ZXLD1370

Document number: DS32165 Rev. 6 - 2

28 of 40

www.diodes.com

December 2016

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ZXLD1370

Figure 30 PWM Dimming from Open Collector Switch

Figure 31 PWM Dimming from MCU

PWM

0V

< 10 ms

0V

2µs

Gate

< 10 ms

2µs

Figure 32 PWM Dimming Minimum and Maximum Pulse

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 0.1% at 500Hz, a minimum pulse width

of 2µs can be achieved resulting in very high resolution and accuracy. While the maximum recommended pulse is for the PWM signal is

10ms (equivalent to 100Hz) (see Figure 32).

The ultimate PWM dimming ratio will be determined by the

switching frequency as the minimum PWM pulse width is

determined by resolving at least 1 switching cycle. The figure to the

right the switching waveforms for a low duty cycle PWM dimming.

As can be seen, when the LED current restarts (blue waveform) it

has to start all the way from zero to the peak level set by

nominal switching frequency would imply.

STATUS

0V

PWM

~15ms

0V

Standby

state

Figure 33 Standby State from PWM Signal

The PWM pin can be used to put the device into standby. Taking

the PWM pin low (<0.4V) for more than 25ms (typically 15ms) the

device will enter its standby state and most of the internal circuitry

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.

When the device restarts from standby mode, a “start-up” time must be allowed for before the device resume full LED current regulation.