Datasheet

( )
+ D
= +
SS offset
OUT2
REF SS
V
V V
R1
V I
SS/TR
TPS54160
EN
PWRGD
SS/ TR
EN
PWRGD
VOUT 1
VOUT 2
R 1
R 2
R3
R 4
TPS54160
TPS54160
SLVS795E –OCTOBER 2008–REVISED SEPTEMBER 2013 www.ti.com
DETAILED DESCRIPTION (continued)
Figure 34 shows a method for ratio-metric start up sequence by connecting the SS/TR pins together. The
regulator outputs ramp up to reach regulation at the same time. When calculating the slow start time the pull up
current source must be doubled in Equation 6. Figure 35 shows the results of Figure 34.
Figure 36. Schematic for Ratiometric and Simultaneous Start-Up Sequence
Ratio-metric and simultaneous power supply sequencing can be implemented by connecting the resistor network
of R1 and R2 shown in Figure 36 to the output of the power supply that needs to be tracked or another voltage
reference source. Using Equation 7 and Equation 8, the tracking resistors can be calculated to initiate the V
OUT2
slightly before, after or at the same time as V
OUT1
. Equation 9 is the voltage difference between V
OUT1
and V
OUT2
at the 95% of nominal output regulation.
The ΔV variable is zero volts for simultaneous sequencing. To minimize the effect of the inherent SS/TR to
VSENSE offset (V
SS(offset)
) in the slow start circuit and the offset created by the pull-up current source (I
SS
) and
tracking resistors, the V
SS(offset)
and I
SS
are included as variables in the equations.
To design a ratio-metric start up in which the V
OUT2
voltage is slightly greater than the V
OUT1
voltage when V
OUT2
reaches regulation, use a negative number in Equation 7 through Equation 9 for ΔV. Equation 9 results in a
positive number for applications which the V
OUT2
is slightly lower than V
OUT1
when V
OUT2
regulation is achieved.
Since the SS/TR pin must be pulled below 40 mV before starting after an EN, UVLO or thermal shutdown fault,
careful selection of the tracking resistors is needed to ensure the device restarts after a fault. Make sure the
calculated R1 value from Equation 7 is greater than the value calculated in Equation 10 to ensure the device can
recover from a fault.
As the SS/TR voltage becomes more than 85% of the nominal reference voltage the V
SS(offset)
becomes larger as
the slow start circuits gradually handoff the regulation reference to the internal voltage reference. The SS/TR pin
voltage needs to be greater than 1.3 V for a complete handoff to the internal voltage reference as shown in
Figure 23.
(7)
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