d123
Advanced Member level 5
Hi,
The circuit shown is intended to make U2 high when current is below 124mA and U2 should be low when the current is over 124mA. I have simulated a multitude of options, none work correctly in simulations nor do the ones I breadboarded to be sure reality matched simulation. Nor does disconnecting LED strings and only having a 10R resistor as a load (real world, not simulation)
The LM193 comparator is definitely within it's common-mode range (INA214 is 100V/V amplification. 100milliOhm * 0.125A = 1.25V; Vref = 1.24V).
Instead of switching the U2 NMOS off, it just goes into current-limiting mode: The Vleds test point voltage falls with rising current and the housekeeping current rises in inverse direction. e.g. for 2 strings with 5R sensing resistors (not the 49.6R they should all be), Vleds drops to ~3.9V and AM1 rises to ~180mA, so housekeeping rises from 2 to 3mA to ~60mA - disastrous waste of finite energy for a battery circuit.
The comparator won't fall to 0V, it drops to ~3V when it is sinking. It has to turn off fully so U2 fully turns off - that is the intention.
I have tried placing a CMOS AND gate between comparator and U2; bipolar and MOS push-pull stages; various other transistor buffers; a comparator after the U3 one with ref level 7V to detect a divided down < 12V, switch to sinking and in this way pull U2 fully to ground.
I feel I can't see the wood for the trees and the problem is something I should understand by now and may be related to the "load" at the MOSFET output affecting how low U3 can sink but I just can't see or find the cause or how to achieve intended functioning.
Why is this happening: "excellent" unintended current limiting but no intended current shutdown?
Thanks.
The circuit shown is intended to make U2 high when current is below 124mA and U2 should be low when the current is over 124mA. I have simulated a multitude of options, none work correctly in simulations nor do the ones I breadboarded to be sure reality matched simulation. Nor does disconnecting LED strings and only having a 10R resistor as a load (real world, not simulation)
The LM193 comparator is definitely within it's common-mode range (INA214 is 100V/V amplification. 100milliOhm * 0.125A = 1.25V; Vref = 1.24V).
Instead of switching the U2 NMOS off, it just goes into current-limiting mode: The Vleds test point voltage falls with rising current and the housekeeping current rises in inverse direction. e.g. for 2 strings with 5R sensing resistors (not the 49.6R they should all be), Vleds drops to ~3.9V and AM1 rises to ~180mA, so housekeeping rises from 2 to 3mA to ~60mA - disastrous waste of finite energy for a battery circuit.
The comparator won't fall to 0V, it drops to ~3V when it is sinking. It has to turn off fully so U2 fully turns off - that is the intention.
I have tried placing a CMOS AND gate between comparator and U2; bipolar and MOS push-pull stages; various other transistor buffers; a comparator after the U3 one with ref level 7V to detect a divided down < 12V, switch to sinking and in this way pull U2 fully to ground.
I feel I can't see the wood for the trees and the problem is something I should understand by now and may be related to the "load" at the MOSFET output affecting how low U3 can sink but I just can't see or find the cause or how to achieve intended functioning.
Why is this happening: "excellent" unintended current limiting but no intended current shutdown?
Thanks.