SS/TR

EN

PWRGD

SS/TR

EN

PWRGD

TPS54521

TPS54521

PWRGD = 2 V / div

EN = 2 V / div

Vout1 = 1 V / div

Vout2 = 1 v / div

Time = 2 msec / div

Time = 2 msec / div

EN = 2 V / div

Vout1 = 1 V / div

Vout2 = 1 V / div

SS/TR

TPS54521

EN

PWRGD

SS/TR

EN

PWRGD

TPS54521

TPS54521

SLVS981 – JUNE 2010

www.ti.com

The sequential method is illustrated in Figure 22 using two TPS54521 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 23 shows the results of Figure 22.

Figure 22. Sequential Start Up Sequence

Figure 23. Sequential Start Up using EN and

PWRGD

Figure 24 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 25 shows the results of Figure 24.

Figure 25. Ratio-metric Startup using Coupled

SS/TR Pins

Figure 24. Ratiometric 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 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.

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