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TPS92640 ROFF formula

paolo bernasconi
Prodigy 210 points

Dear Designers and TI support,

I need to zero the ILED when VIADJ is 0. To do that ROFF has to be connected between Vout and CS. There are 2 equations to to calculate ROFF

 

Setting Vout = 24V (3Vx8LED = 24V), RF=475 Ohm (demo board) RCS=0.2Ohm (demo board) delta ILED=3mA (for example), It results:

ROFF = min [15.5K, 1.1MEG] so ROFF has to be set to 15.5K.

If I do that 15.5K the VOUT ILED is always 0 even varying VIADJ from 0 to 1V. If I set ROFF to 1.2MEG the linearity is good and only if I set 0V as VIADJ ILED is 0. Starting from VIADJ = 0.1V than the ILED  is linear with VIADJ. So the conclusion is the above ROFF selection seems to be wrong and only the max (not min) rule has to apply.

  1. Are you sure of the above formula ?

  2. Connecting ROFF to VCS produces a down shift of the line ILED = k*VIADJ + q, changing the q value. Unfortunately VOUT is not exactly constant so theoretically “q” is not constant. Why don’t connect ROFF to a constant voltage to increase the regulation linearity ?

Thanks

Paolo