Many of the common power supply sequencing methods can be implemented using the SS/TR, EN, and PWRGD pins. The sequential method can be implemented using an open-drain or collector output of a power on reset pin of another device. Figure 7-3 shows the sequential method. The power good is coupled to the EN pin on the TPS54719, which enables the second power supply once the primary supply reaches regulation.

Ratio-metric start-up can be accomplished by connecting the SS/TR pins 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. The ratio metric method is illustrated in Figure 7-5.

Figure 7-3 Sequential Start-Up Sequence

Figure 7-5 Schematic For Ratio-Metric Start-Up Sequence

Figure 7-4 Sequential Start-Up Using EN And PWRGD

Figure 7-6 Ratio-Metric Start-Up With V_OUT1 Leading V_OUT2

Ratio-metric and simultaneous power supply sequencing can be implemented by connecting the resistor network of R1 and R2 shown in Figure 7-5 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 V_OUT2 slightly before, after, or at the same time as V_OUT1. Equation 7 is the voltage difference between V_OUT1 and V_OUT2. The ΔV variable is zero volts for simultaneous sequencing. To minimize the effect of the inherent SS/TR to VSENSE offset (Vssoffset) in the slow 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 ratio-metric start-up in which the V_OUT2 voltage is slightly greater than the V_OUT1 voltage when V_OUT2 reaches regulation, use a negative number in Equation 5 through Equation 7 for ΔV. Equation 7 will result in a positive number for applications where the V_OUT2 is slightly lower than V_OUT1 when V_OUT2 regulation is achieved. As the SS/TR voltage becomes more than 85% of the nominal reference voltage, the Vssoffset becomes larger as the slow start circuits gradually handoff the regulation reference to the internal voltage reference. The SS/TR pin voltage needs to be greater than 0.87 V for a complete handoff to the internal voltage reference as shown in Figure 7-6.

Equation 5.

Equation 6.

Equation 7.

where:

• V_OUT2 is the regulated output of IC2
• V_OUT1 is the output of IC1 at the moment IC2 just reaches its regulation

Figure 7-7 Schematic For Ratio-Metric Start-Up Sequence

Figure 7-8 Ratio-Metric Start-Up Using Coupled SS/TR Pins