ISL97650 Intersil Corporation, ISL97650 Datasheet - Page 14

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

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 the
following equation:
Where I
following table shows some diode recommended.
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.
PI Loop Compensation (Buck Converter)
The buck converter of ISL97650 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 the section on "PI Loop Compensation (Boost
Converter)”.
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 ISL97650. 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 V
device may be unable to turn-on until V
there is a minimum load requirement in this case. The
I
avg
CAPACITOR
TABLE 9. BUCK OUTPUT CAPACITOR RECOMMENDATION
PMEG2020EJ
100µF/6.3V
10µF/6.3V
10µF/6.3V
22µF/6.3V
TABLE 8. BUCK RECTIFIER DIODE RECOMMENDATION
=
DIODE
SS22
(
1 D
o
–
is the output current of buck converter. The
)*I
IN
o
-V
BUCK
V
RATING
SIZE
0805
0805
1210
1206
20V/2A
20V/2A
R
/I
AVG
< 1.5V, the internal MOSFET pull-up
TDK
Murata
TDK
Murata
14
VENDOR
PACKAGE
SOD323F
SMB
LOGIC
C2012X5R0J106M
GRM21BR60J106K
C3216X5R0J226M
GRM31CR60J107M
Philips
Semiconductors
Fairchild
Semiconductor
PART NUMBER
VENDOR
falls. Hence,
(EQ. 15)
ISL97650
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 boot strap capacitor, the MOSFET can’t turn
on. Hence there is minimum load requirement to charge the
bootstrap capacitor properly.
Charge Pump Controllers (V
The ISL97650 includes 2 independent charge pumps (see
charge pump block and connection diagram). The negative
charge pump inverters the V
regulated negative output voltage. The positive charge pump
doubles or triples the V
positive output voltage. The regulation of both the negative
and positive charge pumps is generated by 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 V
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 (V
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 V
P
pump is enabled. This is important for TFT applications
where the negative charge pump output voltage (V
A
The maximum P
from the following equations assuming a 50% switching
duty:
Note: V
function of I
I
2 V
---------------------------------------------------------------------------------------------------------------------- 0.95A
I
3 V
---------------------------------------------------------------------------------------------------------------------- 0.95V
MAX
MAX
VDD
OUT
•
•
SUP
SUP
(
(
2x
3x
supplies need to be established before P
is independent of the voltage on V
(
DIODE
)
)
2
SUP
(
–
–
∼
∼
2
•
2
3
min of 50mA or
min of 50mA or
•
(
MAX
3 R
•
•
(
doubler) and x3 (V
2 R
V
V
•
(2 • I
DIODE
•
DIODE
OUT
ONH
and V
ONH
OFF
MAX
charge pump current can be estimated
(
(
+
2 I
2 I
SUP
DIODE
+
2 R
) is the on-chip diode voltage as a
•
and 20mA for V
•
R
•
MAX
ONL
MAX
voltage and provided a regulated
ONL
SUP
(40mA) < 0.7V.
) V V
)
) V V
)
–
–
)
SUP
SUP
)
voltage and provides a
ON
(
(
ON
ON
Tripler) modes of
(M1, M2 and M3),
and V
)
)
ON
SUP
•
•
.
OFF
until the charge
November 28, 2006
OUT
)
OFF
.
(EQ. 16)
FN9198.3
) and