ISL97671 Intersil, ISL97671 Datasheet - Page 26

• Manufacturer Part Number: ISL97671
• Description: 6-Channel SMBus/I2C or PM Dimming LED Driver
• Manufacturer: Intersil

overheating in the core. The series resistance, DCR,
within the inductor causes conduction loss and heat
dissipation. A shielded inductor is usually more suitable
for EMI susceptible applications, such as LED
backlighting.
The peak current can be derived from the voltage across
the inductor during the Off period, as expressed in
Equation 23:
IL
The choice of 85% is just an average term for the
efficiency approximation. The first term is the average
current, which is inversely proportional to the input
voltage. The second term is the inductor current change,
which is inversely proportional to L and f
for a given switching frequency and minimum input
voltage on which the system operates, the inductor I
must be chosen carefully. At a given inductor size,
usually the larger the inductance, the higher the series
resistance because of the extra winding of the coil. Thus,
the higher the inductance, the lower the peak current
capability. The ISL97671 current limit should also have to
be taken into account.
Output Capacitors
The output capacitor acts to smooth the output voltage
and supplies load current directly during the conduction
phase of the power switch. Output ripple voltage consists
of the discharge of the output capacitor for I
FET On and the voltage drop due to flowing through the
ESR of the output capacitor. The ripple voltage can be
shown as:
ΔV
The conservation of charge principle in Equation 24 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 ESR and enough input ripple current
capability.
Output Ripple
ΔV
small ESR capacitors. In general, Ceramic capacitors are
pk
CO
Co
=
, can be reduced by increasing Co or f
=
(
V
I (
O
O
×
⁄
I
C
O
O
)
⁄
×
(
85%
D f
⁄
S
×
)
+
V
(
I
I (
)
+
O
26
1 2 V
×
⁄
ESR
[
I
)
×
(
V
O
–
V
SW
I
)
SW
⁄
. As a result,
L (
LPEAK
, or using
×
V
O
(EQ. 23)
(EQ. 24)
during
×
ISL97671
f
SW
SAT
)
]
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 ISL97671 can support up to 30mA.
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 CH0 to CH2, this configuration can be
treated as a single string with 90mA current driving
capability.
FIGURE 38. GROUPING MULTIPLE CHANNELS FOR
HIGH CURRENT APPLICATIONS
CH0
CH1
CH2
V
OUT
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June 24, 2010
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