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ISL97671
I LEDmax = ---------------
R SET
For example, if 10 LEDs are used with the worst case V OUT of 35V.
If R 1 and R 2 are chosen such that the OVP level is set at 40V,
then the V OUT is allowed to operate between 25.6V and 40V. If
the requirement is changed to a 6 LEDs 21V V OUT application,
then the OVP level must be reduced and users should follow V OUT
= (64% ~100%) OVP requirement. Otherwise, the headroom
control will be disturbed such that the channel voltage can be
much higher than expected and sometimes it can prevent the
driver from operating properly.
The ratio of the OVP capacitors should be the inverse of the OVP
resistors. For example, if R UPPER /R LOWER = 33/1, then
C UPPER /C LOWER = 1/33 with C UPPER = 100pF and C LOWER = 3.3nF.
Current Matching and Current Accuracy
Each channel of the LED current is regulated by the current
source circuit, as shown in Figure 20.
The LED peak current is set by translating the R SET current to the
output with a scaling factor of 410.5/R SET . 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. On the other hand, the absolute accuracy is additionally
determined by the external R SET , and therefore, additional tolerance
will be contributed by the current setting resistor. A 1% tolerance
resistor is therefore recommended.
voltage will regulate cycle-by-cycle and is always referenced to
the highest forward voltage string in the architecture.
Dimming Controls
The ISL97671 provides SMBus/I 2 C controlled PWM or DC
dimming where the users need to turn the LEDs on through the
Communications” section). On the other hand, the ISL97671 also
provides PWM dimming by external PWM signal where the
SMBCLK and SMBDAT pins need to pull low or grounded.
The ISL97671 allows 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.
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:
410.5
(EQ. 2)
Once R SET is fixed, the LED DC current can be adjusted through
Register 0x07 (BRTDC) as follows:
I LED = 1.58x ( BRTDC ? R SET )
(EQ. 3)
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 I LEDmax . BRTDC can be adjusted dynamically on the fly
during operation. BRTDC = 0 disconnects all channels.
For example, if the maximum required LED current
R SET = 410.5 ? 0.02 = 20.52k Ω
RSET
+
-
REF
+
-
(I LED(max) ) is 20mA, rearranging Equation 2 yields Equation 4:
(EQ. 4)
If BRTDC is set to 200 then:
PWM DIMMING
+
-
I LED = 1.58 ? 200 ? 20100 = 15.7mA
(EQ. 5)
DC DIMMING
FIGURE 20. SIMPLIFIED CURRENT SOURCE CIRCUIT
Dynamic Headroom Control
The ISL97671 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.
When this lowest channel voltage is lower than the short circuit
threshold, V SC , 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 pin 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 programmed current. The output
11
PWM CONTROL
The ISL97671 provides three 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 6channels to
provide an average LED current. During the On periods, the LED
current will be defined by the value of R SET and BRTDC, as
described in Equations 2 and 3. The source of the PWM signal
can be described as follows:
1. Internally generated 256 step duty cycle programmed
through the SMBus/I 2 C.
2. External signal from PWM.
3. DPST mode. Internally generated signal with a duty cycle
defined by the product of the external PWM and SMBus/I 2 C
programmed PWM at the internal setting frequency.
FN7631.3
August 14, 2012
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