ZXLD1350EV4
ZXLD1350EV4 User Guide Iss. 1
6/8
10-04-07
High Frequency PWM Dimming
1. Switch off the power supply.
2. Insert a 0R 0805 resistor across R1 pads.
3. Ensure C3 is not fitted.
4. Connect a PWM signal to the ADJ pin via an open collector NPN transistor, or an open drain N-channel
MOSFET.
5. Alternatively, drive the ADJ pin directly with a PWM signal. However, make sure the PWM signal voltage
levels do not violate the ADJ pin voltage rating. Driving the ADJ pin above 1.25V will exceed the maximum
set current for the value of Rs and may damage the device or LED.
6. Set the PWM frequency to between 10KHz and 50KHz. The cut-off frequency of the internal filter is 4kHz,
and exceeding the 50kHz may cause modulation with the switching regulator.
7. The dimming ratio will be about 6:1, similar to the DC dimming. The nominal LED current (output current),
I
OUT,
is given by
I
OUT
= 0.1*D/Rs where I
OUT
= the nominal LED current.
Rs = the sense resistor value in ohms.
Do not use a resistor value lower than 0.27
Ω.
D = the duty cycle of the PWM dimming frequency.
0.1V is the nominal sense voltage with ADJ open circuit or set to 1.25V.
Note: The ADJ pin is internally referenced to 1.25V. This pin should be left floating for normal operation
without dimming. Please refer to the datasheet for PWM frequency.
8. Follow the ‘ZXLD1350EV4 Power Up’ sequence.
Low Frequency PWM Dimming
1. Switch off the power supply.
2. Insert a 0R 0805 resistor across R1 pads.
3. Make sure C3 is not fitted.
4. Connect a PWM signal to the ADJ pin via an open collector NPN transistor or an open drain N-channel
MOSFET.
5. Alternatively, drive the ADJ pin directly with a PWM signal. However, make sure the PWM signal voltage
levels do not violate the ADJ pin voltage rating. Driving the ADJ pin above 1.25V will exceed the maximum
set current for the value of Rs and may damage the device or LED
6. The PWM frequency can be low; around 100Hz or up to 1kHz.
7. The ZXLD1350 is now effectively being turned on and off at the PWM frequency. The dimming ratios are in
the region of 100:1, much greater than the DC dimming ratio. The average l LED current (output current),
I
OUT,
is given by
I
OUT
= 0.1*D/Rs where I
OUT
= the average LED current.
Rs = the sense resistor value in ohms.
Do not use a resistor value lower than 0.27
Ω.
D = the duty cycle of the PWM dimming frequency.
0.1V is the nominal sense voltage with ADJ open circuit or set to 1.25V.
8. Follow the ‘ZXLD1350EV4 Power Up’ sequence.
Soft-start
1. Switch off the power supply.
2. Insert a 0R 0805 resistor across R1 pads.
3. Fit a capacitor at C3 to decouple the pin. The value of C3 will determine the soft-start time setting. Please
see the datasheet for calculation of the capacitor value.
4. Follow the ‘ZXLD1350EV4 Power Up’ sequence.
Switching the output current off
1. Switch off the power supply.
2. Insert a 0R 0805 resistor across R1 pads.
3. Follow the ‘ZXLD1350EV4 Power Up’ sequence.
4. Connect the ADJ pin to GND to turn off the output current.
5. Follow the ‘ZXLD1350EV4 Power Up’ sequence. The ZXLD1350 internal switch remains switched off (output
current off) whilst the ADJ pin is pulled to GND.
