Diodes ZXLD1321 - Manual (Page 11)

ZXLD1321
Issue 1 - January 2008 11 www.zetex.com
© Zetex Semiconductors plc 2008
Application notes
Setting peak coil current
The peak current in the coil is set by the resistor (R
SENSE
) 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 R
M
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 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 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 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 R
ADJ
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 (R
M
). 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).
Rsense
mV
I
SWpeak
50
=
M
LED
R
mV
nom I
100
) (
=