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ADD5211 FET + Rsense heating

Question asked by hamasaki1818 on Feb 26, 2015
Latest reply on Feb 26, 2015 by Jon.Kraft

Hiya,

 

I've got a small 4-string LED driver board made using AD5211.

It's schematic is basically the 'application circuit' from current datasheet and from Eval_ADD5211_Calculation.xlsx

Rfreq = 49.9K (default)

Rc/Cc = 100R, 2.2uF (default)

Rset = 12.5k (120mA string)

Rramp, Rcs are 6.8/0.1R

 

Coil, NFet and diode are from 5211 EVAL BOM:

IRLR120NTRPBF + Wurth 744771133 + SK310B-LTP. Sense resistor is WSL2010R1000FEA18

The board is powered by 14-15V, with the following spec for leds;

120mA/string

51.9V/string

My R/OVP is 220K/10K @ 57.5V

Anything else not mentioned is per datasheet.

Dimming freq is 20kHz.

 

At 50% duty, the setup is drawing aroudn 1A from 14V supply.

 

Rsense (and eventually the FET) are running at around 90C.

I think the AD5211-EVAL was made for 100mA strings, but still going from 100 to 120 shouldn't matter that much, right?

What is the normal operating condition of the components in 5211-EVAL? My board's layout is basically what's shown in Eval_ADD5211_Bill_Of_Materials.xlsx page (the coil + fet + input/output cap + diode section. Going to 95% duty heats it up even more, so that's not really acceptable.

 

When running, I don't  expect this to be completely cold, but I don't expect 90C either.

The AD5211 itself is warm, but I can't tell if that's heat radiated by the Rsense or comes from the driver itself. Anyway, its significantly less than what fet + rsense are running. Components sourced from Digikey and checked for correct values (R + C, anyway, I've had them swap in different parts before).

 

Any ideas? What's the rated wattage 5211-EVAL runs at, while staying reasonably not-hot?

Do I need to change these two components for something with higher ratings, or could I be doing something else wrong?

 

Thanks.

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