SLVSFL2A May 2021 – August 2021 TPS7H4002-SP
PRODUCTION DATA
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.
Ratiometric and simultaneous power-supply sequencing can be implemented by connecting the resistor network of R_{1} and R_{2} (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 VOUT_{2} slightly before, after, or at the same time as VOUT_{1}. Equation 8 is the voltage difference between VOUT_{1} and VOUT_{2}.
To design a ratiometric start-up in which the VOUT_{2} voltage is slightly greater than the VOUT_{1} voltage when VOUT_{2} 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 VOUT_{2} is slightly lower than VOUT_{1} when VOUT_{2} 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 R_{1} value from Equation 6 must be greater than the value calculated in Equation 9.