Zxld1370, Application information – Diodes ZXLD1370 User Manual
Page 28
ZXLD1370
Document number: DS32165 Rev. 5 - 2
28 of 39
September 2012
© Diodes Incorporated
ZXLD1370
A Product Line of
Diodes Incorporated
Application Information
(cont.)
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
V
SENSE
/R
S*
1.15. So the first pulse is always longer than the
nominal switching frequency would imply.
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.