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LM3644EVM: in pass mode, VLED is not close to Vin.

Part Number: LM3644EVM
Other Parts Discussed in Thread: LM3644

Sir.,

My customer use LM3644 EVM and try to get 300mA LED current. but it only gets 240mA.

system condition:

-pass mode

-Vin 4.4V

-BRC=24(292mA)

from waveform below, the ILED is only 241mA.

CH1: VLED1

Ch2: Vin

CH3: ILED

if changing LED current setting to 240mA, it looks ok.

it means that the Max LED current is only 240mA and the VLED max is only 3.7V?

in datasheet, VLED should be very close to Vin, right? or there is min voltage drop across the current source? what do I miss?

In Pass mode the difference between (VIN − ILED ×RPMOS) and the voltage across the LED is dropped across the current source.

by the way, the LED current looks ok if using normal mode. due to they only need 100us on time, boost is too slow for them.

BR,

frank