ZXLD1360

Issue 1 - March 2007

25

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© Zetex Semiconductors plc 2007

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.

PWM voltage

Ton

0V

Toff

200mV

0V

Output current

0

Filter output