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

The sequential method is shown in Figure 7-7 using two TPS7H4003-SEP devices. The PWRGD pin of the first device is coupled to the EN pin of the second device, which enables the second power supply after the primary supply reaches regulation.

Figure 7-7 Sequential Start-Up Sequence

Figure 7-8 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 soft-start time, the pullup current source must be doubled in Equation 5 as there is only one SS/TR capacitor. A similar situation applies if a resistor divider is used in the EN pin, that is, only one resistor divider is needed and the factor of 2 must be taken into account when calculating the resistor values. This ratiometric connection is the one used in primary mode as described in the Adjustable Switching Frequency and Synchronization (SYNC) section.

Figure 7-8 Ratiometric Start-Up Sequence

Ratiometric and simultaneous power-supply sequencing can be implemented by connecting the resistor network of R₁ and R₂ (shown in Figure 7-9) 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 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 6 and Equation 7 for ΔV. Equation 8 results in a positive number for applications where the VOUT₂ is slightly lower than VOUT₁ when VOUT₂ regulation is achieved.

The ΔV variable is 0 V for simultaneous sequencing. To minimize the effect of the inherent SS/TR to VSENSE offset (V_SS-OFFSET = 30 mV) in the soft-start circuit and the offset created by the pullup current source (I_SS = 2.5 μA) and tracking resistors, the V_SS-OFFSET and I_SS 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.

Equation 6.

Equation 7.

Equation 8.

Equation 9.