EL7640, EL7641, EL7642
The following table gives typical values (margins are
considered 10%, 3%, 20%, 10% and 15% on VIN, VO, L, f
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.
S
and I
:
LMT
TABLE 2.
L (µH)
NOTE: Capacitors have a voltage coefficient that makes their
effective capacitance drop as the voltage across them increases.
V
(V)
V
(V)
f
(MHz)
1.2
I
(mA)
IN
O
S
OMAX
898
3.3
3.3
3.3
5
9
6.8
6.8
6.8
6.8
6.8
6.8
C
in the equation above assumes the effective value of the
OUT
capacitor at a particular voltage and not the manufacturer’s stated
value, measured at zero volts.
12
15
9
1.2
622
458
1360
944
694
1.2
Compensation
1.2
The EL7640, EL7641, EL7642 can operate in either P mode
or PI mode. Connecting COMP pin directly to V will enable
5
12
15
1.2
IN
P mode; For better load regulation, use PI mode with a
2.2nF capacitor and a 180Ω 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.
5
1.2
Input Capacitor
The input capacitor is used to supply the current to the
converter. It is recommended that C be larger than 10µF.
IN
The reflected ripple voltage will be smaller with larger C
The voltage rating of input capacitor should be larger than
maximum input voltage.
.
IN
Boost Feedback Resistors
As the boost output voltage, V
, is reduced below 12V
BOOST
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
Boost Inductor
The boost inductor is a critical part which influences the
output voltage ripple, transient response, and efficiency.
Value of 3.3µH to 10µH inductor is recommended in
applications to fit the internal slope compensation. The
inductor must be able to handle the following average and
peak current:
proportionally, as V
is reduced, by means of a series
BOOST
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.
I
O
-------------
I
=
LAVG
1 – D
R7 = ((1/0.1 x R2) – 1/R1)^-1
C7 = 1/(2 x 3.142 x fp x R7)
∆I
L
--------
+
2
I
= I
LPK
LAVG
Linear-Regulator Controllers (V
and V )
OFF
ON
Rectifier Diode
The EL7640, EL7641, EL7642 include 2 independent
linear-regulator controllers, in which there is one positive
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 voltage (V ), and one negative voltage (V
). The
ON
OFF
V
and V linear-regulator controller function diagram,
OFF
ON
application circuit and waveforms are shown in Figure 19
and Figure 20 respectively.
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.
V
– V
I
O
1
O
IN
----------------------- --------------- ----
V
= I
× ESR +
LPK
×
×
RIPPLE
V
C
f
O
OUT
S
FN7415.1
September 26, 2005
12
INTERSIL [ Intersil ]
