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Zxld1322 – Diodes ZXLD1322 User Manual

Page 10

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ZXLD1322

ZXLD1322

Document number: DS32166 Rev. 3 - 2

10 of 17

www.diodes.com

April 2010

© Diodes Incorporated

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:

I

SWPEAK

=

50mV

R

SENSE


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:

I

LED

(NOM) =

100mV

R

M


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 R

ADJ

from the ADJ pin to

ground to vary the DC voltage at the ADJ pin. R

ADJ

forms the lower part of a resistive divider and the internal 100k

Ω resistor

between the ADJ and V

REF

pins forms the upper part. A value of 1M

Ω for R

ADJ

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

R

ADJ

value of approximately 5k

Ω. If required, an end-stop resistor in series with R

ADJ

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 R

ADJ

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 I

LED

(nom).






















Fig. 1