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[FAQ] LP8861-Q1: Different ISET current setting equation

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Part Number: LP8861-Q1

For the devices LP8861-Q1, TPS61193-Q1, TPS61194-Q1, the maximum current for the LED current sinks is controlled with external Riset resistor at ISET pin. But there are two different equations to calculate Iset current and Riset resistor.

In the datasheet, the equation is Riset= 2342 / (IOUT-2.5). But in the EVM user's guide, the equation is Riset=2000*Vbg/IOUT. In my application, if the required channel current is 20mA, the Riset can be 120kohm or 134kohm. Which one is right?

  • Both equations could be used within the optimum ILED range (for good rise time and current matching). As you operate at lower ILED currents then the difference between both equations is more noticeable. Both equations should yield similar results from ~30-40mA to 100mA (where current matching and rise time is expected to be good). So, it may be good to keep in mind that 20mA is pushing a little bit the limits for which some aspects of its performance may not be the best (but IC should still work well). For lower current levels I think that you could use Riset=2000*Vbg/IOUT.

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