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 7-5 using two TPS7H4002-SP devices. The power good 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 7-6 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 5.

Figure 7-6 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 7-7) 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 slow-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.

Figure 7-7 Ratiometric and Simultaneous Start-Up Sequence