4 years, 2 months ago.

LPC824 Brownout


I have a situation where I am restricting the voltage imput from 7V to 30v. If voltage drops below 7V my buck convertor (TPS54240) will cut off power to device. If it goes over a 30v spike my varistor will cut off the device.

So heres the problem: When the voltage drops below 7V to say 6 and then comes back to 7 the micro controller doesnt start. If i remove the device and plug it back in it works in some cases. If i "short the power and earth" in the device to take care of any residual capcacitance it always works wheen i put it back in

I have 2 1mF on the output side of the buck convertor and am guessing that is holding the power and the micro controller is stuck in some state. To prove this connected a TTL to see if micro controller is dong somehting. However when TTL is connected it works correctly as expected. Guessing the TTL is continuing to provide some power or is draining power causing the MCU to have a "clean break"

Can anyone throw some light? After this rambling :)

Best Regards Suerndar

Can you put a multimeter on the supply pins of the LPC824 when this happens?

posted by Erik - 01 Feb 2017


As always, your efforts to helping this forum is so "reassuring" for us. I have not said it in the past. But folks in our company go "Dont know the answer ?? Ask Erik" and Erik in this situation stands for mbed.

Back to your question

1) When volage is 8v input we get a constant 4.15v out and things are working

2) Voltage is 7: theres probably one cycle of reduction in voltage then it comes back up and drops again. The voltmeter shows a dramatic reduction to 0.6v and then exponentially slow reduction to about 0.1v

3) Voltage back to 8+v the output of tps54240 goes back 4.15 (which is then dropped to 3.3v using a LDO). But MCU never comes up

I have not checked the actual voltage at the MCU level. Will do and let you know. MAYBE the LDO isnt doing its job?

However I have seen some of this hang happen. My partly relation questions are 1) Is it related to Brownout

2) If i register for brownout will the function be called continuously called when voltage is at Brownout levels or just once and one

3) What is normally expected to be done in the brownout code

4) If I need to reset on BrownOut how do i do it? But what if the device boots and BrownOut condition is still valid?

thanks a bunch

posted by siva surendar 02 Feb 2017

Erik We found the following

When the power goes down to cut off voltage and comes up live 8v the TPS54240 correctly converts to 4.15 volts which is then converted by MCP1801 to 3.3v

1) The out of MCP 1801 is correct at 3.3v

2) One of the TX pins of the MCP is also correct at 3.3v

3) But the MCU is still not working. Is not executing any code

posted by siva surendar 02 Feb 2017

Further info TPS54240 is set to cut off output voltage if input is brought to about 7.3v (or less)

1) When I bring the input to 7.3/7.2/7.1 and keep it there (even for about 10 min) take the voltage back to 8 device MCU doesnt start by itself

2) If I bring input to 3 and take it back up to 8 device starts correctly

3) When device doesnt start when i hit the reset button the MCU starts

posted by siva surendar 03 Feb 2017
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