ZXLD1350

ZXLD1350

Document number: DS33468 Rev. 7 - 2

15 of 22

www.diodes.com

November 2010

© Diodes Incorporated

A Product Line of

Diodes Incorporated

Application Information (Continued)

High frequency PWM mode

At PWM frequencies above 10kHz and for duty cycles above 0.16, the output of the internal low pass filter will contain a

DC component that is always above the shutdown threshold. This will maintain continuous device operation and the nominal

average output current will be proportional to the average voltage at the output of the filter, which is directly proportional to the

duty cycle. (See Figure 3 – High frequency PWM operating waveforms). For best results, the PWM frequency should be

maintained above the minimum specified value of 10kHz, in order to minimize ripple at the output of the filter. The shutdown

comparator has approximately 50mV of hysteresis, to minimize erratic switching due to this ripple. An upper PWM frequency

limit of approximately one tenth of the operating frequency is recommended, to avoid excessive output modulation and to avoid

injecting excessive noise into the internal reference.

Figure 3. High Frequency PWM operating waveforms

The nominal average value of output current in this mode is given by:

This mode will give minimum output ripple and reduced radiated emission, but with a reduced dimming range (approx.5:1). The

restricted dimming range is a result of the device being turned off when the DC component on the filter output falls below

200mV.

Shutdown mode

Taking the ADJ pin to a voltage below 0.2V for more than approximately 100

μs, will turn off the output and supply current will

fall to a low standby level of 15

μA nominal.

100% nominal average value. (See graphs for details).