If a tracking function is desired, the SS/TR pin can be used for this purpose by connecting it to an external tracking voltage. The output voltage tracks that voltage. If the tracking voltage is between 50 mV and 1.2 V, the FB pin will track the SS/TR pin voltage as described in Equation 11 and shown in Figure 9-2.

Equation 11.

Figure 9-2 Voltage Tracking Relationship

Once the SS/TR pin voltage reaches about 1.2 V, the internal voltage is clamped to the internal feedback voltage and device goes to normal regulation. This works for rising and falling tracking voltages with the same behavior, as long as the input voltage is inside the recommended operating conditions. For decreasing SS/TR pin voltage, the device does not sink current from the output. So, the resulting decrease of the output voltage may be slower than the SS/TR pin voltage if the load is light. When driving the SS/TR pin with an external voltage, do not exceed the voltage rating of the SS/TR pin, which is V_IN + 0.3 V.

If the input voltage drops into undervoltage lockout or even down to zero, the output voltage goes to zero, independent of the tracking voltage. Figure 9-3 shows how to connect devices to get ratiometric and simultaneous sequencing by using the tracking function.

Figure 9-3 Schematic for Ratiometric and Simultaneous Start-Up

The resistive divider of R1 and R2 can be used to change the ramp rate of VOUT2 faster, slower, or the same as that of VOUT1.

A sequential start-up is achieved by connecting the PG pin of VOUT1 to the EN pin of VOUT2. A ratiometric start-up sequence happens if both supplies share the same soft-start capacitor. Equation 10 calculates the soft-start time, though the SS/TR current must be doubled. Details about these and other tracking and sequencing circuits are found in Sequencing and Tracking With the TPS621-Family and TPS821-Family Application Report.