There are several methods to enable the TPS7H500x-SEP through the EN pin. The pin can be tied directly to VLDO, which would allow for the device to be enabled as soon as the voltage on VLDO surpasses the rising edge voltage threshold of the EN pin. The pin can also be driven with an externally generated signal or a compatible PGOOD signal for instances in which sequencing is desired. Lastly, two resistors can be used to program the controller to enable when VIN surpasses a user determined threshold, as shown in Figure 8-5. The two resistors are configured as a divider, with one between VIN and EN and the other between EN and AVSS.

Use Equation 1 to calculate the value for R_{UVLO_TOP} for a chosen value of R_{UVLO_BOT} based on the desired maximum start-up voltage for the device. With these selected resistors, Equation 2 is used to determine the minimum start-up voltage.

Equation 1.

Equation 2.

In the two-resistor configuration of Figure 8-5, the controller also shuts down due to undervoltage lockout when the input voltage falls below a particular threshold. This is due to the hysteresis of the EN pin. In order to determine the voltages at which shutdown is expected to occur, use Equation 3 and Equation 4.

Equation 3.

Equation 4.

It is important to take care when selecting the values for R_{UVLO_TOP} and R_{UVLO_BOT}. It is recommended to optimize the selection of these resistors for start-up in order to ensure proper operation. The UVLO value must be approximately 75% or less of the input voltage in order to ensure that the device turns on as expected under all circumstances. Setting the UVLO any higher may cause issues with the turn-on of the device. Figure 8-6 shows the expected start-up and UVLO voltages on a 12-V rail where the maximum start-up voltage is 90% of the nominal input voltage. In this instance, a turn-off will occur when the input voltage falls to between 75% and 65% of its nominal value.