UC3842/3 Current Sense pin - Page 1
Forum section: electronics / projects

author: hesam.moshiri
author_rank: Regular Contributor
timestamp: October 20, 2020, 11:43:22 am
content: I want to design a constant current supply based on the UC3843.I see the chip provides the current sense input, can I use this pin as the current feedback to limit the current?The mentioned threshhold voltage is 1V. I'm not sure exceeding 1V will turn-off the PWM (short circuit use case) or reduce PWM duty cycle as voltage feedback error amplifier does (2.5V reference).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 20, 2020, 03:04:51 pm
content: Constant current where?The UC3843 is a peak current mode controller. Notice the Error Amplifier sets the sense threshold. The comparator feeds a latch, which is set by the oscillator every cycle and reset by the comparator. It fundamentally works by sensing current here, on a cycle by cycle basis, and proportional to the error amp's command.For something like an LED supply, you would typically use peak current mode operation as normal, then use the error amp to regulate output voltage and current.Here's an example I made some years ago:https://www.seventransistorlabs.com/Images/LED_Light.pngIt has adjustable output, overvoltage protection, and thermal limiting.Tim

author: Phoenix
author_rank: Frequent Contributor
timestamp: October 20, 2020, 11:27:53 pm
content: Quote from: hesam.moshiri on October 20, 2020, 03:26:44 pmAs Tim mentioned, this is a peak mode current control IC. The operation is that the error amplifier of the output voltage is used to set the peak current limit. The current sense pin is fed into a comparator to compare against the error amplifier's peak limit. Once the peak current is reached the comparator switches and trips the flip-flop off turning off the PWM. the flip-flop is then reset by the internal oscillator. This continues on a cycle by cycle basis.What Tim has done is feed an external current regulation loop as the feedback instead of a voltage. With this arrangement you still use the peak current control, but the peak value is set by the external current control loop. This could achieve what you are after.You can also trick the chip into becoming a simple PWM generator without peak current control. You sort of reverse the operation by feeding a ramp into the current sense pin and a "target duty cycle" value into the feedback pin. You then use your external current/voltage loop to create the duty cycle. This is Similar to Tim's concept but without the additional internal peak controller.In any case you must use the current sense pin for something as it is integral to the PWM generation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 21, 2020, 09:14:59 am
content: Quote from: hesam.moshiri on October 20, 2020, 03:26:44 pmWhat output?You seem to have a lot of preconceptions about how this chip works. Did you read the block diagram you posted? It says everything you need to know. (Are you familiar with logic diagrams? Would you like us to talk through the circuit?)I said,Quote from: T3sl4co1l on October 20, 2020, 03:04:51 pmIf you grounded that pin, it would fundamentallynot work!Tim