ISL97650 Intersil Corporation, ISL97650 Datasheet - Page 12

• Manufacturer Part Number: ISL97650
• Description: 4-Channel Integrated LCD Supply
• Manufacturer: Intersil Corporation

Boost Inductor
The boost inductor is a critical part which influences the
output voltage ripple, transient response, and efficiency.
Values of 3.3µH to 10µH are to match the internal slope
compensation. The inductor must be able to handle the
following average and peak current:
Some inductors are recommended in Table 3.
Rectifier Diode (Boost Converter)
A high-speed diode is necessary due to the high switching
frequency. Schottky diodes are recommended because of
their fast recovery time and low forward voltage. The reverse
voltage rating of this diode should be higher than the
maximum output voltage. The rectifier diode must meet the
output current and peak inductor current requirements. The
following table is some recommendations for boost converter
diode.
Output Capacitor
The output capacitor supplies the load directly and reduces
the ripple voltage at the output. Output ripple voltage
consists of two components: the voltage drop due to the
inductor ripple current flowing through the ESR of output
capacitor, and the charging and discharging of the output
capacitor.
For low ESR ceramic capacitors, the output ripple is
dominated by the charging and discharging of the output
I
I
V
LAVG
LPK
INDUCTOR
RIPPLE
4.55A
2.9A
DIODE
TABLE 4. BOOST CONVERTER RECTIFIER DIODE
3A
SS23
SL23
6.8µH/
6.8µH/
5.2µH/
=
PEAK
PEAK
TABLE 3. BOOST INDUCTOR RECOMMENDATION
=
PEAK
I
LAVG
------------ -
1 D
=
I
–
O
I
LPK
RECOMMENDATION
V
RATING
+
DIMENSIONS
30V/2A
30V/2A
7.6X7.6X3.0 Sumida
10x10.1x3.8 Cooper
R
7.3x6.8x3.2
ΔI
--------
/I
2
×
AVG
L
(mm)
ESR
+
V
----------------------- -
PACKAGE
O
V
TDK
Bussmann
SMB
SMB
–
VENDOR
O
12
V
IN
×
--------------- -
C
OUT
Fairchild Semiconductor
Vishay Semiconductor
I
O
RLF7030T-6R8N3R0
CDR7D28MNNP-6R8NC
CD1-5R2
×
PART NUMBER
--- -
f
1
s
VENDOR
(EQ. 5)
(EQ. 6)
(EQ. 7)
ISL97650
capacitor. The voltage rating of the output capacitor should
be greater than the maximum output voltage.
Note: Capacitors have a voltage coefficient that makes their
effective capacitance drop as the voltage across then
increases. C
effective value of the capacitor at a particular voltage and not
the manufacturer's stated value, measured at zero volts.
The following table shows some selections of output
capacitors.
PI Loop Compensation (Boost Converter)
The boost converter of ISL97650 can be compensated by a
RC network connected from CM1 pin to ground. C3 = 4.7nF
and R1 = 10k RC network is used in the demo board. A
higher resistor value can be used to lower the transient
overshoot - however, this may be at the expense of stability
to the loop.
The stability can be examined by repeatedly changing the
load between 100mA and a max level that is likely to be
used in the system being used. The A
examined with an oscilloscope set to AC 100mV/div and the
amount of ringing observed when the load current changes.
Reduce excessive ringing by reducing the value of the
resistor in series with the CM1 pin capacitor.
Boost Converter Feedback Resistors and
Capacitor
An RC network across feedback resistor R5 may be required
to optimize boost stability when A
than 12V. This network reduces the internal voltage
feedback used by the IC. This RC network sets a pole in the
control loop. This pole is set to approximately fp = 10kHz for
C
adding a small capacitor (20-100pF) in parallel with R5 (i.e.
R16 = short) may help to reduce A
regulation, particularly if high value feedback resistors are
used.
R16
C18
CAPACITOR
TABLE 5. BOOST OUTPUT CAPACITOR RECOMMENDATION
OUT
10µF/25V
10µF/25V
=
=
= 10µF and fp = 4kHz for C
⎛
⎝
------------------------------------------------------ -
(
2
⎛
⎝
------------------------- -
0.1 R5
×
3.142
OUT
1
×
SIZE
1210
1210
1
×
in the equation above assumes the
⎞
⎠
fp
–
×
------- -
R3
1
R5
TDK
Murata
⎞
⎠
)
–
VENDOR
1
VDD
OUT
VDD
VDD
voltage is set to less
= 30µF. Alternatively,
noise and improve
C3225X7R1E106M
GRM32DR61E106K
voltage should be
PART NUMBER
November 28, 2006
FN9198.3
(EQ. 8)
(EQ. 9)