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LDS8866
mode and returning to 1-x mode is called the 1-x
mode transition hysteresis (about 600 mV).
Operation of PWM-based LED Current Control
The maximum current value in each of the
LDS8866’s three LED banks is factory preset; to set
each ILED below this value, a PWM (a duty cycle
based) control signal can be applied at the
PWM1/PWM2 pins.
Using a PWM control technique guarantees stable
WLED color temperature over a wide range of LED
currents. The LED color temperature set at the
factory preset maximum LED current does not vary
with respect to the average LED current unlike
conventional 1-wire LED current control methods.
The LDS8866 ’s PWM logic control circuits have been
designed to operate from 100 Hz to 100 kHz with
duty cycles higher than (0.02*F)% and lower than
(100 – 0.02*F)%, where F is the PWM control
frequency in kHz. The brightness dynamic dimming
range at 200 Hz is 25,000 : 1. PWM control
frequencies lower than 100 Hz are not recommended
(especially with short duty cycles) because LED
flicker may become visible.
When PWM current control is enabled, the LED
current is modulated from zero to 100% over a single
PWM period. For example, when PWM1/PWM2 is
logic high, the LED current is set equal to the
maximum factory preset value. When PWM1/PWM2
is logic low, the LED current is zero. The average
LED current level is then determined by the PWM
duty cycle that may be adjusted as described above.
Figure 3. LDS8866 Timing Diagram
Note:
Timing diagram represents condition when LED forward voltage Vf is higher than Charge Pump Mode times(1.5) input voltage minus voltage
drop on current regulator VPCR and minus voltage drop on charge pump output resistance Rcp at Iled current through N LEDs.
Vf > CPM X Vin – Vd – Rcp x Iled x N; PWM duty Cycle = T PWM ON / (T PWM ON + T PWM OFF )
When LED current control is enabled at the
PWM1/PWM2 inputs, the LDS8866’s maximum input
current is determined by the factory preset maximum
LED current multiplied by number of LED used, the
charge pump operating mode (1-x, 1.33-x, 1.5-x, or
2-x), and divided by charge-pump driver’s efficiency.
For example, if six LEDs are used and the charge
pump is operating in 2-x mode, the maximum pulse
current at V IN would then be 400 mA (= 30 mA/LED x
? 2009 IXYS Corp.
Characteristics subject to change without notice
7
Doc. No. 8866DS, Rev. N2.0
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