Power good (PG) pin on LD39100

[lamikr]

Hi

I am looking some guidance how to use the PG pin to indicate the power good mode with fixed versions of LD39100. (3.3 and 1.8V versions.) I plan to use the 3.3V output as an input for the fixed 1v8 ldo version, so ideally all I need to monitor to verify the functionality is the PG pin from 1.8V version of LDO.

PG pins I plan to pull-up high to Vin and datasheet suggest using a resistor between 100kOhm and 1Mohm. I assume that with the fixed 3.3V version of LD39100, the voltage should be high enough to power up the LED directly but with the 1.8V ldo case, I should use it more like an IO pin and then supply the LED voltage from LDO 3.3V?

[Peter BENSCH · Accepted solution]

Welcome @lamikr, to the community!

If I understand your description correctly, you want to connect an LED to each PG to display the status, right?

In this case, the pull-up would be the current limiting resistor of the respective LED, so that it can be calculated instead of the recommended 100k...1M depending on the voltage used (max current 6mA). For the regulator for 3.3V VIN must be greater by the dropout voltage, so it should be sufficient even for white LEDs.

For the 1.8V regulator, you should indeed connect the LED to 3.3V, i.e. VIN of the 1.8V regulator. However, this still excludes white LEDs, as PG has a drop of about 0.4V at 6mA. The resulting approx. 2.9V for the LED and its series resistor can then only be used for red, yellow and some green LEDs, because some variants of green have too high and therefore unsuitable forward voltages.

However, you should bear in mind that PG will drive the LED if the voltage is not correct. This means that the LED does not light up when the voltage is at the correct value, but when it is below 80%.

Hope that answers your question?

Regards
/Peter