ISL97646 Просмотр технического описания (PDF) - Intersil
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ISL97646 Datasheet PDF : 15 Pages
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ISL97646
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 used to match the internal
slope compensation. The inductor must be able to handle
the following average and peak current:
ILAVG
=
----I--O-------
1–D
ILPK
=
IL
AVG
+
-Δ----I--L-
2
(EQ. 5)
Some inductors are recommended in Table 3.
TABLE 3. BOOST INDUCTOR RECOMMENDATION
DIMENSIONS
INDUCTOR
(mm)
MFG
PART NUMBER
6.8µH/3APEAK 7.3x6.8x3.2
10µH/4APEAK 8.3x8.3x4.5
5.2µH/4.55APEAK 10x10.1x3.8
TDK
RLF7030T-6R8N3R0
Sumida CDR8D43-100NC
Cooper CD1-5R2
Bussmann
Rectifier Diode
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. Table 4 lists some
recommendations for boost converter diode.
TABLE 4. BOOST CONVERTER RECTIFIER DIODE
RECOMMENDATION
DIODE
SS23
VR/IAVG RATING
30V/2A
PACKAGE
MFG
SMB
Fairchild
Semiconductor
MBRS340
40V/3A
SMC
International
Rectifier
SL23
30V/2A
SMB
Vishay
Semiconductor
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.
VRIPPLE
=
ILP
K
×
E
S
R
+
V-----O-----–-----V----I--N--
VO
×
------I--O-------
COUT
×
-1--
fs
(EQ. 6)
For low ESR ceramic capacitors, the output ripple is
dominated by the charging and discharging of the output
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.
COUT in Equation 6 above assumes the effective value of the
capacitor at a particular voltage and not the manufacturer’s stated
value, measured at zero volts.
Table 5 lists some selections of output capacitors.
TABLE 5. BOOST OUTPUT CAPACITOR RECOMMENDATION
CAPACITOR SIZE MFG
PART NUMBER
10µF/25V
1210 TDK
C3225X7R1E106M
10µF/25V
1210 Murata GRM32DR61E106K
Compensation
The boost converter of ISL97646 can be compensated by a
RC network connected from the CM1 pin to ground. 4.7nF
and 10k RC network is used in the demo board. The larger
value resistor and lower value capacitor can lower the
transient overshoot, however, at the expense of stability of
the loop.
Cascaded MOSFET Application
A 20V N-Channel MOSFET is integrated in the boost
regulator. For the applications where the output voltage is
greater than 20V, an external cascaded MOSFET is needed,
as shown in Figure 15. The voltage rating of the external
MOSFET should be greater than AVDD.
VIN
AVDD
INTERSIL
ISL97646
LX
FB
FIGURE 15. CASCADED MOSFET TOPOLOGY FOR HIGH
OUTPUT VOLTAGE APPLICATIONS
Linear-Regulator (LDO)
The ISL97646 includes a LDO with adjustable output, and it
can supply current up to 350mA. The output voltage is
adjusted by connection of ADJ pin. When ADJ pin is
connected to ground, the output voltage is set to 3.3V; when
ADJ pin is floating, the output voltage is set to 2.85V, and
when ADJ pin is connected to LDO_OUT pin, the output
voltage is set to 2.5V.
11
FN6265.1
December 14, 2007
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