12
FN6436.0
June 18, 2007
Table 2 gives typical values (margins are considered 10%,
3%, 20%, 10% and 15% on V
IN
, V
O
, L, f
S
and I
LMT
:
Input Capacitor
The input capacitor is used to supply the current to the
converter. It is recommended that C
IN
be larger than 10F.
The reflected ripple voltage will be smaller with larger C
IN
.
The voltage rating of input capacitor should be larger than
the maximum input voltage.
Boost Inductor
The boost inductor is a critical part which influences the
output voltage ripple, transient response, and efficiency.
Value of 3.3H to 10H inductor is recommended in
applications to fit the internal slope compensation. The
inductor must be able to handle the following average and
peak current:
Rectifier Diode
A high-speed diode is desired due to the high switching
frequency. Schottky diodes are recommended because of
their fast recovery time and low forward voltage. The rectifier
diode must meet the output current and peak inductor
current requirements.
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.
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 them increases.
C
OUT
in the Equation 6 assumes the effective value of the capacitor
at a particular voltage and not the manufacturer’s stated value,
measured at zero volts.
Compensation
The ISL97642 incorporates a transconductance amplifier in
its feedback path to allow the user some adjustment on the
transient response and better regulation. The ISL97642
uses current mode control architecture, which has a fast
current sense loop and a slow voltage feedback loop. The
fast current feedback loop does not require any
compensation. The slow voltage loop must be compensated
for stable operation. The compensation network is a series
RC network from COMP pin to ground. The resistor sets the
high frequency integrator gain for fast transient response
and the capacitor sets the integrator zero to ensure loop
stability. For most applications, a 2.2nF capacitor and a
180 resistor are inserted in series between COMP pin and
ground. To improve the transient response, either the
resistor value can be increased or the capacitor value can be
reduced, but too high resistor value or too low capacitor
value will reduce loop stability.
Boost Feedback Resistors
As the boost output voltage, V
BOOST
, is reduced below 12V,
the effective voltage feedback in the IC increases the ratio of
voltage to current feedback at the summing comparator
because R
2
decreases relative to R
1
. To maintain stable
operation over the complete current range of the IC, the
voltage feedback to the FBB pin should be reduced
proportionally (as V
BOOST
is reduced) by means of a series
resistor-capacitor network (R
7
and C
7
) in parallel with R
1
,
with a pole frequency (fp) set to approximately 10kHz for C
2
(effective) = 10µF and 4kHz for C
2
(effective) = 30µF.
Linear-Regulator Controllers (V
ON
and V
OFF
)
The ISL97642 includes 2 independent linear-regulator
controllers, in which there is one positive output voltage
(V
ON
) and one negative voltage (V
OFF
). The V
ON
and V
OFF
linear-regulator controller function diagram, application
circuit and waveforms are shown in Figures 18 and 19
respectively.
TABLE 2.
V
IN
(V) V
O
(V) L (µH) f
S
(MHz) I
OMAX
(mA)
3.3 9 6.8 1.2 890
3.3 12 6.8 1.2 666
3.3 15 6.8 1.2 530
5 9 6.8 1.2 1350
5126.81.2 1000
5156.81.2 795
I
LAVG
I
O
1D–
-------------
=
(EQ. 5)
I
LPK
I
LAVG
I
L
2
--------
+=
V
RIPPLE
I
LPK
ESR
V
O
V
IN
–
V
O
------------------------
I
O
C
OUT
----------------
1
f
S
-----
+=
(EQ. 6)
R
7
10.1 R
2
1R
1
^-1–=
(EQ. 7)
C
7
1 2 3.142 fp R
7
=
(EQ. 8)
ISL97642
