jmgelba
- 12th December 2010, 04:23
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.
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.