D

´

V out2 +

V

V ssoffset

R1 =

V ref

Iss

´

D

-

Vref

R1

R2 =

Vout2 +

V

Vref

V = Vout1

Vout 2

D

-

>

´

-

´ D

R1

2800

Vout1 180

V

SS/TR

TPS54521

EN

PWRGD

SS/TR

TPS54521

EN

PWRGD

VOUT1

VOUT 2

R1

R2

R3

R4

Time = 2 msec / div

EN = 2 V / div

Vout1 = 1 V / div

Vout2 = 1 V / div

TPS54521

www.ti.com

SLVS981 – JUNE 2010

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 26. Ratiometric and Simultaneous Startup Sequence

Figure 27. Ratio-metric Startup with Vout1 Leading Vout2

Copyright © 2010, Texas Instruments Incorporated

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