ISL97635AIRZ-TK Intersil, ISL97635AIRZ-TK Datasheet - Page 11

• Manufacturer Part Number: ISL97635AIRZ-TK
• Description: IC LED DRVR WHT/RGB BCKLGT 24QFN
• Manufacturer: Intersil
• Type: Backlight, White LED, RGBr

Specifications of ISL97635AIRZ-TK

• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 6
• Internal Driver: Yes
• Type - Primary: Automotive, Backlight
• Type - Secondary: RGB, White LED
• Frequency: 600kHz, 1.2MHz
• Voltage - Supply: 6 V ~ 24 V
• Voltage - Output: 34.5V
• Mounting Type: Surface Mount
• Package / Case: 24-VQFN
• Operating Temperature: -40°C ~ 85°C
• Current - Output / Channel: 35mA
• Internal Switch(s): Yes
• Efficiency: 91%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: ISL97635AIRZ-TK

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 ISL97635A 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 applications 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 ISL97635A depends on the
type of LED chosen in the application. The ISL97635A is
capable of boosting up to 34.5V and typically driving 9 LEDs
in series for each of the 6 channels, enabling a total of 54
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, 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
contributed by the current setting resistor. A 1% tolerance
resistor is therefore recommended.
SET
, and therefore, additional tolerance will be
11
SET
. The source
SET
current
ISL97635A
Dynamic Headroom Control
The ISL97635A 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
programmed 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 ISL97635A allows two ways of controlling the LED
current, and therefore, the brightness. They are:
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
maximum possible LED current, as shown in Equation 1:
DC CURRENT ADJUSTMENT
Once R
through register 0x07 (BRTDC), as shown in Equation 2:
I
I
LEDmax
LED
1. DC current adjustment.
2. PWM chopping of the LED current defined in Step 1.
FIGURE 17. SIMPLIFIED CURRENT SOURCE CIRCUIT
=
SET
2.87
=
RSET
RSET
-------------- -
R
is fixed, the LED DC current can be adjusted
SC
×
733
PWM DIMMING
DC DIMMING
SET
BRTDC R
, such voltage will be used as the feedback
+
-
IN
REF
REF
voltage is lower than the short circuit
⁄
SET
SET
. This should be chosen to fix the
+
+
-
-
+
+
-
-
December 22, 2008
(EQ. 1)
(EQ. 2)
FN6564.2