ISL97653A Intersil Corporation, ISL97653A Datasheet - Page 13

• Manufacturer Part Number: ISL97653A
• Description: 5-Channel Integrated LCD Supply
• Manufacturer: Intersil Corporation

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and source of M0. In addition, a resistor can be connected
between PROT and the gate of M0, in order to limit the
maximum V
Buck Converter
The buck converter is a step down converter supplying
power to the logic circuit of the LCD system. The ISL97653A
integrates a high voltage N-channel MOSFET to save cost
and reduce external component count. In the continuous
current mode, the relationship between input voltage and
output voltage as expressed in Equation 8:
Where D is the duty cycle of the switching MOSFET.
Because D is always less than 1, the output voltage of a
buck converter is lower than input voltage.
The peak current limit of buck converter is set to 2.5A, which
restricts the maximum output current (average) based on
Equation 9:
Where ΔI
shown in Equation 10:
Where L is the buck inductor, f
(680kHz).
Feedback Resistors
The buck converter output voltage is determined by
Equation 11:
Where R
converter loop to set the output voltage Current drawn by
the resistor network should be limited to maintain the overall
converter efficiency. The maximum value of the resistor
network is limited by the feedback input bias current and the
potential for noise being coupled into the feedback pin. A
resistor network in the order of 1kΩ is recommended.
Buck Converter Input Capacitor
Input capacitance should support the maximum AC RMS
current which occurs at D = 0.5 and maximum output
current.
V
--------------------- -
I
ΔI
V
I
OMAX
acrms
LOGIC
LOGIC
V
pp
IN
=
(
C
=
V
--------------------- -
=
=
IN
13
LOGIC
L f
2.5A ΔI
P-P
⋅
R
-------------------------- -
)
D
GS
and R
s
14
=
R
is the ripple current in the buck inductor as
–
+
14
of M0 at all times.
⋅
D
(
R
1 D
14
P-P
13
⋅
(
–
1 D
×
are the feedback resistors in the buck
–
)
V
FBL
)
⋅
I
O
13
s
is the switching frequency
(EQ. 10)
(EQ. 12)
(EQ. 11)
(EQ. 8)
(EQ. 9)
ISL97653A
Where I
shows some recommendations for input capacitor.
Buck Inductor
A 3.3µH to 10µH inductor range is recommended for the
buck converter. Besides the inductance, the DC resistance
and the saturation current are also factors that need to be
considered when choosing a buck inductor. Low DC
resistance can help maintain high efficiency. Saturation
current rating should be higher than 2A. Here are some
recommendations for buck inductor.
Rectifier Diode (Buck Converter)
A Schottky diode is recommended for fast recovery and low
forward voltage. The reverse voltage rating should be higher
than the maximum input voltage. The peak current rating is
2.5A, and the average current is given by Equation 13:
Where I
following table shows some diode recommended.
I
avg
CAPACITOR
INDUCTOR
2.7A
PMEG2020EJ
TABLE 6. INPUT CAPACITOR (BUCK) RECOMMENDATION
10µF/16V
10µF/10V
22µF/16V
3A
TABLE 8. BUCK RECTIFIER DIODE RECOMMENDATION
4.7µH/
6.8µH/
=
PEAK
DIODE
SS22
PEAK
TABLE 7. BUCK INDUCTOR RECOMMENDATION
(
o
o
1 D
–
is the output current of the buck converter. Table 6
is the output current of buck converter. The
)*I
DIMENSIONS
o
5.7x5.0x4.7
7.3x6.8x3.2
SIZE
1206
0805
1210
(mm)
V
RATING
20V/2A
20V/2A
R
/I
TDK
Murata
Murata
AVG
VENDOR
Murata
TDK
VENDOR
PACKAGE
SOD323F
SMB
LQH55DN4R7M01K
RLF7030T-6R8M2R8
Philips
Semiconductors
Fairchild
Semiconductor
C3216X7R1C106M
GRM21BR61A106K
C3225X7R1C226M
PART NUMBER
PART NUMBER
VENDOR
February 21, 2008
(EQ. 13)
FN6367.1