Foldback current limiting of voltage regulator - Page 1
Forum section: electronics / beginners

author: khatus
author_rank: Regular Contributor
timestamp: October 08, 2022, 04:40:53 pm
content: https://www.codrey.com/learn/foldback-current-limiting-little-secrets/QuoteHello guys, I want to know if the output current of the voltage regulator crosses its maximum output current limit(2.2 A) then what will be the value of the output current after foldback? It is said here less than normal load condition but how much less?

author: golden_labels
author_rank: Super Contributor
timestamp: October 08, 2022, 05:51:56 pm
content: “It depends”That is limited so not to exceed thermal limits of the regulator. Instantaneously that 7805 may dump as much as 2.2A. But that is transient current, possibly only for a split second before the feedback loop kicks in and limits power dissipation. After that the regulator will settle for some value below 2.2A, but that value is not specified. If you could keep the junction at 25°C, you can achieve at least 750mA in the worst case scenario. On one hand you are unlikely to achieve that temperature,(1)leading to current below 750mA. On the other hand you are unlikely to be operating in in maximum dropout condition, so you can get more than 750mA.In practice 1A to 1.5A is achievable. But that comes from accumulated experience of people using 78xx regulators and observing what other manufacturers offer. As an example, the relevant figure from STMicro’s datasheet. Just an example, as some producers offer better or worse characteristics.(1)Would require a cooling system more expensive than simply adding a transistor to carry excess current.

author: EPAIII
author_rank: Super Contributor
timestamp: October 09, 2022, 05:32:21 am
content: That spec sheet IS telling you what the current will be after foldback occurs. The line just above the one you outlined in red states that the Typical Short Circuit Current is 0.75 A.It should remain at that value until the power is interrupted for long enough for the input side, filter capacitors to discharge. The output Voltage is not specified, but it will be held low enough so that this short circuit current is not exceeded.As for keeping the temperature at or below 25°C, that is a confusing point in my mind. It is the junction temperature, not the ambient or case temperature. Good luck unless a real good heat sink is used AND the ambient temperature is a good amount below that. I would think at least 10°C or more below but that's just an educated guess.PS: The Texas Instruments data sheet gives the same values.

author: golden_labels
author_rank: Super Contributor
timestamp: October 09, 2022, 08:02:57 pm
content: Quote from: EPAIII on October 09, 2022, 05:32:21 amDatasheet entries have a very important, yet so often ignored column called “test conditions”. It is not to be skipped. 750mA is atypicalcurrentfor about 30V dropout, assuming you can somehow extract 23W of heat to keep thejunction at 25°C. The current will be different with different temperature and dropout voltage. In particular this is not an LDO regulator, so with little output-input difference it exhibits sudden drop in maximum current. This is why the chart I quoted and collective knowledge is relevant.

author: bdunham7
author_rank: Super Contributor
timestamp: October 09, 2022, 09:07:37 pm
content: Just for educational purposes, I tried an old STI L7805CV that I had in a bin. I used a PSU with 32V output and up to about 4A max current. Turning the current up quickly with a knob, I got a peak current of ~2.2A and a continuous short current of ~30mA. The foldback is very quick and does not seem to vary too much thermally, at least not on this variant. Doing a disconnect and reconnect at full power resulted in me not being able to see any large peak current even using a meter with 250us peak detect. I didn't try a quick-connect at lower voltages. The regulator was not heatsinked and never got particularly warm to the touch.