ZXLD1371Q

ZXLD1371Q

Document number: DS37116 Rev. 1 - 2

33 of 42

www.diodes.com

September 2015

© Diodes Incorporated

ZXLD1371Q

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 ZXLD1371Q 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

STATUS

0V

PWM

~15ms

0V

Standby

state