ZXLD1322DCCTC datasheet(10/17 Pages) DIODES

10 / 17 page

ZXLD1322

Document number: DS32166 Rev. 3 - 2

10 of 17

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April 2010

A Product Line of

Diodes Incorporated

Application Notes

Setting Peak Coil Current

The peak current in the coil is set by the resistor (RSENSE) between the switch emitter and ground according to:

ISWPEAK

50mV

RSENSE

The minimum peak current will depend on operating mode, coil inductance and supply voltage range. The maximum peak

current must not exceed the specified value for the switch. (See Application circuits for details)

Setting LED Current

The nominal average LED current is given by:

ILED (NOM) =

100mV

Where RM is the external resistor connected between pins M_VIN and VIN.

This current can be adjusted to a lower value by applying a dc control voltage or PWM control signal to the ADJ pin.

DC Control

The LED current can be adjusted over a 10% to 100% range by connecting a variable resistor RADJ from the ADJ pin to

ground to vary the DC voltage at the ADJ pin. RADJ forms the lower part of a resistive divider and the internal 100kΩ resistor

between the ADJ and VREF pins forms the upper part. A value of 1MΩ for RADJ will therefore give a maximum current of

91% of ILED (nom) and the device will be turned off when the voltage on the ADJ pin falls below 28mV, corresponding to an

RADJ value of approximately 5kΩ. If required, an end-stop resistor in series with RADJ can be used to maintain the voltage on

the ADJ pin above the turn-on threshold.

Using a logarithmic potentiometer for RADJ will give an approximately linear variation of output current with shaft rotation.

(Fig 1)

If required, the maximum output current can be restored to 100% by adjusting the value of the LED current monitor resistor

(RM). The tolerance of the internal 100k resistor and RADJ should be taken into account when calculating output current.

The ADJ pin is clamped internally to a voltage of 575mV (nom), to limit maximum average output current to approximately

115% of ILED(nom).

Fig. 1

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