isl97636 Intersil Corporation, isl97636 Datasheet - Page 10

• Manufacturer Part Number: isl97636
• Description: 8-channel Led Driver
• Manufacturer: Intersil Corporation

Theory of Operation
PWM Boost Converter
The current mode PWM boost converter produces the
minimal voltage needed to enable the LED stack with the
highest forward voltage drop to run at the programmed
current. The ISL97636 employes current mode control boost
architecture that has a fast current sense loop and a slow
voltage feedback loop. Such architecture achieves a fast
transient response that is essential for the notebook
backlight application where the power can be a series of
drained batteries or instantly changed to an AC/DC adapter
without rendering a noticeable visual nuisance. The number
of LEDs that can be driven by ISL97636 depends on the
type of LED chosen in the application. The ISL97636 is
capable of boosting up to 34.5V and typically driving 9 LEDs
in series for each of the 8 channels, enabling a total of 72
pieces of the 3.5V/30mA type of LEDs.
Enable and PWMI
The EN/PWMI pin serves dual purposes; it is used as an
enable signal and can be used for PWM input signal for
dimming. If a PWM signal is applied to this pin, the first pulse of
minimum 40µs will be used as an Enable signal. If there is no
signal for longer than 28ms, the device will enter shutdown.
The EN/PWMI pin cannot be floating, thus a 10kΩ pull-down
resistor may need to be added.
Current Matching and Current Accuracy
Each channel of the LED current is regulated by the current
source circuit, as shown in Figure 17.
The LED peak current is set by translating the R
to the output with a scaling factor of 733/R
terminals of the current source MOSFETs are designed as
100mV to minimize the power loss. The sources of errors of
the channel-to-channel current matching come from the
op amp’s offset, internal layout and reference and these
parameters are optimized for current matching and absolute
current accuracy. The absolute accuracy is also determined
by the external R
should be used.
SET
, and therefore a 1% tolerance resistor
10
SET
. The source
SET
current
ISL97636
Dynamic Headroom Control
The ISL97636 features a proprietary Dynamic Headroom
Control circuit that detects the highest forward voltage string
or effectively the lowest voltage from any of the IIN pins.
When this lowest I
threshold, V
signal for the boost regulator. The boost makes the output to
the correct level such that the lowest IIN pin is at the target
headroom voltage. Since all LED stacks are connected to
the same output voltage, the other IIN 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 ISL97636 allows two ways of controlling the LED
current, and therefore, the brightness. They are:
MAXIMUM DC CURRENT SETTING
The initial brightness should be set by choosing an
appropriate value for R
maximum possible LED current:
DC CURRENT ADJUSTMENT
R
DC current adjustment but minimum resistance should not
be lower than 21kΩ for a maximum of 35mA.
I
LEDmax
1. DC current adjustment
2. PWM chopping of the LED current defined in Step 1.
SET
FIGURE 17. SIMPLIFIED CURRENT SOURCE CIRCUIT
can be a DCP (Digitally Controlled Potentiometer) for
=
RSET
RSET
-------------- -
R
SC
733
PWM DIMMING
SET
, such voltage will be used as the feedback
+
-
IN
REF
REF
voltage is lower than the short circuit
SET
. This should be chosen to fix the
+
+
-
-
+
+
-
-
May 9, 2008
(EQ. 1)
FN6570.0