ISL97673IRZ-TK Intersil, ISL97673IRZ-TK Datasheet - Page 25

• Manufacturer Part Number: ISL97673IRZ-TK
• Description: IC LED DVR PWM CTRL 6CH 20QFN
• Manufacturer: Intersil

Specifications of ISL97673IRZ-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: 4.5 V ~ 26.5 V
• Voltage - Output: *
• Mounting Type: Surface Mount
• Package / Case: 20-VFQFN Exposed Pad
• Operating Temperature: -40°C ~ 85°C
• Current - Output / Channel: 40mA
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

a current rating, saturation in the core will cause the
effective inductor value to fall, leading to an increase in
peak to average current level, poor efficiency and
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 19:
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 ISL97673 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 Equation 20:
The conservation of charge principle in Equation 20 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.
The choice of X7R over Y5V ceramic capacitor is highly
recommend because X7R capacitor is less sensitive to
capacitance change over voltage but the Y5V capacitor
exhibits very high capacitance coefficient such that its
absolute capacitance can be reduced to 10~20% to the
rated capacitance at maximum voltage.
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.
ΔV
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 (
+
25
O
1 2 V
×
⁄
ESR
[
I
×
)
(
V
O
–
V
SW
I
)
SW
⁄
. As a result,
L (
LPEAK
, or using
×
V
O
(EQ. 19)
(EQ. 20)
during
×
f
SW
SAT
ISL97673
)
]
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 one to two 4.7µF/50V ceramic capacitors are
suitable for netbook to 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 ISL97673 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 37. GROUPING MULTIPLE CHANNELS FOR
SMBDAT/SDA
SMBCLK/SCL
FIGURE 38. MULTIPLE DRIVERS OPERATION
EN
HIGH CURRENT APPLICATIONS
CH0
CH1
CH2
SMBCLK
SMBDAT
EN/PWM
V
OUT
SMBCLK
SMBDAT
EN/PWM
June 24, 2010
FN7633.0