ISL97675IRZ-TK Intersil, ISL97675IRZ-TK Datasheet - Page 18

• Manufacturer Part Number: ISL97675IRZ-TK
• Description: IC LED DVR PWM CTRL 4CH 20QFN
• Manufacturer: Intersil
• Type: Backlight, White LEDr

Specifications of ISL97675IRZ-TK

• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 4
• Internal Driver: Yes
• Type - Primary: Backlight
• Frequency: 600kHz, 1.2MHz
• Voltage - Supply: 4.5 V ~ 26 V
• Voltage - Output: 45V
• Mounting Type: Surface Mount
• Package / Case: 20-VQFN
• Operating Temperature: -40°C ~ 85°C
• Current - Output / Channel: 30mA
• Internal Switch(s): Yes
• Efficiency: 92%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: ISL97675IRZ-TKTR

Inductor
The selection of the inductor should be based on its
maximum current (I
dissipation (DCR), EMI susceptibility (shielded vs
unshielded), and size. Inductor type and value influence
many key parameters, including ripple current, current
limit, efficiency, transient performance and stability.
The inductor’s maximum current capability must be large
enough to handle the peak current at the worst case
condition. If an inductor core is chosen with a lower
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 17:
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, minimum input voltage
must be used to calculate the input/inductor current as
shown in Equation 17. Fora given inductor size, the larger
the inductance value, the higher the series resistance
because of the extra number of turns required, thus,
higher conductive losses. The ISL97675 current limit
should be less than the inductor saturation current.
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 during the FET
ton period and the voltage drop due to load current
flowing through the ESR of the output capacitor. The
ripple voltage is shown in Equation 18:
Equation 18 shows the importance of using a low ESR
output capacitor for minimizing output ripple.
The choice of X7R over Y5V ceramic capacitors is highly
recommended because the former capacitor is less
sensitive to capacitance change over voltage as shown in
Figure 25. Y5V’s absolute capacitance can be reduced to
10%~20% of its rated capacitance at the maximum
voltage. In any case, Y5V type of ceramic capacitor
should be avoided.
IL
ΔV
peak
CO
=
=
(
V
I (
O
O
×
⁄
C
I
O
O
)
×
⁄
(
D f
85%
⁄
S
SAT
)
×
+
V
) characteristics, power
(
I
I (
)
18
O
+
×
1 2 V
ESR
⁄
[
I
)
×
(
V
O
–
SW
V
I
)
. As a result,
⁄
L (
×
V
(EQ. 18)
(EQ. 17)
O
×
ISL97675
f
SW
)
]
Here are some recommendations for various
applications:
For 20mA applications with V
type) is sufficient.
For 20mA applications with V
type) is required in some configurations.
FIGURE 25. X7R AND V5Y TYPES CERAMIC
Channel Capacitor
It is recommended to use at least 1.5nF capacitors from
CH pins to
ripple at boost frequency, but will degrade transient
performance at high PWM frequencies. The best value is
dependant on PCB layout. Up to 4.7nF is sufficient for
most configurations.
Output Ripple
ΔV
small ESR capacitors as shown in Equation 18. 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 as much on the next
on period which minimizes transient current. The output
capacitor is also needed for compensation, and, in
general one to two 4.7µF/50V ceramic capacitors are
needed for netbook or 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, therefore, a suitable
current rated Schottky diode must be used.
Co
3.0
2.5
2.0
1.5
1.0
0.5
, can be reduced by increasing Co or f
0
0
V
5
OUT
CAPACITORS
. Larger capacitors will reduce LED current
10
POLY. (CERAMIC Y5V 2.2µF 50V CAP)
POLY. (CERAMIC X7R 2.2µF 50V CAP)
APPLIED VOLTAGE (V)
15
20
IN
IN
> 7V, 1 x 4.7µF (X7R
< 7V, 2 x 4.7µF (X7R
25
30
SW
35
, or using
May 19, 2010
40
FN7630.0
45