Zxld1371, Applications information – Diodes ZXLD1371 User Manual
Page 34
ZXLD1371
ZXLD1371
Document number: DS35436 Rev. 1 - 2
34 of 42
February 2012
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Applications Information
(cont.)
LED current can be adjusted digitally, by applying a low
frequency PWM logic signal to the PWM pin to turn the
controller on and off. This will produce an average output
current proportional to the duty cycle of the control signal.
During PWM operation, the device remains powered up
and only the output switch is gated by the control signal.
The PWM signal can achieve very high LED current
resolution. In fact, dimming down from 100% to 0, a
minimum pulse width of 2µs can be achieved resulting in
very high accuracy. While the maximum recommended
pulse is for the PWM signal is10ms.
Figure 12. PWM Dimming Minimum and
Maximum Pulse
The device can be put in standby by taking the PWM pin to ground, or pulling it to a voltage below 0.4V with a suitable open
collector NPN or open drain NMOS transistor, for a time exceeding 15ms (nominal). In the shutdown state, most of the
circuitry inside the device is switched off and residual quiescent current will be typically 90µA. In particular, the Status pin
will go down to GND while the FLAG and REF pins will stay at their nominal values.
Figure 13. Stand-by State From PWM Signal
Thermal Control of LED Current
For thermal control of the LEDs, the ZXLD1371 monitors the voltage on the TADJ pin and reduces output current if the
voltage on this pin falls below 625mV. An external NTC thermistor and resistor can therefore be connected as shown below
to set the voltage on the TADJ pin to 625mV at the required temperature threshold. This will give 100% LED current below
the threshold temperature and a falling current above it as shown in the graph. The temperature threshold can be altered by
adjusting the value of Rth and/or the thermistor to suit the requirements of the chosen LED.
The Thermal Control feature can be disabled by connecting TADJ directly to REF.
Here is a simple procedure to design the thermal feedback circuit:
1) Select the temperature threshold T
threshold
at which the current must start to decrease
2) Select the Thermistor TH1 (both resistive value at 25˚C and beta)
3) Select the value of the resistor R
th
as R
th
= TH at T
threshold
PWM
0V
< 10 ms
0V
2µs
Gate
< 10 ms
2µs