LTC7860 limits current if sense threshold breached?

Status
Not open for further replies.

Flyback

Well-Known Member
Hi,
The LTC7860 has short circuit protection.
But the datasheet does not make clear if this is brought on by the Vout being low, or by the sense resistor voltage going above 0.09V? (or both?)

We have a 600W Power module with a LTC7860 upstream of it.
Vin is 18-36V
The LTC7860 sense resistor is 3 milliohms

The normal running current for the power module is 35Amps at 18Vin.
35A puts 0.105V across the sense resistor...thats above the sense threshold of LTC7860.

So what happens?

Does the LTC7860 start switching the PFETs or not?
Datasheet doesn't make it clear
Maybe the LTC7860 waits for the output to fall below X volts before "crying wolf " and going into short circuit protection mode?

Do you know?

Is there any circumstance at which the LTC7860 would "ignore" the sense resistor voltage going above 0.09V?

LTC7860
 
I think that the LTC7860 will limit the current. There doesn't seem to be anything in the data sheet to say that the protection only applies in an over-voltage condition. The current limit will protect the output wiring, the inductor, the MOSFET and the sensing resistor, so I can't see why it would ever be disabled.

The data sheet says:- "An inductor current limit condition is detected when VSENSE exceeds 95mV. When the current limit threshold is exceeded, the P-channel MOSFET is immediately turned off by pulling the GATE voltage to VIN regardless of the controller input."

The 600 W power module will only take 35 A at full load with an 18 V supply. Any less load or higher supply voltage will result in less current being taken.

The design appears to be marginal for 18 V / 600 W load, but it could be that those conditions don't need to be met. If the load is less than the power module's maximum rating, 35 A won't ever be consumed.

You could try making a circuit with a much lower rating. Just use a 3 Ohm resistor and the current limit will be 35 mA and it will be much easier to test the overload conditions.
 
Status
Not open for further replies.
Cookies are required to use this site. You must accept them to continue using the site. Learn more…