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ISL97692, ISL97693, ISL97694A
Current Matching and Current Accuracy
Each channel of the LED current is regulated by a current sink
circuit.
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.
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
VIN
UVLO
(RISING)
of the channel-to-channel current matching come from internal
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 , and so a 0.1% tolerance resistor is
recommended.
Dynamic Headroom Control
The ISL97692/3/4A feature a proprietary Dynamic Headroom
Control circuit that detects the highest forward voltage string or
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 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
VO
SOFT-START
(7ms)
EN
PWMI
ICHn
Power-Off Sequence
FEEDBACK
REGULATION
ESTABLISHED
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.
Soft-Start
Once the ISL97692/3/4A is powered up and the EN pin is taken
high, 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 ISL97692/3/4A
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
VIN
VO
EN
PWMI
ICHn
V UVLO - 150mV
BOOST CONVERTER
TURNED OFF
t OFF AFTER REACHING
{V UVLO - 150mV}
DEPENDS ON APPLICATION
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.
17
Operation with Input Voltage Greater than
5.5V
The ISL97692/3/4A 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 26 for a typical application schematic
adopting this solution.
FN7839.4
December 20, 2012
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