12
FN7415.2
February 22, 2006
The following table 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
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 above assumes the effective value of the
capacitor at a particular voltage and not the manufacturer’s stated
value, measured at zero volts.
Compensation
The EL7640, EL7641, EL7642 can operate in either P mode
or PI mode. Connecting COMP pin directly to V
IN
will enable
P mode; For better load regulation, use PI mode with a
2.2nF capacitor and a 180 resistor 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 R2 decreases relative to R1. 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 (R7 and C7) in parallel with R1,
with a pole frequency (fp) set to approximately 10kHz. for C2
effective = 10µF and 4kHz for C2 (effective) = 30µF.
R7 = ((1/0.1 x R2) – 1/R1)^-1
C7 = 1/(2 x 3.142 x fp x R7)
Linear-Regulator Controllers (V
ON
and V
OFF
)
The EL7640, EL7641, EL7642 include 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 Figure 19
and Figure 20 respectively.
TABLE 2.
V
IN
(V) V
O
(V) L (µH) f
S
(MHz) I
OMAX
(mA)
3.3 9 6.8 1.2 898
3.3 12 6.8 1.2 622
3.3 15 6.8 1.2 458
5 9 6.8 1.2 1360
5126.81.2 944
5156.81.2 694
I
LAVG
I
O
1D–
-------------
=
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
-----
+=
EL7640, EL7641, EL7642
