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ISL97675
PWM Control
The ISL97675 has a high speed 8-bit digitizer that decodes an
incoming PWM signal and converts it into four channels of 8-bit
PWM current with a phase shift function that will be described later.
During the PWM On period, the LED peak current is defined by the
value of R ISET resistor, the average LED current of each channel is
controlled by I LEDmax and the PWM duty cycle in percent as:
ILED4-20mA
ILED3-20mA
ILED2-20mA
I LED ( ave ) = I LEDmax × PWM
(EQ. 5)
ILED1-20mA
ILED_Total_80mA
When the PWM input = 0, all channels are disconnected and the
I LED is guaranteed to be <10μA in this state.
The PWM dimming frequency is adjusted by a resistor at the
RFPWM pin, which will be described in “PWM Dimming
Frequency Adjustment” on page 14.
5 10
TIME (ms)
FIGURE 22. CONVENTIONAL LED DRIVER PWM DIMMING CHANNEL
AND TOTAL CURRENT AT 50% DUTY CYCLE
ILED1-20mA
ILED2-20mA
ILED3-20mA
ILED4-20mA
ILED_Total_80mA
ILED4-20mA
ILED3-20mA
ILED2-20mA
5
10
15
ILED1-20mA
TIME (ms)
FIGURE 20. CONVENTIONAL 4-Ch LED DRIVER WITH 10% PWM
DIMMING CHANNEL CURRENT (UPPER) AND TOTAL
CURRENT (LOWER)
ILED_Total_40mA
5
10
TIME (ms)
FIGURE 23. EQUAL PHASE SHIFT LED DRIVERpwm DIMMING
ILED1-20mA
ILED2-20mA
ILED3-20mA
ILED4-20mA
ILED_Total_20mA
CHANNEL AT 50% DUTY CYCLE
Phase Shift Control
The ISL97675 is capable of delaying the phase of each current
source. Conventional LED drivers pose the worst load transients
to the boost circuit by turning on all channels simultaneously as
shown in Figure 20. In contrast, the ISL97675 phase shifts each
channel by turning them on once during each PWM dimming
period as shown in Figure 21. At each dimming duty cycle except
5
10
at 100%, the sum of the phase shifted channel currents will be
TIME (ms)
FIGURE 21. PHASE SHIFT 4-Ch LED DRIVER WITH 10% PWM
DIMMING CHANNEL CURRENT (UPPER) AND TOTAL
less than a conventional LED driver as shown in Figure 21 and
23. Equal phase means there is fixed delay between channels
and such delay can be calculated as:
t D1 = ------------------ x ? ---------- ?
t D2 = ------------------ x ? ( 255 ) – ( N – 1 ) ? ---------- ? ?
CURRENT (LOWER)
t FPWM 255
( 255 ) ? N ?
t FPWM 255
255 ? ? N ? ?
(EQ. 6)
(EQ. 7)
where ( 255/N ) in Equation 6 and Equation 7 can only be integer
because the PWM dimming is controlled by an internal 8-bit
13
FN7630.1
September 14, 2011
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