Zxld1322 – Diodes ZXLD1322 User Manual
Page 10
ZXLD1322
ZXLD1322
Document number: DS32166 Rev. 3 - 2
10 of 17
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