Datasheet

SS /TR
TPS5401
EN
PWRGD
SS / TR
EN
VSENSE
V
OUT1
V
OUT2
R1
R2
R3
R4
TPS5401
PWRGD
OUT2 ssoffset
ref ss
V + V V
R1 = ×
V I
D
ref
OUT2 ref
V R1
R2 =
V + V V
´
D -
TPS5401
SLVSAB0 –DECEMBER 2010 www.ti.com
DETAILED DESCRIPTION (continued)
Figure 34. Schematic for Ratiometric and Simultaneous Start-Up Sequence
Ratiometric and simultaneous power supply sequencing can be implemented by connecting the resistor network
of R1 and R2 shown in Figure 34 to the output of the power supply that must be tracked or to another voltage
reference source. Using Equation 5 and Equation 6, the tracking resistors can be calculated to initiate the V
OUT2
slightly before, after, or at the same time as V
OUT1
. Equation 7 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
ssoffset
) in the slow-start circuit and the offset created by the pullup current source (I
ss
) and
tracking resistors, the V
ssoffset
and I
ss
are included as variables in the equations. Figure 37 shows the result when
ΔV = 0 V.
To design a ratiometric 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 5 through Equation 7 for ΔV. Equation 7 results in a
positive number for applications in which V
OUT2
is slightly lower than V
OUT1
when V
OUT2
regulation is achieved.
Figure 35 and Figure 36 show the start-up waveforms for negative and positive ΔV, respectively.
Because 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 can restart after a fault. Make sure
the calculated R1 value from Equation 5 is greater than the value calculated in Equation 8 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
ssoffset
becomes larger as
the slow-start circuits gradually hand off the regulation reference to the internal voltage reference. The SS/TR pin
voltage must be greater than 1.3 V for a complete handoff to the internal voltage reference as shown in
Figure 23.
(5)
(6)
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