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C3225X7R1E106M Datasheet PDF : 20 Pages
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ISL97650B
Rectifier Diode (Buck Converter)
A Schottky diode is recommended due to fast recovery and
low forward voltage. The reverse voltage rating should be
higher than the maximum input voltage. The peak current
rating is 2A, and the average current should be as shown in
Equation 15:
Iavg = (1 – D)*Io
(EQ. 15)
Where IO is the output current of buck converter. Table 8
shows some diode recommended.
TABLE 8. BUCK RECTIFIER DIODE RECOMMENDATION
DIODE
VR/IAVG
RATING PACKAGE
VENDOR
PMEG2020EJ
20V/2A
SOD323F Philips
Semiconductors
SS22
20V/2A
SMB
Fairchild
Semiconductor
Output Capacitor (Buck Converter)
Four 10µF or two 22µF ceramic capacitors are recommended
for this part. The overshoot and undershoot will be reduced
with more capacitance, but the recovery time will be longer.
TABLE 9. BUCK OUTPUT CAPACITOR RECOMMENDATION
CAPACITOR
(µF/V)
SIZE
VENDOR PART NUMBER
10/6.3
0805 TDK
C2012X5R0J106M
10/6.3
0805 Murata GRM21BR60J106K
22/6.3
1210 TDK
C3216X5R0J226M
100/6.3
1206 Murata GRM31CR60J107M
PI Loop Compensation (Buck Converter)
The buck converter of ISL97650B can be compensated by a
RC network connected from CM2 pin to ground. C9 = 4.7nF
and R2 = 2k RC network is used in the demo board. The
larger value resistor can lower the transient overshoot,
however, at the expense of stability of the loop.
The stability can be optimized in a similar manner to that
described in “PI Loop Compensation (Boost Converter)” on
page 12.
Bootstrap Capacitor (C16)
This capacitor is used to provide the supply to the high driver
circuitry for the buck MOSFET. The bootstrap supply is
formed by an internal diode and capacitor combination. A
1µF is recommended for ISL97650B. A low value capacitor
can lead to overcharging and in turn damage the part.
If the load is too light, the on-time of the low side diode may
be insufficient to replenish the bootstrap capacitor voltage. In
this case, if VIN - VBUCK < 1.5V, the internal MOSFET
pull-up device may be unable to turn-on until VLOGIC falls.
Hence, there is a minimum load requirement in this case.
The minimum load can be adjusted by the feedback
resistors to FBL.
The bootstrap capacitor can only be charged when the
higher side MOSFET is off. If the load is too light which can
not make the on-time of the low side diode be sufficient to
replenish the bootstrap capacitor, the MOSFET can’t turn on.
Hence there is minimum load requirement to charge the
bootstrap capacitor properly.
Charge Pump Controllers (VON and VOFF)
The ISL97650B includes 2 independent charge pumps (see
charge pump block and connection diagram, Figure 17). The
negative charge pump inverters the VSUP voltage and
provides a regulated negative output voltage. The positive
charge pump doubles or triples the VSUP voltage and
provided a regulated positive output voltage. The regulation
of both the negative and positive charge pumps is generated
by the internal comparator that senses the output voltage
and compares it with the internal reference.
The pumps use pulse width modulation to adjust the pump
period, depending on the load present. The pumps can
provide 30mA for VOFF and 20mA for VON.
Positive Charge Pump Design Consideration
The positive charge pump integrates all the diodes (D1, D2
and D3 shown in the “Block Diagram” on page 9) required
for x2 (VSUP doubler) and x3 (VSUP Tripler) modes of
operation. During the chip start-up sequence the mode of
operation is automatically detected when the charge pump is
enabled. With both C7 and C8 present, the x3 mode of
operation is detected. With C7 present, C8 open and with
C1+ shorted to C2+, the x2 mode of operation will be
detected.
Due to the internal switches to VSUP (M1, M2 and M3),
POUT is independent of the voltage on VSUP until the charge
pump is enabled. This is important for TFT applications
where the negative charge pump output voltage (VOFF) and
AVDD supplies need to be established before POUT.
The maximum POUT charge pump current can be estimated
from Equations 16 and 17 assuming a 50% switching duty:
IMAX(2x) ∼ min of 50mA or
2-----•-----V----S----U----P----(-–--2--2--•---•--(--2-V----D•----IR--O---O-D---N--E---H-(--2--+---•--R--I--MO-----AN----XL---)-)--)-–-----V----(--V-----O----N-----) • 0.95A
(EQ. 16)
IMAX(3x) ∼ min of 50mA or
3-----•-----V----S----U----P-----–----3-----•-----V----D----I--O----D-----E---(---2-----•----I--M-----A----X----)---–-----V----(--V-----O----N----)-
(2 • (3 • RONH + 2 • RONL))
•
0.95
A
(EQ. 17)
Note: VDIODE (2 • IMAX) is the on-chip diode voltage as a
function of IMAX and VDIODE (40mA) < 0.7V.
14
FN6748.1
March 22, 2010
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