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ISL97682, ISL97683, ISL97684
Maximum DC Current Setting
The initial brightness should be set by choosing an appropriate
value for R SET . This should be chosen to fix the maximum
possible LED current as shown in Equation 2 for ISL97682 and
Equation 3 for ISL97683 and ISL97684:
ILED1-20mA
ILED2-20mA
I LEDmax = ---------------
( 804 )
R SET
(EQ. 2)
ILED3-20mA
ILED4-20mA
I LEDmax = ---------------
( 402 )
R SET
(EQ. 3)
ILED_Total_20mA
5
10
DC Current Adjustment
Once R SET is fixed, the LED DC current can be adjusted.
For example, in the 4-channel ISL97684, if the maximum required
LED current (I LED(max) ) is 20mA, rearranging Equation 3 yields
Equation 4:
TIME (ms)
FIGURE 21. PHASE SHIFT 4-Ch LED DRIVER WITH 10% PWM
DIMMING CHANNEL CURRENT (UPPER) AND TOTAL
CURRENT (LOWER)
R SET = ( 402 ) ? 0.02 = 20.1k Ω
(EQ. 4)
ILED4-20mA
PWM Control
The ISL97682, ISL97683, ISL97684 have high speed 8-bit
digitizers that decode the incoming PWM signal and convert it into
2- 3- or 4- 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 R SET resistor value. The
average LED current of each channel is controlled by I LEDmax and
the PWM duty cycle in percent shown by Equation 5:
ILED3-20mA
ILED2-20mA
ILED1-20mA
ILED_Total_80mA
I LED ( ave ) = I LEDmax × PWM
(EQ. 5)
5
10
TIME (ms)
When the PWM input = 0, all channels are disconnected and the
I LED is guaranteed to be <5μA in this state.
FIGURE 22. CONVENTIONAL LED DRIVER PWM DIMMING CHANNEL
AND TOTAL CURRENT AT 50% DUTY CYCLE
The PWM dimming frequency is adjusted by a resistor at the
ILED4-20mA
ILED1-20mA
ILED2-20mA
ILED3-20mA
ILED4-20mA
ILED_Total_80mA
ILED3-20mA
ILED2-20mA
ILED1-20mA
ILED_Total_40mA
5
10
15
5
10
TIME (ms)
FIGURE 20. CONVENTIONAL 4-Ch LED DRIVER WITH 10% PWM
DIMMING CHANNEL CURRENT (UPPER) AND TOTAL
CURRENT (LOWER)
11
TIME (ms)
FIGURE 23. EQUAL PHASE SHIFT LED DRIVER PWM DIMMING
CHANNEL AT 50% DUTY CYCLE
FN7689.1
May 16, 2012
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制造商:Intersil 功能描述:ISL97687 EVAL