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ISL97691
PWM DIMMING
The ISL97691 employs direct PWM dimming such that the output
PWM dimming follows directly with the input PWM signal without
modifying the input frequency. PWM dimming for channels 1 and 2
are set by the PWMI_1 pin, and channels 3 and 4 are set by the
VIN
UVLO
(RISING)
PWMI_2 pin. These two PWM inputs are independent of the two
1-Wire interfaces 1-WIRE_1 and 1-WIRE_2, and any combination
of these 4 inputs may be active at the same time. The average
VO
SOFT-START
(7ms)
FEEDBACK
REGULATION
ESTABLISHED
LED current of each channel can be calculated as Equation 4:
I LED ( ave ) = I LED × PWM
(EQ. 4)
1_WIRE_n
Where:
I LED is the current set by 1-WIRE_n interface (A)
PWM is the duty of the signal at the PWMI_n pin
During the PWM off-time, the two 1-WIRE data will remain at the
previous programmed levels.
Current Matching and Current Accuracy
Each channel of the LED current is regulated by a current sink
circuit.
The LED peak current is set by the external R SET resistor
according to Equation 1. The current sink MOSFETs in each LED
driver channel output are designed to run at ~300mV to optimize
power loss versus accuracy requirements. The sources of errors
of the channel-to-channel current matching come from internal
PWMI
ICHn
FIGURE 19.
Note that there will be also an initial in-rush current to C OUT when
V IN is applied. This is determined by the ramp rate of V IN and the
values of C OUT and L.
Power-Off Sequence
VIN
V UVLO - 150mV
amplifier offsets, internal layout and reference accuracy. These
parameters are optimized for current matching and absolute
current accuracy. Absolute accuracy is also determined by the
external resistor R SET , so a 0.1% tolerance resistor is
recommended.
Dynamic Headroom Control
The ISL97691 features a proprietary Dynamic Headroom Control
circuit that detects the highest forward voltage string or
VO
1_WIRE_n
BOOST CONVERTER
URNED OFF
t OFF AFTER
REACHIN
(V UVLO - 150mV)
DEPENDS ON
APPLICATION
effectively the lowest voltage on any of the channel pins. When
this lowest channel voltage is lower than the short circuit
threshold, V SC , such voltage will be used to help set the output
voltage of the boost regulator. The boost regulates 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 sink circuit on each channel
will ensure that each channel has the same current.
Soft-Start
Once the ISL97691 is powered up, the boost regulator will begin
to switch and the current in the inductor will ramp-up. The current
in the boost power switch is monitored and the switching is
terminated in any cycle where the current exceeds the current
limit. The ISL97691 includes a soft-start feature where this
current limit starts at a low value (350mA). This is stepped up to
the final 2.8A current limit in 7 further steps of 350mA. These
steps will happen over typically 7ms, and will be extended at low
LED PWM frequencies if the LED duty cycle is low. This allows the
output capacitor to be charged to the required value at a low
current limit and prevents high input current for systems that
have only a low to medium output current requirement.
12
PWMI_n
CHn
FIGURE 20.
Operation with Input Voltage Greater than
5.5V
The ISL97691 boost regulator can operate from an input voltage
higher than 5.5V, and up to 23V, as long as an additional supply
voltage between 2.4V and 5.5V is available for the VIN pin.
Please refer to Figure 21 for a typical application schematic
adopting this solution.
FN7840.1
February 1, 2013
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