# Voltage Regulator U3V50F24 for High Power LEds

Hello,
I would like to know if someone used the Pololu voltage Regulator U3V50F24 in order to supply High power LEDs ( 7 x Cree XM-L Led in series). Vin voltage is done by 4x Li-Ion battery format 18650 ( 4 x 3.7V = 14.8V or 16.8V full charged, current 2600mAh)? I plan to use a 3.75 ohms 5W resistor at output in serie with the LEDs to adjust the current around 800mA?
Cheers

Hello,

-Derrill

I donâ€™t mean to hijack this topic but Iâ€™m eager to see the analysis and Iâ€™m not sure where the OP went. Isnâ€™t the solution simply a matter of calculating if the regulator can deliver at least 800mA without overheating? Assume that the load consumes 24V @ 800mA = 19.2W. The datasheet says the regulator can handle up to 5A at the input, so at 16.8V thatâ€™s 84W. So the regulatorâ€™s max power is over 4 times the power to the load, which seems like a safe bet even at the worst case efficiency of 80%. One could also look at the efficiency graphs to see that the efficiency at 12V and 18V input is over 90% @ 800mA output and assume that thatâ€™s probably an OK load because you graphed it. Thatâ€™s obviously not analysis, but itâ€™s evidence, right? Are either of these ways of thinking about the problem correct?

Hereâ€™s where I get confused: were this a â€śnormalâ€ť regulator like an LM317, Iâ€™d be asking how much power the regulator has to dissipate and looking to see if the package can handle it. But a step-up regulator has a negative â€śvoltage dropâ€ť across it. How do you calculate the power dissipated by a step up regulator? Why isnâ€™t max power dissipation specified in the datasheet?

Hello.

We do not have enough information to verify that the current draw will be 800 mA yet, but if we presume that is accurate, then your process for determining the regulator can handle that is valid.

You can determine the power dissipation of the regulator from the efficiency graphs: the difference between 100% and the percent efficiency is the percent of power dissipated.

We do not specify a maximum the board can handle because that varies by operating condition (different parts heat up more at different voltages and currents). On newer regulators such as our U3V70x family of step-up regulators, we specify maximum output current under different conditions. If you really care what the power dissipation is at those maximums, you could plug that maximum current back into the efficiency graphs and determine what the power dissipated must be at that operating point.

-Patrick

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