ISL97674 Просмотр технического описания (PDF) - Intersil
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ISL97674 Datasheet PDF : 28 Pages
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ISL97674
+
-
+- REF
RSET
+
-
PWM DIMMING
DC DIMMING
FIGURE 19. SIMPLIFIED CURRENT SOURCE CIRCUIT
The LED peak current is set by translating the RSET
current to the output with a scaling factor of 401.8/RSET.
The source terminals of the current source MOSFETs are
designed to run at 500mV to optimize power loss versus
accuracy requirements. The sources of errors of the
channel-to-channel current matching come from the
op amps offset, internal layout, reference, and current
source resistors. These parameters are optimized for
current matching and absolute current accuracy.
However, the absolute accuracy is additionally
determined by the external RSET. A 1% tolerance resistor
is recommended.
Dynamic Headroom Control
The ISL97674 features a proprietary Dynamic Headroom
Control circuit that detects the highest forward voltage
string or effectively the lowest voltage from any of the
CH0-CH5 pins digitally. When the lowest channel voltage
is lower than the short circuit threshold, VSC, such
voltage will be used as the feedback signal for the boost
regulator. The boost makes the output to the correct
level such that the lowest channel is at the target
headroom voltage. Since all LED stacks are connected to
the same output voltage, the other channel pins will have
a higher voltage, but the regulated current source circuit
on each channel will ensure that each channel has the
same current. The output voltage will regulate cycle-by-
cycle and it is always referenced to the highest forward
voltage string in the architecture.
Dimming Controls
The ISL97674 allow two ways of controlling the LED
current, and therefore, the brightness. They are:
1. DC current adjustment
2. PWM chopping of the LED current defined in Step 1.
There are various ways to achieve DC or PWM current
control, which will be described in the following.
where BRT is the PWM brightness level programmed in
the Register 0x00. BRT ranges from 0 to 255 in decimal
MAXIMUM DC CURRENT SETTING
The initial brightness should be set by choosing an
appropriate value for RSET. This should be chosen to fix
the maximum possible LED current:
ILEDmax
=
-(--4---0----1---.--8----)
RSET
(EQ. 1)
DC CURRENT ADJUSTMENT
Once RSET is fixed, the LED DC current can be adjusted
through Register 0x07 (BRTDC) as follows:
ILED = 1.58x(BRTDC ⁄ RSET)
(EQ. 2)
BRTDC can be programmed from 0 to 255 in decimal and
defaults to 255 (0xFF). If left at the default value, LED
current will be fixed at ILEDmax. BRTDC can be adjusted
dynamically on the fly during operation. BRTDC = 0
disconnects all channels.
For example, if the maximum required LED current
(ILED(max)) is 20mA, rearranging Equation 1 yields
Equation 3:
RSET = 401.8 ⁄ 0.02 = 20.1kΩ
(EQ. 3)
If BRTDC is set to 200 then:
ILED = 1.58 • 200 ⁄ 20100 = 15.7mA
(EQ. 4)
PWM Control
The ISL97674 provides two different PWM dimming
methods, as described in the following. Each of these
methods results in PWM chopping of the current in the
LEDs for all 6 channels to provide an average LED
current. During the On periods, the LED current will be
defined by the value of RSET and BRTDC, as described in
Equations 1 and 2. The source of the PWM signal can be
described as follows:
1. SMBus/I2C generated 256 level duty cycle
programmed through the SMBus/I2C.
2. External signal from PWM.
The default PWM dimming is in SMBus/I2C mode. In both
methods, the average LED current of each channel is
controlled by ILED and the PWM duty cycle in percent as:
ILED(ave) = ILED × PWM
(EQ. 5)
Method 1 (SMBus/I2C controlled PWM)
To use this mode, users need to set Register0x01 to 0x05
with EN/PWM in logic high.
The average LED current of each channel is controlled
by the SMBus/I2C setting as
ILED(ave) = ILED × (BRT ⁄ 255)
(EQ. 6)
and defaults to 255 (0xFF). BRT = 0 disconnects all
channels.
11
FN7634.0
June 25, 2010
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