UC3842/3 Current Sense Pin

Thread Starter

SAM-Mo

Joined Jan 19, 2019
41
I want to design a constant current supply based on the UC3842/3.

I see the chip provides the current sense input, can I use this pin as the current feedback to limit the current?
The mentioned threshold voltage is 1V. I'm not sure exceeding 1V will turn-off the PWM (short circuit use case) or reduce PWM duty cycle as the voltage feedback error amplifier does (2.5V reference).

2020-10-20_15-10-19.jpg
 

Thread Starter

SAM-Mo

Joined Jan 19, 2019
41
Yes it will shut down the output, until the current sense pin goes less than 1 V , then it will start up again, .
So it just turns-off the PWM and it does not reduce the PWM duty cycle as the Feedback pin does. the use case for detecting the short circuit.
if we don't want to use it, we can connect it to ground. Am I right?
 

Thread Starter

SAM-Mo

Joined Jan 19, 2019
41
Yes if it's not required to sense current just tie it to the ground.
I want to sense the current by the output shunt resistor and opamp, however, I have a doubt whether this pin only used to turn-off the chip by overcurrent, or I could use it to keep the current constant and compensate it by reducing the voltage (constant current). if that's not applicable, then I'm gonna tie it to the ground and use the feedback pin for sensing both voltage and current like many other chips.
 

crutschow

Joined Mar 14, 2008
25,664
So it just turns-off the PWM and it does not reduce the PWM duty cycle
It can reduce the duty-cycle if the current drops below 1V before the start of each cycle (as is likely).
Note that the current-sense op amp is connected to the PWM latch.
It can thus act to generate a constant-current by reducing the duty-cycle.
 

jtn

Joined Mar 27, 2017
22
Current sense pin 3 is not just for current limit circuit. It is part of the current mode feedback control circuit of the UC3842. Make sure in normal, the I sense voltage is less than 1V to avoid current limiting kicks in. To design for a constant current source the feedback voltage is not sensing from the output. Add a small resistor at the output in the power return path and convert this Rsense voltage through an amplifier circuit. Feed this voltage to Error Amp of UC3842.
 
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