isl97635 Intersil Corporation, isl97635 Datasheet - Page 25

• Manufacturer Part Number: isl97635
• Description: Smbus 8-channel Led Driver
• Manufacturer: Intersil Corporation

output capacitor. The ripple voltage can be shown as
Equation 21:
The conservation of charge principle also brings up the fact
that during the boost switch Off-period, the output capacitor
is charged with the inductor ripple current minus a relatively
small output current in boost topology. As a result, the user
needs to select an output capacitor with low ESD and
enough input ripple current capability.
Output Ripple
ΔV
small ESR capacitors. In general, ceramic capacitors are the
best choice for output capacitors in small to medium sized
LCD backlight applications due to their cost, form factor, and
low ESR.
A larger output capacitor will also ease the driver response
during PWM dimming Off-period due to the longer sample
and hold effect of the output drooping. The driver does not
need to boost harder in the next On-period that minimizes
transient current. The output capacitor is also needed for
compensation, and, in general 2x4.7µF/50V ceramic
capacitors are suitable for notebook display backlight
applications.
Schottky Diode
A high speed rectifier diode is necessary to prevent
excessive voltage overshoot, especially in the boost
configuration. Low forward voltage and reverse leakage
current will minimize losses, making Schottky diodes the
preferred choice. Although the Schottky diode turns on only
during the boost switch Off-period, it carries the same peak
current as the inductor, and therefore, a suitable current
rated Schottky diode must be used.
Applications
High Current Applications
Each channel of the ISL97635 can support up to 35mA. For
applications that need higher current, multiple channels can
be grouped to achieve the desirable current. For example,
the cathode of the last LED can be connected to IIN0 to IIN2,
this configuration can be treated as a single string with
105mA current driving capability.
ΔV
Co
CO
, can be reduced by increasing Co or f
=
I (
O
⁄
C
O
×
D f
⁄
S
)
+
(
I (
O
25
×
ESR
)
SW
, or using
(EQ. 21)
ISL97635
.
Multiple Drivers Operation
For large LCD panels where more than 8 channels of LEDs
are needed, multiple ISL97635s with each driver having its
own supporting components can be controlled together with
the common SMBus. While the ISL97635 does not have
extra pins strappable slave address feature, separate EN
signal can be applied to each driver for asynchronous
operation. A trade-off of such scheme is that an exact faulty
channel cannot be identified if the PWMI/EN signal is
common to all drivers.
SMBCLK
SMBDAT
FIGURE 30. GROUPING MULTIPLE CHANNELS FOR HIGH
EN1
EN2
FIGURE 32. 16-BIT DIMMING ILLUSTRATION
FIGURE 31. MULTIPLE DRIVERS OPERATION
CURRENT APPLICATIONS
IIN0
IIN1
IIN2
SMBCLK
SMBDAT
EN/PWMI
STEP 255 PWM CONTROL
STEP 0~255 DC CONTROL
STEP 254 PWM CONTROL
STEP 1 PWM CONTROL
STEPS 0~255 DC
STEP 0 PWM CONTROL
V
OUT
December 22, 2008
SMBCLK
SMBDAT
EN/PWMI
FN6434.2