Electronics > Projects, Designs, and Technical Stuff
Voltage controlled variable miliohm resistor
<< < (2/2)
zero0d:
Hi All,

first thank you for the replays.

on the sketched circuit that i attached  is a buck-boost controller that already has integrated averaging current sense limit ( LM5176), the controller is sensing with ISNS pins and when the current limit occurs the controller pulls down SS pin, but this is fixed and doesn't depend on input voltage ( or in my case constant input power).

So the idea is to replace the miliohm sense resistor RSNS with a circuit so i can adjust (the let say it so) RSNS with a DAC ( I will also measure with ADC the input voltage in order to know what max current can i have that meets the power limit, this is controlled with MCU)

1) Do maybe some one have an idea?

2) In the mean time i got a proposed circuit from TI, but it uses a lot of components and i' not shore how it works, please look attached file, and if you have time or understand how it works please comment

BR.
Zero999:
That can't be the complete schematic. The LM5176 is more complicated than that.
http://www.ti.com/lit/ds/symlink/lm5176.pdf

Anyway, you're going about this the wrong way. Voltage controlled resistors are notoriously difficult to implement, especially low Ohm values, which work up to any decent frequency. Variable current limiting should be done by adjusting the reference voltage, rather than the sense resistor. If the controller you're using doesn't support this, then find one which does.
zero0d:
Hi,

this is the part of the schematic that is used to limit the input power.

i'm not aware of a controller that supports input power limit out of a box.
Kleinstein:
There is no need to really simulate a milli-ohms resistor. It is enough to modify the sense voltage, so that the controller chips sees a different voltage. For a current limit it should be enough to just add some offset and no multiplication needed. So the Ti circuit is kind of the right way but rather complicated.

For a large range one could consider switching shunts, as the dynamic range is limited: smaller shunts result in very small voltages so that noise and offset drift can become limiting. Too large a shunt would result in excessive heat and thus thermal problems.
David Hess:

--- Quote from: zero0d on March 18, 2020, 01:02:31 pm ---The thing is my buck-boost converter that senses input current it is always comparing the sensed voltage across sens resistor with 50mV and then trips the over current. And i have for different input voltages different input currents and thus cant use the same Rsns resistor for all scenarios. ( Image attached)
--- End quote ---

The usual solution is to add attenuation or gain, maybe jumper selectable, between the current shunt and trigger circuit.
Navigation
Message Index
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod