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ISL97651 Datasheet PDF : 19 Pages
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ISL97651
Some inductors are recommended in Table 3.
TABLE 3. BOOST INDUCTOR RECOMMENDATION
DIMENSIONS
INDUCTOR
(mm)
VENDOR
PART NUMBER
6.8µH/ 12.95x9.4x5.21 Coilcraft DO3316P-682ML
4.6APEAK
10µH/
5.5APEAK
10x10x5 Sumida CDR10D48MNNP-100NC
5.2µH/ 10x10.1x3.8 Cooper CD1-5R2
4.55APEAK
Bussmann
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.
Table 4 shows some recommendations for boost converter
diode.
TABLE 4. BOOST CONVERTER RECTIFIER DIODE
RECOMMENDATION
DIODE
VR/IAVG
RATING
PACKAGE
VENDOR
SS23
30V/2A
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
=
IL
P
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 assumes the effective value
of the capacitor at a particular voltage and not the
manufacturer’s stated value, measured at 0V.
Table 5 shows some selections of output capacitors.
TABLE 5. BOOST OUTPUT CAPACITOR RECOMMENDATION
CAPACITOR SIZE VENDOR
PART NUMBER
10µF/25V
1210 TDK
C3225X7R1E106M
10µF/25V
1210 Murata
GRM32DR61E106K
PI Loop Compensation (Boost Converter)
The boost converter of ISL97651 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 AVDD voltage should be
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 AVDD voltage is set to less
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
COUT = 10µF and fp = 4kHz for COUT = 30µF. Alternatively,
adding a small capacitor (20pF to 100pF) in parallel with R5
(i.e. R17 = short) may help to reduce AVDD noise and
improve regulation, particularly if high value feedback
resistors are used.
R 17
=
⎛⎛
⎝⎝
--0---.--1----1-×----R-----5---⎠⎞
– R----1-3----⎠⎞ –1
(EQ. 7)
C18 = (---2-----×-----3---.--1---4---2---1--×-----f--p-----×-----R----5-----)
(EQ. 8)
Cascaded MOSFET Application
An 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 12. The voltage rating of the external
MOSFET should be greater than AVDD.
12
FN7493.3
April 24, 2009
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