14
FN9198.4
April 17, 2009
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:
Where I
o
is the output current of buck converter. Table 8
shows some recommended diodes.
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 “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 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
IN
-V
BUCK
< 1.5V, the internal MOSFET pull-up
device may be unable to turn-on until V
LOGIC
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 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
ON
and V
OFF
)
The ISL97650 includes 2 independent charge pumps (see
charge pump block and connection diagram, Figure 17). The
negative charge pump inverts the V
SUP
voltage and
provides a regulated negative output voltage. The positive
charge pump doubles or triples the V
SUP
voltage and
provides a regulated 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
OFF
and 20mA for V
ON
.
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
SUP
doubler) and x3 (V
SUP
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 V
SUP
(M1, M2 and M3),
P
OUT
is independent of the voltage on V
SUP
until the charge
pump is enabled. This is important for TFT applications
where the negative charge pump output voltage (V
OFF
) and
A
VDD
supplies need to be established before P
OUT
.
The maximum P
OUT
charge pump current can be estimated
from the following equations assuming a 50% switching
duty:
Note: V
DIODE
(2 • I
MAX
) is the on-chip diode voltage as a
function of I
MAX
and V
DIODE
(40mA) < 0.7V.
TABLE 8. BUCK RECTIFIER DIODE RECOMMENDATION
DIODE
V
R
/I
AVG
RATING PACKAGE VENDOR
PMEG2020EJ 20V/2A SOD323F Philips
Semiconductors
SS22 20V/2A SMB Fairchild
Semiconductor
TABLE 9. BUCK OUTPUT CAPACITOR RECOMMENDATION
CAPACITOR SIZE VENDOR PART NUMBER
10µF/6.3V 0805 TDK C2012X5R0J106M
10µF/6.3V 0805 Murata GRM21BR60J106K
22µF/6.3V 1210 TDK C3216X5R0J226M
100µF/6.3V 1206 Murata GRM31CR60J107M
I
avg
1D–*I
o
=
(EQ. 15)
I
MAX
2xmin of 50mA or
2V
SUP
2– V
DIODE
2I
MAX
VV
ON
–
22r
ONH
r
ONL
+
----------------------------------------------------------------------------------------------------------------------
0.95A
I
MAX
3xmin of 50mA or
3V
SUP
3– V
DIODE
2I
MAX
VV
ON
–
23r
ONH
2r
ONL
+
----------------------------------------------------------------------------------------------------------------------
0.95V
(EQ. 16)
ISL97650
