isl97652 Intersil Corporation, isl97652 Datasheet - Page 14

• Manufacturer Part Number: isl97652
• Description: 4-channel Integrated Lcd Supply With Dual Vcom Amplifiers
• Manufacturer: Intersil Corporation

chip. Therefore, three separate fault mechanisms are
operated.
Buck Converter
The buck converter is the step down converter, which
supplies the current to the logic circuit of the LCD system.
The ISL97652 integrates an 20V N-Channel MOSFET to
save cost and reduce external component count. In the
continuous current mode, the relationship between input
voltage and output voltage is as follows:
Where D is the duty cycle of the switching MOSFET.
Because D is always less than 1, the output voltage of 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 the
following equation:
Where ΔI
following equation:
Where L is the buck inductor, f
V
--------------------- -
I
ΔI
OMAX
1. The SWO output range is constantly monitored and
2. As well as monitoring any rise in the voltage at SWO, the
3. Once the ISL97652 has successfully sequenced the
LOGIC
V
PP
IN
expected to rise if the PFET is in current limit. The rate of
rise at SWO can be calculated from the current limit and
the capacitance on SWO by using the equation
dV/dt = Ilimit/Cavdd. The SWO voltage range is split into
sections of approximately 0.7V such that every time the
output rises by this amount the circuit detects that the
voltage is rising. Should the circuit remain in current limit
for more than 100µs with no such rise taking place the
circuit will fault out. In this scenario, the PFET will
immediately switch itself off and the rest of the ISL97652
will later fault out due to the boost voltage at A
away.
circuit also monitors any falls in this level. If the output
falls by more than a certain amount while it is in current
limit the circuit will fault out immediately. This amount
varies from about 1V to about 1.4V depending on the
output level before the fall. In this scenario, the PFET will
immediately switch itself off and the rest of the ISL97652
will later fault out due to the boost voltage falling away.
boost on and the boost soft-start capacitor has charged
up, a third fault check is also added. After this point if the
PFET enters current limit for greater than the global
timeout of 40µs then the chip will fault out. In this scenario
the whole chip will be disabled with the PFET
immediately switched off.
=
=
V
--------------------- -
=
LOGIC
2.5A ΔI
L f
PP
D
⋅
s
is the ripple current in the buck inductor as the
–
⋅
(
1 D
PP
–
)
14
s
is the switching frequency.
VDD
(EQ. 10)
falling
(EQ. 8)
(EQ. 9)
ISL97652
Feedback Resistors
The buck converter output voltage is determined by the
following equation:
Where R11 and R12 are the feedback resistors of buck
converter 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
The capacitor should support the maximum AC RMS current
which happens when D = 0.5 and maximum output current.
Where I
following table shows some recommendations for input
capacitor.
Buck Inductor
A 3.3µH to 10µH inductor is the good choice for the buck
converter. Besides the inductance, the DC resistance and
the saturation current are also the factor needed to be
considered when choosing buck inductor. Low DC
resistance can help maintain high efficiency, and the
saturation current rating should be 2.5A. Here are some
recommendations for buck inductor.
Rectifier Diode (Buck Converter)
A Schottky diode is recommended due to fast recovery and low
forward voltage. The reverse voltage rating should be higher
V
I
ACRMS
INDUCTOR
LOGIC
CAPACITOR
2.7A
2.4A
TABLE 6. INPUT CAPACITOR (BUCK) RECOMMENDATION
3A
10µF/16V
10µF/10V
22µF/16V
4.7µH/
6.8µH/
10µH/
PEAK
PEAK
PEAK
TABLE 7. BUCK INDUCTOR RECOMMENDATION
(
o
=
C
is the output current of the buck converter. The
IN
R
-------------------------- -
11
)
R
=
DIMENSIONS
12.95x9.4x3.0 Coilcraft
+
12
5.7x5.0x4.7
7.3x6.8x3.2
R
D
(mm)
12
SIZE
1206
0805
1210
⋅
×
(
1 D
V
–
FBB
TDK
Murata
Murata
)
VENDOR
⋅
I
Murata
TDK
O
VENDOR
LQH55DN4R7M01K
RLF7030T-6R8M2R8
DO3308P-103
GRM21BR61A106K
C3216X7R1C106M
C3225X7R1C226M
PART NUMBER
PART NUMBER
November 2, 2007
(EQ. 12)
(EQ. 11)
FN9287.1