ZXLD1321 datasheet(11/20 Pages) DIODES

11 / 20 page

ZXLD1321

Issue 1 - January 2008

www.zetex.com

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

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

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

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 100k

Ω resistor between the ADJ and VREF pins forms the upper

part. A value of 1M

Ω for RADJ will therefore give a maximum current of 91% of I

LED (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).

Rsense

SWpeak

LED

nom

100

)

(

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