Current sensing at high voltages

[jmgelba]

I have an input referenced buck boost LED driver that I designed to drive 5 high power LED's in parallel. Each LED requires 2.1A @ around 25.5V. Total output is limited to 11A @26V. Because this is an input referenced design, the cathodes of the LED's are connected to the positive input supply and the anodes see a boosted voltage biased by the input voltage. So although Vled is 26V, the output referenced to ground could be up to 66V (Vin max is 40V.)

With me??

Ok, so I have high side current sense resistors for each LED just for the purpose of looking at what each LED is pulling. The voltage at the resistor is a max of 66.8V at which point the over voltage protection cuts in and turns off the driver.

My question: as my PIC is referenced to ground, how do I get the current sense voltage at the resistors down to a max of 5V? Opamp's out there that work rail to rail up to 70V with ultra low offset? With a 0.1r resistor at 2.1A I'd only see 22mV across the resistor. I'd need a gain of 1000. Power would be 0.441W x 5 = a total of 2.2W wasted.

Any better, elegant and small solutions? Board space is very tight.

[jmgelba]

Found a solution. LTC6101 high side current sense amplifier. HV version allows up to 100V and depending on resistor combination can use a very low value sense resistor thus wasting very little power.

Datasheet example: 0.1r cs, 100r rin 4.99K rout will give 4.99Vout @10A load.

http://cds.linear.com/docs/Datasheet/6101fg.pdf