D

´

V out2 +

V

V ssoffset

R1 =

V ref

Iss

SS/TR

TPS54821

EN

PWRGD

SS/TR

EN

PWRGD

TPS54821

SS/TR

EN

PWRGD

SS/TR

EN

PWRGD

TPS54821

TPS54821

16

TPS54821

SLVSB14B – OCTOBER 2011 – REVISED FEBRUARY 2016

www.ti.com

Product Folder Links: TPS54821

Submit Documentation Feedback

Copyright © 2011–2016, Texas Instruments Incorporated

Device Functional Modes (continued)

The sequential method is illustrated in Figure 22 using two TPS54821 devices. The power good of the first

device is coupled to the EN pin of the second device which enables the second power supply once the primary

supply reaches regulation.

Figure 22. Sequential Start Up Sequence

Figure 23 shows the method implementing ratio-metric sequencing by connecting the SS/TR pins of two devices

together. The regulator outputs ramp up and reach regulation at the same time. When calculating the slow start

time the pull-up current source must be doubled in Equation 5.

Figure 23. Ratio-metric Start Up Sequence

Ratio-metric and simultaneous power supply sequencing can be implemented by connecting the resistor network

of R1 and R2 shown in Figure 24 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. .

The deltaV variable is zero volt 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, 2.3

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

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)