Multi-input latching BJT circuit

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samihawasli

Joined Oct 19, 2020
10
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I am working to create an over current protection circuit for an 8V 3A power supply circuit. The block diagram is above (ignore the block box for now). A battery drives a microcontroller, the power supply circuit, and the over current monitor. To extend the life of the battery, the 8V regulator isn’t always on, it is enabled when needed using the u-controller. The buck regulator chip, lmr33640, within the regulator CTK has an enable pin which I planned on using for this task. When the enable pin is high, the regulator is active.
The over current monitor schematic is below. The circuit uses a dedicated different amp, AD8205, with a gain of 50 to measure the voltage drop across a 0.01 Ohm current shunt. A general purpose op-amp (LM358) is used as a comparator to monitor when the current through the shunt reaches a certain threshold, 3A*0.01Ohm*50V/V=1.5V. A simulation shows the protection circuit’s output as a function of load resistance R2 (also plotting load current). Below the 3A limit the current monitor outputs the positive rail. Above the 3A limit the monitor output drops to zero.
Since the output of the current monitoring circuit is high under normal operation, my plan was to use or mimic an AND gate with the u-controller’s enable output. In theory, under normal operation below 3 amps, the u-controller’s output would turn the 8V regulator on/off. However, if the over current monitor is tripped and its output it zero, the AND gate would always output 0.
The block diagram as is would definitely oscillate in many cases. I was trying to think of a clever latching BJT circuit that could sit in the black block to avoid this problem, I’m open to using a single momentary switch. I’ve searched the internet and have seen many examples of soft latching BTJ switching circuits, but this is slightly different? Bonus: I don’t really want an AND gate, if it can all be done with cheapo jelly beans parts that would great.
Happy to provide more context,
Thanks in advance,
Sami
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sghioto

Joined Dec 31, 2017
2,395
One way is to add a diode to the output of the comparator that will clamp the reference voltage to about .7 volts when low. Set a offset voltage (V4) on the AD8205 above this value, say 1 volt and adjust the reference voltage or V2 as needed.
EEE LM358 Latch.png
 
Last edited:

sghioto

Joined Dec 31, 2017
2,395
Reset can be through the micro on the offset voltage V4. May need to add a cap and resistor on the minus input of the comparator depending on the V2 reference circuit. I would assume V2 and V4 are simple voltage dividers.
EDIT: offset voltage is V4
 
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