7.3.18 Sequencing (SS/TR)

Many of the common power supply sequencing methods can be implemented using the SS/TR, INH and PWRGD pins. The sequential method is illustrated in Figure 35 using two LMZ31710 devices. The PWRGD pin of the first device is coupled to the INH pin of the second device which enables the second power supply once the primary supply reaches regulation. Figure 36 shows sequential turn-on waveforms of two LMZ31710 devices.

Figure 35. Sequencing Schematic

Figure 36. Sequencing Waveforms

Simultaneous power supply sequencing can be implemented by connecting the resistor network of R1 and R2 shown in Figure 37 to the output of the power supply that needs to be tracked or to another voltage reference source. The tracking voltage must exceed 750mV before V_OUT2 reaches its set-point voltage. The PWRGD output of the V_OUT2 device may remain low if the tracking voltage does not exceed 1.4 V. Figure 38 shows simultaneous turn-on waveforms of two LMZ31710 devices. Equation 3 and Equation 4 calculate the values of R1 and R2.

Equation 3.

Equation 4.

Figure 37. Simultaneous Tracking Schematic

Figure 38. Simultaneous Tracking Waveforms