Datasheet
Soft-Start and Powergood
V
OCL
V
VDDQ
V
PGOOD
V
S5
80%
87%
100%
85 µs
45 µs
UDG−04066
T
VDDQSS
+
2 C
VDDQ
V
VDDQ
0.8
I
VDDQOCP
) 85 ms
(2)
T
VTTSS
+
C
VTT
V
VTT
I
VTTOCL
(3)
TPS51116
www.ti.com
................................................................................................................................................................ SLUS609H – MAY 2004 – REVISED JULY 2009
The soft start function of the SMPS is achieved by ramping up reference voltage and two-stage current clamp. At
the starting point, the reference voltage is set to 650 mV (87% of its target value) and the overcurrent threshold
is set half of the nominal value. When UVP comparator detects VDDQ become greater than 80% of the target,
the reference voltage is raised toward 750 mV using internal 4-bit DAC. This takes approximately 85 µ s. The
overcurrent threshold is released to nominal value at the end of this period. The powergood signal waits another
45 µ s after the reference voltage reaches 750 mV and the VDDQ voltage becomes good (above 95% of the
target voltage), then turns off powergood open-drain MOSFET.
The soft-start function of the VTT LDO is achieved by current clamp. The current limit threshold is also changed
in two stages using an internal powergood signal dedicated for LDO. During VTT is below the powergood
threshold, the current limit level is cut into 60% (2.2 A).This allows the output capacitors to be charged with low
and constant current that gives linear ramp up of the output. When the output comes up to the good state, the
overcurrent limit level is released to normal value (3.8 A). TPS51116 has an independent counter for each
output, but the PGOOD signal indicates only the status of VDDQ and does not indicate VTT powergood
externally. See Figure 1 .
Figure 1. VDDQ Soft-Start and Powergood Timing
Soft-start duration, T
VDDQSS
, T
VTTSS
are functions of output capacitances.
where I
VDDQOCP
is the current limit value for VDDQ switcher calculated by Equation 5 .
where, I
VTTOCL
= 2.2 A (typ). In each of the two previous calculations, no load current during start-up are
assumed. Note that both switchers and the LDO do not start up with full load condition.
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