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SC4509

Highly Integrated Step-Up Converter for White LED Applications in MLPD-W8-2x2

Semtech Corporation 

POWER MANAGEMENT

Applications Information

Programming the LED Current

Please refer to the Typical Application Circuit on page 1.

The LED current is programmed by connecting a resistor

(R1) between the FB pin and GND. The LED current in

mA is given by the following equation:

ILED = 199/R1

where R1 value is in Ohms.

Setting the Output Voltage

The non-inverting input of the internal error amplifier is

tied to a 199mV voltage reference. A simple voltage divider

(R1 at the top and R2 at the bottom) sets the converter

output voltage. Please refer to the Application Circuit for

OLED Panels on the next page. The voltage feedback gain

h = 0.199/Vo is related to the divider resistors value as

h R2

R1 R2

Once either R1 or R2 is chosen, the other can be calcu-

lated for the desired output voltage Vo. Since the number

of standard resistance values is limited, the calculated

resistance may not be available as a standard value resis-

tor. As a result, there will be a set error in the converter

output voltage. This non-random error is caused by the

feedback voltage divider ratio. It cannot be corrected by

the feedback loop.

SC4509

© 2007 Semtech Corp.

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