Diodes ZXLD1362 - Manual (Page 22)

ZXLD1362
ZXLD1362
Document number: DS33472 Rev. 5 - 2
22 of 24
www.diodes.com
May 2012
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Application Information (cont.)
ADJ Pin
The ADJ pin is a high impedance input for voltages up to 1.35V
so, when left floating, PCB tracks to this pin should be as short as
possible to reduce noise pickup. A 100nF capacitor from the ADJ
pin to ground will reduce frequency modulation of the output
under these conditions. An additional series 3.3k Ω resistor can
also be used when driving the ADJ pin from an external circuit
(see below). This resistor will provide filtering for low frequency
noise and provide protection against high voltage transients.
High Voltage Tracks
Avoid running any high voltage tracks close to the ADJ pin, to
reduce the risk of leakage currents due to board contamination.
The ADJ pin is soft-clamped for voltages above 1.35V to
desensitize it to leakage that might raise the ADJ pin voltage and
cause excessive output current. However, a ground ring placed
around the ADJ pin is recommended to minimize changes in
output current under these conditions.
Evaluation PCB
ZXLD1362 evaluation boards are available on request and
provide quick testing of the ZXLD1362 device.
Dimming Output Current Using PWM
Low Frequency PWM Mode
When the ADJ pin is driven with a low frequency PWM signal
(eg 100Hz), with a high level voltage V
ADJ
and a low level of
zero, the output of the internal low pass filter will swing
between 0V and V
ADJ
, causing the input to the shutdown circuit
to fall below its turn-off threshold (200mV nom) when the ADJ
pin is low. This will cause the output current to be switched on
and off at the PWM frequency, resulting in an average output
current I OUTavg proportional to the PWM duty cycle.
(See Figure 4 - Low frequency PWM operating waveforms).
Figure 4. Low Frequency PWM Operating Waveforms
The average value of output current in this mode is given by:
I
OUTavg
= 0.1D
PWM
/R
S
[for D
PWM
>0.001]
This mode is preferable if optimum LED 'whiteness' is required.
It will also provide the widest possible dimming range (approx.
1000:1) and higher efficiency at the expense of greater output
ripple.
GND
Z XLD1362 ADJ
3.3k
100nF
GND