ZXLD1356 datasheet(18/29 Pages) DIODES

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ZXLD1356

Document number: DS33470 Rev. 3 - 2

18 of 29

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December 2010

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Diodes Incorporated

Application Information

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:

IOUTnom = 0.2/RS [for RS > 0.36Ω]

The table below gives values of nominal average output current for several preferred values of current setting resistor (RS) in the

typical application circuit shown on page 1:

RS (Ω)

Nominal average output

current (mA)

0.36

555

0.56

357

1.33

150

The above values assume that the ADJ pin is floating and at a nominal voltage of VREF (=1.25V). Note that RS = 0.36V is the

minimum allowed value of sense resistor under these conditions to maintain switch current below the specified maximum value.

It is possible to use different values of RS if the ADJ pin is driven from an external voltage. (See next section).

Output current adjustment by external DC control voltage

The ADJ pin can be driven by an external dc voltage (VADJ), as shown, 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 (0.2/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 550mA maximum.

The input impedance of the ADJ pin is 50k

Ω ±25% for voltages below VREF and 14.2kΩ ±25% for voltages above VREF +100mV.

GND

ZXLD1356

ADJ

GND

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