Datasheet

VOUT1

VOUT2

VOUT1

VOUT2

R1 > 2800 Vout1 180 deltaV´ - ´

deltaV = Vout1 Vout2-

VREF R1

R2 =

Vout2 + deltaV VREF

Vout2 + deltaV Vssoffset

R1 =

VREF Iss

TPS54240

SLVSAA6A –APRIL 2010–REVISED DECEMBER 2012

www.ti.com

DETAILED DESCRIPTION (continued)

Ratio-metric 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 needs to be tracked or another voltage

reference source. Using Equation 7 and Equation 8, the tracking resistors can be calculated to initiate the Vout2

slightly before, after or at the same time as Vout1. Equation 9 is the voltage difference between Vout1 and Vout2

at the 95% of nominal output regulation.

The deltaV variable is zero volts for simultaneous sequencing. To minimize the effect of the inherent SS/TR to

VSENSE offset (Vssoffset) in the slow start circuit and the offset created by the pullup current source (Iss) and

tracking resistors, the Vssoffset and Iss are included as variables in the equations.

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 7 through Equation 9 for deltaV. Equation 9 will result in a

positive number for applications which the Vout2 is slightly lower than Vout1 when Vout2 regulation is achieved.

Since the SS/TR pin must be pulled below 40mV before starting after an EN, UVLO or thermal shutdown fault,

careful selection of the tracking resistors is needed to ensure the device will restart 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 Vssoffset 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 1.3V for a complete handoff to the internal voltage reference as shown in

Figure 23.

(7)

(8)

(9)

(10)

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Figure 35. Ratio-metric Startup with Tracking Figure 36. Ratiometric Startup with Tracking

Resistors Resistors

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