FC1360PRG datasheet(4/7 Pages) FS

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Application Notes

Setting nominal average output current with external resistor RS

The nominal average output current in the LED(s) is determined by the value of the external

current sense resistor (RS) connected between VIN and ISENSE and is given by:

IOUT = 0.1/RS (A)

The table below gives values of nominal average output current for several preferred values of

current setting resistor (RS) in the typical application circuit:

RS (ohm)

Nominal Average Output Current (mA)

0.1

1000

0.13

760

0.15

667

The above values assume that the ADJ pin is floating and at a nominal voltage of VREF

(1.25V).Note that RS = 0.1 ohm is the minimum allowed value of sense resistor under these

conditions to maintain switch current below the specified maximum value.

Output current adjustment by external DC control voltage

The ADJ pin can be driven by an external dc voltage (VADJ), to adjust the output current to a

value above or below the nominal average value defined by RS.

The nominal average output current in this case is given by:

IOUTdc = (VADJ /1.25) x 100mV x RS (for 0.3< VADJ <2.5V)

Note that 100% brightness setting corresponds to VADJ = VREF. When driving the ADJ pin

above 1.25V, RS must be increased in proportion to prevent IOUTdc exceeding 1A maximum.

The input impedance of the ADJ pin is 200k_ ±25% for voltages below VREF and 20k_ ±25%

for voltages above VREF +100mV.

Inherent open-circuit LED protection

If the connection to the LED(s) is open-circuited, the coil is isolated from the LX pin of the

chip, so the device will not be damaged, unlike in many boost converters, where the back EMF

may damage the internal switch by forcing the drain above its breakdown voltage.

FC1360

2009. 07. 10

4/7

Revision No : 0

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