscope probes.
Connecting such regular probes to SS1 or SS2 changes the behavior of the soft-start
function and no valid waveform can be monitored.
To monitor these terminals, use high-impedance probes, such as active FET probes.
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