Voltage supervisory ic not working! - Page 1
Forum section: electronics / beginners

author: taimur2001
author_rank: Newbie
timestamp: November 16, 2020, 12:35:28 am
content: Hello,I am using supervisory IC PN:MAX809MEXRT (Vth =4.38) but its not working as expected. Push-pull active low reset output. Using it to enable the 3.3V switcher.Supervisory IC connected to 5V battery powered supply (Basically using Large capacitance as battery.) When the board is powered up, it take some time to reach 4.38V but its not enabling 3.3V switcher because I am seeing 0V at the output (reset pin). After recycling power 3 or 4 times, supervisory IC start working as expected.Please see the attached schematic circuit. Any suggestion?

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 16, 2020, 01:51:53 am
content: Are you sure that TP3 reaches 4.38 V? Maybe you have a measurement error? Try applying a slightly higher voltage of 5.2 V, for example. Where is the U16 ground connected? Maybe a powerful capacitor eats up some of the voltage along the ground track?

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 16, 2020, 01:57:42 am
content: Such a large capacity is actually a short circuit during charging. The current of this circuit is limited by two resistors with a total resistance of 3.4 Ohms, with this resistance, the current consumption will be 1.47 A. Perhaps the power supply can not provide 4.38 V at this current, or even goes into protection.

author: taimur2001
author_rank: Newbie
timestamp: November 16, 2020, 03:39:34 am
content: Quote from: S. Petrukhin on November 16, 2020, 01:51:53 amU16 GND is connected to main ground using small GND shape.Yes, Once the capacitors are fully charged it voltage reaches 4.78V (PMIC should work on 4.38V). I will increase the voltage to 5.2V and see if works or not.

author: taimur2001
author_rank: Newbie
timestamp: November 16, 2020, 03:41:52 am
content: Quote from: S. Petrukhin on November 16, 2020, 01:57:42 amYes, thats a valid point too because 5V LDO maximum current output is 1A. Should i try with one resistor, with 6.8ohm? This will bring the current consumption to 0.7A? Or what value do you suggest?

author: djococaud
author_rank: Regular Contributor
timestamp: November 16, 2020, 08:40:45 am
content: The MAX809 needs a Pull-up, not a pull-downEDIT : not sure, actually...I need to check the datasheet...EDIT2 : Nervermind, I've checked the datasheet, the pull-down is OK.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 16, 2020, 11:25:55 am
content: Quote from: djococaud on November 16, 2020, 08:40:45 amMAX803/809 - have a nReset output and must be pull-down, but MAX810 have a Reset output and must be pull-up.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 16, 2020, 11:30:37 am
content: Quote from: taimur2001 on November 16, 2020, 03:41:52 amTry to leave one resistor, temporarily remove the parallel one. This will unload the power supply. It may well be that the protection is triggered, which after time turns on, checks the status and re-turns off. With these jerks of checking, it gradually charges the capacitor.Or supply power from the lab unit.