Diodes ZXLD1360 - Manual (Page 23)

ZXLD1360
ZXLD1360
Document number: DS33471 Rev. 4 - 2
23 of 25
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March 2011
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A Product Line of
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Application Information (cont.)
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 4 – 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 4. High Frequency PWM Operating Waveforms
The nominal average value of output current in this mode is given by:
I
OUT
nom »0.1D
PWM
/R
S
[for D
PWM
>0.16]
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
V
ADJ
Ton
0V
V
ADJ
Toff
200mV
0V
Output current
0.1/R
S
0
I OUTnom
Filter output