el7585il-t7 Intersil Corporation, el7585il-t7 Datasheet - Page 10

• Manufacturer Part Number: el7585il-t7
• Description: Tft-lcd Power Supply
• Manufacturer: Intersil Corporation

The following table gives typical values (margins are
considered 10%, 3%, 20%, 10%, and 15% on V
and I
Input Capacitor
An input capacitor is used to supply the peak charging
current to the converter. It is recommended that C
larger than 10µF. The reflected ripple voltage will be smaller
with larger C
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.
Values of 3.3µH to 10µH are to match the internal slope
compensation. The inductor must be able to handle the
following average and peak current:
I
I
Rectifier Diode
A high-speed diode is necessary 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.
V
For low ESR ceramic capacitors, the output ripple is
dominated by the charging and discharging of the output
LAVG
LPK
RIPPLE
V
IN
3.3
3.3
3.3
OMAX
5
5
5
=
(V)
=
I
LAVG
------------ -
1 D
=
I
–
O
:
I
LPK
IN
V
+
. The voltage rating of input capacitor should
O
∆I
--------
12
15
12
15
9
9
2
×
(V)
L
ESR
+
V
----------------------- -
L (µH)
TABLE 2.
O
6.8
6.8
6.8
6.8
6.8
6.8
V
–
O
10
V
IN
×
--------------- -
C
(MHz)
OUT
I
O
f
1
1
1
1
1
1
S
×
---- -
f
1
S
IN
1.040686
0.719853
0.527353
1.576797
1.090686
0.79902
, V
I
IN
OMAX
O
be
, L, f
S
,
EL7585
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
capacitor at a particular voltage and not the manufacturer’s stated
value, measured at zero volts.
Compensation
The EL7585 can operate in either P mode or PI mode.
Connecting the C
For better load regulation, use PI mode with a 4.7nF
capacitor in series with a 10K resistor between C
ground. This value may be reduced to improve transient
performance, however, very low values will reduce loop
stability.
Boost feedback resistors
As the boost output voltage, A
effective voltage feedback in the IC increases the ratio of
voltage to current feedback at the summing comparator
because R
operation over the complete current range of the IC, the
voltage feedback to the FBB pin should be reduced
proportionally, as A
resistor-capacitor network (R
with a pole frequency (f
effective = 10µF and 4kHz for C
R
C
PI mode C
The IC is designed to operate with a minimum C
of 4.7nF and a minimum C
Note that, for high voltage A
ceramic capacitors (C
greatly; a 16V 10µF ceramic can drop to around 3µF at 15V.
To improve the transient load response of A
a resistor may be added in series with the C
larger the resistor the lower the overshoot but at the expense
of stability of the converter loop - especially at high currents.
With L = 10µH, A
should have a capacitance of greater than 10µF. R
can have values up to 5kΩ for C
up to 10K for C
Larger values of R
A
ensure A
maximum desired current and then the transient load
response of A
maximum value of R
VDD
OUT
7
7
= ((1/0.1 x R
= 1/(2 x 3.142 x f
in the equation above assumes the effective value of the
load currents less than the current limit are used. To
VDD
2
INT
decreases relative to R
stability, the IC should be operated at the
VDD
2
2
(C
) - 1/R
(effective) up to 30µF.
VDD
INT
INT
should be used to determine the
VDD
23
p
INT
pin directly to V
) and R
2
x R
= 15V, C
(R
) reduces their effective capacitance
1
p
.
is reduced, by means of a series
) set to approximately 10kHz for C
)^-1
7
7
) may be possible if maximum
)
2
VDD
(effective) = 10µF.
7
VDD
INT
and C
23
2
, the voltage coefficient of
2
(effective) = 30µF.
= 4.7nF, C
, is reduced below 12V the
(effective) up to 20µF and
(R
1
7
IN
. To maintain stable
10
) in parallel with R
will enable P mode;
)
23
VDD
2
(effective)
capacitor. The
23
in PI mode,
INT
March 9, 2006
INT
capacitor
and
FN7345.2
(R
1
7
,
)
2