Voltage supervisor output oscillation - Page 1
Forum section: electronics / beginners

author: Koen
author_rank: Frequent Contributor
timestamp: January 08, 2017, 11:01:13 pm
content: Hello,I've just discovered a disturbance in the force of a circuit I had always assumed good and wonder how to best solve it.I use MAX809 (active-low push-pull RESET voltage supervisor) to output HIGH when the input voltage (from a battery) is above a certain threshold (3.08V) and LOW if under. The supervisor output toggles regulators enable pins so when the battery is under 3.08V, everything is shutdown.It worked well enough. Come today, and I discover that when the battery arrives at 3.08V, the supervisor output oscillates HIGH/LOW instead of staying LOW. Supervisor goes high, regulators try to start, supervisor goes low, regulators stop and so on. Surely this is draining the battery more than it should until the battery voltage is too low and the supervisor finally stays LOW.How best could I solve this ? I thought of an RC filter on the supervisor output but I have the impression a RESET-monitor isn't the tool for the job. Yet, I can't find anything better googling it.Thank you very much, Koen

author: Benta
author_rank: Super Contributor
timestamp: January 08, 2017, 11:24:36 pm
content: I don't think the MAX809 is the problem, that would be extremely unusual.It sounds like your battery supply is weak. Circuit turns on, starts consuming power, voltage drops, MAX809 resets, battery recovers, circuits starts up, battery voltage drops again etc.You have a basic problem somewhere. Do not add RC filters etc. to the MAX, that will only muddle things.

author: Koen
author_rank: Frequent Contributor
timestamp: January 08, 2017, 11:36:43 pm
content: When the regulators start, the overall load on the battery is only 300µA so I don't think voltage drops so low that the MAX809 completely resets. I'll verify this tomorrow, thank you !

author: tautech
author_rank: Super Contributor
timestamp: January 08, 2017, 11:37:11 pm
content: What allowance have you made for hysteresis in your design ?

author: Koen
author_rank: Frequent Contributor
timestamp: January 08, 2017, 11:47:44 pm
content: If I understood the question right and according to the ONSemi MAX809 datasheet, it would be 3.04V-3.11V or 70mV.

author: tautech
author_rank: Super Contributor
timestamp: January 09, 2017, 12:06:53 am
content: Quote from: Koen on January 08, 2017, 11:47:44 pmNo, they are the parameter range that a devicewillmeet.The hysteresis of your device might be very small which could indeed to oscillation of the output if the output was loaded excessively. The datasheet states the output can supply 20 mA and if supplying a high capacitive load that could make it oscillate. Try adding a series resistor to the output to soften the load.Failing that, another device in the same batch may behave differently, if you have more try another.http://www.onsemi.com/pub_link/Collateral/MAX809S-D.PDF

author: eblc1388
author_rank: Frequent Contributor
timestamp: January 09, 2017, 01:09:45 am
content: If your sole aim is to stop the oscillation, then there is a possible way to achieve that.Use the max810 series instead which outputs a HIGH when battery voltage drops to 3.08V and use this signal to trigger a SCR. The purpose of the SCR is to pull low the regulator enable pin, stopping it. Make sure the SCR anode current flowing is larger than its holding current requirement such that the SCR will latch ON, until the battery is replaced.

author: Koen
author_rank: Frequent Contributor
timestamp: January 09, 2017, 04:08:51 am
content: Benta was right. One of the active-low load switches after the main regulator was pulled up to the regulator output instead of the battery and would rise with the regulator, turning on an MCU whose internal supervisor was left unconfigured too low and which (the MCU) would in turn toggle the load switch active because startup conditional statements were removed from software to debug something else earlier. It's the little things. Thank you very much !