Adding output current clamp to Buck?

[cupoftea]

Hi,
Doing Buck converter....7V4 input, 300kHz, and Iout is 0-20A. (Vout 1v)
Is it OK to add the current clamp as shown to clamp the output current to 20A?
I cant see any reason why the ITH pin cant be pulled down like this?
LTspice and jpeg as attached.

LTC7803

https://www.analog.com/media/en/technical-documentation/data-sheets/ltc7803.pdf

(incidentally, LTC1871 datasheet shows pull down on ITH pin on page 33....

https://www.analog.com/media/en/technical-documentation/data-sheets/1871fe.pdf

)

 

[scopeprobe]

I'd question whether the LT1006 Output voltage with a series diode would be low enough to limit the current as the Max voltage on Ith is only 2V. I've not evaluated the circuit in great detail but it maybe easier to feed a current into R2 when trying to limit the current such that the IC interprets the rise in voltage accross R2 and winds off the duty cycle.

 

[cupoftea]

Thanks, what you say was kind of my worries too....though then i looked at the block diagram, and i believe the device with "0.65V" and "2.7V" into it indicates that 0.65V is the zero duty cycle voltage. Datasheet doesnt say this...but i assume it, please correct me if i am wrong.

 

[mtwieg]

One of the headaches with LT's controllers is that they don't clearly state the relationships between the error amplifier circuitry and the PWM control circuitry (mainly because of their sophisticated slope compensation scheme). For most applications this knowledge isn't necessary, but if you want to augment their chip's functionality with additional circuitry you have to do some trial and error experiments.

The simplest approach to implementing a hard output current limit is to leverage the max threshold of its current sense comparator Vsense(max). For this chip it's 50mV. So for a peak current limit of 20A, pick Rsense=2.5mohm. If you have 10A of ripple current and want to limit average Iout to 20A, pick Rsense=2mohm, and so on.

 

[cupoftea]

The simplest approach to implementing a hard output current limit is to leverage the max threshold of its current sense comparator Vsense(max). For this chip it's 50mV. So for a peak current limit of 20A, pick Rsense=2.5mohm. If you have 10A of ripple current and want to limit average Iout to 20A, pick Rsense=2mohm, and so on.

Thanks, you are right, but sorry i should have declared...this will be used for charging a supercapacitor...and so when its at 0V output, the inductor current "staircases" well high unless we have an output current clamp.....so unfortunately , in our case, we cant do the "sense resistor thing".