ZXLD1371

ZXLD1371

Document number: DS35436 Rev. 1 - 2

34 of 42

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February 2012

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Diodes Incorporated

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)

2)

Select the Thermistor TH1 (both resistive value at 25˚C and beta)

3)

PWM

0V

< 10 ms

0V

2µs

Gate

< 10 ms

2µs