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LDS8865
If the device detects a sufficient input voltage is
present to drive all LED currents in 1x mode, it will
change automatically back to 1x mode. This only
applies for changing back to the 1x mode. The
difference between the input voltage when exiting 1x
mode and returning to 1x mode is called the 1x mode
transition hysteresis (about 600 mV).
Operation of PWM-based LED Current Control
The maximum current value in each of the
LDS8865’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 LDS8865’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. PW M 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 LDS8865 Timing Diagram
Note:
1. 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 )
2. Timing Diagram is not to scale
? 2009 IXYS Corp.
Characteristics subject to change without notice
7
Doc. No. 8865DS, Rev. N2.1
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