isl97635a Intersil Corporation, isl97635a Datasheet - Page 25

• Manufacturer Part Number: isl97635a
• Description: Smbus 6-channel Led Driver
• Manufacturer: Intersil Corporation

and the voltage drop due to flowing through the ESR of the
output capacitor. The ripple voltage can be shown as
Equation 21:
The conservation of charge principle in Equation 19 also
brings up a 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 users need to select an output capacitor with
low ESD and with a enough input ripple current capability.
Output Ripple
ΔV
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 respond
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 the 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’s, and therefore, a suitable current
rated Schottky diode must be used.
Applications
High Current Applications
Each channel of the ISL97635A 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 C
=
I (
O
⁄
C
O
×
D f
⁄
S
)
+
(
I (
O
25
×
ESR
)
O
or f
S
, or using small
(EQ. 21)
ISL97635A
.
Multiple Drivers Operation
For large LCD panels where more than 6 channels of LEDs
are needed, multiple ISL97635As with each driver having its
own supporting components can be controlled together with
the common SMBus. While the ISL97635A does not have
extra pins strappable slave address feature, a 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 EN/PWMI signal is
common to all drivers.
16-Bit Dimming
The SMBus controlled PWM and DC dimmings can be
combined to effectively provide 16 bits of dimming capability,
which can be valuable for automotive and avionics display
applications. Figure 32 illustrates one programming example
where 256 steps of PWM dimming can be programmed
between each DC dimming steps or vice versa.
SMBCLK
SMBDAT
FIGURE 30. GROUPING MULTIPLE CHANNELS FOR HIGH
EN1
EN2
FIGURE 31. MULTIPLE DRIVERS OPERATION
CURRENT APPLICATIONS
IIN0
IIN1
IIN2
SMBCLK
SMBDAT
EN/PWMI
V
OUT
December 22, 2008
SMBCLK
SMBDAT
EN/PWMI
FN6564.2