14
FN7493.3
April 24, 2009
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 ISL97651 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 ISL97651. 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.
Linear-Regulator Controllers (V
ON
and V
OFF
)
The ISL97651 include 2 independent charge pumps (see
Figure 13). The negative charge pump inverters 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
.
The positive charge pump can generate double or triple
V
SUP
voltage depending on the configuration of C2+ and
C2- pins. If the C2+ pin connects to C1+, it is the voltage
doubler, and if C2+ connects C2- via a capacitor, it
configured a voltage tripler.
Positive Charge Pump Design Consideration
The positive charge pump integrates all the diodes (D1, D2
and D3 shown in the block diagram in Figure 13) 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 Equation 15 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.
In voltage doubler configuration, the maximum V
ON
is as
given by Equation 16:
For Voltage Tripler:
V
ON
output voltage is determined by Equation 18:
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
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. 15)
V
ON_MAX(2x)
2V
SUP
V
DIODE
– 2I
OUT
2r
ONH
r
ONL
+–=
(EQ. 16)
V
ON_MAX(3x)
3V
SUP
V
DIODE
– 2I
OUT
3r
ONH
2r
ONL
+–=
(EQ. 17)
V
ON
V
FBP
1
R
8
R
9
-------
+
=
(EQ. 18)
ISL97651
