7.3.13 Sequencing (SS/TR)

Many of the common power-supply sequencing methods can be implemented using the SS/TR, EN, and PWRGD pins.

Figure 20 shows the sequential method using two TPS54320 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 21 shows the results of Figure 20.

Figure 20. Sequential Start-Up Sequence SPACE

Figure 21. Sequential Start-Up Using EN and PWRGD

Figure 22 shows the method implementing ratiometric 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 pullup current source must be doubled in Equation 4. Figure 23 shows the results of Figure 22.

Figure 22. Ratiometric Start-Up Sequence SPACE

Figure 23. Ratiometric Start-Up Using Coupled SS/TR Pins

Ratiometric 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 5 and Equation 6, the tracking resistors can be calculated to initiate the Vout2 slightly before, after, or at the same time as Vout1. Equation 7 is the voltage difference between Vout1 and Vout2.

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 5 and Equation 6 for ΔV. Equation 7 results in a positive number for applications where the Vout2 is slightly lower than Vout1 when Vout2 regulation is achieved. Figure 25 and Figure 26 show the results for positive ΔV and negative ΔV respectively.

The ΔV variable is 0 V for simultaneous sequencing. To minimize the effect of the inherent SS/TR to VSENSE offset (Vssoffset, 29 mV) in the slow-start circuit and the offset created by the pullup current source (Iss, 2.3 μA) and tracking resistors, the Vssoffset and Iss are included as variables in the equations. Figure 27 shows the result when ΔV = 0 V.

To ensure proper operation of the device, the calculated R1 value from Equation 5 must be greater than the value calculated in Equation 8.

Equation 5.

Equation 6.

Equation 7.

Equation 8.

Figure 24. Ratiometric and Simultaneous Start-Up Sequence

Figure 25. Ratiometric Start-Up With Vout1 Leading Vout2

Figure 26. Ratiometric Start-Up With Vout2 Leading Vout1

Figure 27. Simultaneous Start-Up