TPS54341_15 datasheet(17/48 Pages) TI

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OUT1

2800

180

´ D

OUT1

OUT2

D =

REF

OUT2

REF

+ D -

OUT2

SSoffset

REF

+ D

SS/TR

TPS54341

PWRGD

SS / TR

PWRGD

VOUT 1

VOUT 2

R 1

R 2

R 4

TPS54341

www.ti.com

SLVSC61 – NOVEMBER 2013

DETAILED DESCRIPTION (continued)

Figure 33. Schematic for Ratiometric and Simultaneous Startup Sequence

Ratiometric and simultaneous power-supply sequencing is implemented by connecting the resistor network of R1

and R2 shown in Figure 33 to the output of the power supply that must be tracked or another voltage reference

source. Using Equation 6 and Equation 7, calculate the tracking resistors to initiate the VOUT2 slightly before,

after, or at the same time as VOUT1. Equation 8 is the voltage difference between VOUT1 and VOUT2 at the 95% of

nominal output regulation.

The

ΔV variable is 0 V for simultaneous sequencing. To minimize the effect of the inherent SS/TR to FB offset

(VSSoffset) in the soft-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 ratiometric 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 through Equation 8 for

ΔV. Equation 8 results in a

positive number for applications which the VOUT2 is slightly lower than VOUT1 when VOUT2 regulation is achieved.

Because the SS/TR pin must be pulled below 54 mV before starting after an EN, UVLO, or thermal shutdown

fault, careful selection of the tracking resistors is required to ensure the device restarts after a fault. The

calculated R1 value from Equation 6 must be greater than the value calculated in Equation 9 to ensure the

device recovers from a fault.

As the SS/TR voltage becomes more than 85% of the nominal reference voltage, the VSSoffset becomes larger as

the soft-start circuits gradually hands-off the regulation reference to the internal voltage reference. The SS/TR pin

voltage must be greater than 1.5 V for a complete handoff to the internal voltage reference.

(6)

(7)

(8)

(9)

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