zxld1350 Zetex Semiconductors plc., zxld1350 Datasheet - Page 22

• Manufacturer Part Number: zxld1350
• Description: 350ma Led Driver With Internal Switch
• Manufacturer: Zetex Semiconductors plc.

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.
The nominal average value of output current in this mode is given by:
I
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.
Issue 6 - April 2007
© Zetex Semiconductors plc 2007
OUTnom
Output current
Filter output
PWM voltage
≈0.1D
PWM
0.1/R
200mV
/R
V
V
ADJ
ADJ
S
Figure 3 High frequency PWM operating waveforms
S
0V
0V
0
[for D
PWM
>0.16]
Ton
22
Toff
ZXLD1350
www.zetex.com
I
OUTnom