Datasheet

D
´
Vout2 + V Vssoffset
R1 =
Vref Iss
´
D -
Vref R1
R2 =
Vout2 + V Vref
V = Vout1 Vout2D -
> ´ - ´DR1 2800 Vout1 180 V
Time = 2 msec / div
EN = 2 V / div
Vout1 = 1 V / div
Vout2 = 1 V / div
SS/TR
TPS54320
EN
PWRGD
SS/TR
TPS54320
EN
PWRGD
VOUT1
VOUT 2
R1
R2
R3
R4
TPS54320
SLVS982A –AUGUST 2010–REVISED SEPTEMBER 2010 www.ti.com
Ratio-metric and simultaneous power supply sequencing can be implemented by connecting the resistor network
of R1 and R2 shown in Figure 26 to the output of the power supply that needs to be tracked or another voltage
reference source. Using Equation 6 and Equation 7, the tracking resistors can be calculated to initiate the Vout2
slightly before, after or at the same time as Vout1. Equation 8 is the voltage difference between Vout1 and
Vout2.
To design a ratio-metric start up in which the Vout2 voltage is slightly greater than the Vout1 voltage when Vout2
reaches regulation, use a negative number in Equation 6 and Equation 7 for deltaV. Equation 8 results in a
positive number for applications where the Vout2 is slightly lower than Vout1 when Vout2 regulation is achieved.
Figure 27 and Figure 28 show the results for positive deltaV and negative deltaV respectively.
The deltaV variable is zero volts for simultaneous sequencing. To minimize the effect of the inherent SS/TR to
VSENSE offset (Vssoffset, 29mV) in the slow start circuit and the offset created by the pull-up current source
(Iss, 2.3mA) and tracking resistors, the Vssoffset and Iss are included as variables in the equations. Figure 29
shows the result when deltaV = 0V.
To ensure proper operation of the device, the calculated R1 value from Equation 6 must be greater than the
value calculated in Equation 9.
(6)
(7)
(8)
(9)
Figure 27. Ratio-metric Startup with Vout1 Leading
Vout2
Figure 26. Ratiometric and Simultaneous Startup
Sequence
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