8.1.3 Using Precision Enable and Power Good

The LM26420 device precision EN and PG pins address many of the sequencing requirements required in today's challenging applications. Each output can be controlled independently and have independent power good. This allows for a multitude of ways to control each output. Typically, the enables to each output are tied together to the input voltage and the outputs ratiometrically ramp up when the input voltage reaches above UVLO rising threshold. There may be instances where it is desired that the second output (V_OUT2) does not turn on until the first output (V_OUT1) has reached 90% of the desired setpoint. This is easily achieved with an external resistor divider attached from V_OUT1 to EN₂, see Figure 31.

Figure 31. V_OUT1 Controlling V_OUT2 with Resistor Divider

If it is not desired to have a resistor divider to control V_OUT2 with V_OUT1, then the PG₁ can be connected to the EN₂ pin to control V_OUT2, see Figure 32. R_PG1 is a pullup resistor on the range of 10 kΩ to 100 kΩ, 50 kΩ is the suggested value. This turns on V_OUT2 when V_OUT1 is approximately 90% of the programmed output.

Figure 32. PG₁ Controlling V_OUT2

Another example might be that the output is not to be turned on until the input voltage reaches 90% of desired voltage setpoint. This verifies that the input supply is stable before turning on the output. Select R_EN1 and R_EN2 such that the voltage at the EN pin is greater than 1.12 V when reaching the 90% desired set-point.

Figure 33. V_OUT Controlling V_IN

The power good feature of the LM26420 is designed with hysteresis in order to ensure no false power good flags are asserted during large transient. Once power good is asserted high, it is not pulled low until the output voltage exceeds ±14% of the setpoint for a during of approximately 7.5 µs (typical), see Figure 34.

Figure 34. Power Good Hysteresis Operation