LMZ31520: How to drive LED by using PWRGD-pin

Takahashi-san,

There are more options than what I am showing, but see below for a couple of options for logic high, i.e. LED ON when PWRGD is high. In both cases, the LED ON current is approximately (V_LEDPU-V_LED)/R_LED, when PWRGD is high/open.

I would recommend the left option that uses an external FET, because it provides more flexibility in setting the LED current. For the option at the right, you are limited by the PWRGD pull down capability. Per the Power Good electrical characteristics listed on datasheet page 4, when PWRGD is pulling low with 2mA of current, there can be up to 0.3V at PWRGD. That means when PWRGD is low and the LED should be off, there can still be some voltage across the LED and potentially enough current for the LED to be lit dimly. You may have to look harder for a suitable LED for using the option at the right without an external FET.

Please note I have used another V_LEDPU rail for the LED string. In theory, you could also tie the LED string to the V5V LDO for low LED currents, but we do not have an explicit datasheet spec on V5V external drive current capability, so I would caution against using V5V for any LED currents >5mA or so without the customer thoroughly testing on an EVM or prototype board. The V5V LDO is used to power the internal circuitry of the LMZ31520.