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I'm building a power supply, and I'm trying to see if the LM2679 will fit my needs. I like how it's similar to the LM2596 as well as the fact that it has a pin to limit the switch current. However, I have a question regarding the relationship switch current limit and the max output current.

For reference, my test schematic is below. It outputs 5 volts ideally, and it is set to give out 1 A. I adjust the current to the load with switches and resistors, incrementing the current to the load in 0.25 increments until it hits 1A or goes over it. Looking at the waveforms, as it hit 1A, the output voltage lowers in order to adjust for the increasing load, which is what I expect. However, what I'm confused about is the relationship between the switch current and the output current going to the load.

The datasheet states that the peak switch current is equal to a factor of 37,125 divided by the adjustment resistor, Radj. It recommends that for the full temperature range, this limit must be greater than 50% of the max load current. So, assuming my max load is 1 amp, that means the max switch current is 1.5A. Using that value, Radj = 24.75kOhms. I would think that ideally, the load current would be able to tolerate that limit, but as you can see, the voltage is dropping as the load is reaching 1 amp, perhaps even before that. As the load goes past 1 amp, it peaks but the current goes down and kind of settles. I understand that the peak switch current is not the max current limit that can go to the load, but is there a general rule-of-thumb of sorts with regards to the relationship between max load current and the switch mode current?

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In the datasheet the following is also stated

The peak switch current is equal to a factor of 37,125 divided by RADJ. A resistance of 5.6 kΩ sets the current limit to typically 6.3 A and an RADJ of 8.25 kΩ reduces the maximum current to approximately 4.4 A. For predictable control of the current limit, TI recommends keeping the peak switch current greater than 3 A. For lower current applications a 3-A switching regulator with adjustable current limit, the LM2673, is available.

meaning that for a peak current lower than 3 A, a lower precision should be expected regarding the current peak detection.

As for the relationship between the switch and load current, there is some dependence on a couple of parameters and the mode of operation as follows:

Discontinuous Mode

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enter image description here

Continuous Mode

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Source

Since the the LM2679 can operate in both modes, DCM and CCM, I would say it is not straight forward to come up with a rule-of thumb for the peak current value. As long as the peak current was kep above 3A, your best option would be to follow the ratio set by RADJ.

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  • \$\begingroup\$ Thanks for your reply. I suppose that I'll have to take that into account, or I'll have to use the LM2673. Just need to find a model of it first. Thanks for your references. I'll have a look at the equations and see if I can't work with them. \$\endgroup\$ – BestQualityVacuum May 2 '20 at 1:13
  • \$\begingroup\$ @user101402 If this answer satisfies, please "accept" the answer. \$\endgroup\$ – Huisman May 2 '20 at 5:31

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