add8754dacpz-reel7 Analog Devices, Inc., add8754dacpz-reel7 Datasheet - Page 14

• Manufacturer Part Number: add8754dacpz-reel7
• Description: Lcd Panel Power, Vcom, And Gate Modulation
• Manufacturer: Analog Devices, Inc.

Choosing the Input and Output Capacitors
The ADD8754 requires input and output bypass capacitors to
supply transient currents while maintaining a constant input
and output voltage. Use a low effective series resistance (ESR)
10 μF or greater input capacitor to prevent noise at the
ADD8754 input. Place the capacitors between VIN_1, VIN_2,
and GND and as close as possible to the ADD8754. Ceramic
capacitors are preferred because of their low ESR character-
istics. Alternatively, use a high value, medium ESR capacitor in
parallel with a 0.1 μF low ESR capacitor as close as possible to
the ADD8754.
The output capacitor maintains the output voltage and supplies
current to the load while the ADD8754 switch is on. The value
and characteristics of the output capacitor greatly affect the
output voltage ripple and stability of the regulator. Use a low
ESR output capacitor; ceramic dielectric capacitors are
preferred.
For very low ESR capacitors such as ceramic capacitors, the
ripple current due to the capacitance is calculated as follows.
Because the capacitor discharges during the on time, t
charge removed from the capacitor, Q
multiplied by the on time. Therefore, the output voltage ripple
( Δ V
where:
C
I
Choose the output capacitor based on the following equation:
Table 10. Capacitor Manufacturers
Vendor
AVX
Murata
Sanyo
Taiyo Yuden
t
D
L
ON
OUT
is the average inductor current.
=
OUT
=
V
is the output capacitance.
C
Δ
OUT
V
OUT
) is
f
V
D
SW
OUT
OUT
−
≥
V
=
f
IN
SW
I
C
Q
L
OUT
×
×
C
(
V
V
OUT
=
OUT
I
L
×
C
−
Web Address
www.avxcorp.com
www.murata.com
www.sanyovideo.com
www.t-yuden.com
×
Δ
OUT
V
t
V
ON
IN
OUT
)
C
, is the load current
ON
, the
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Diode Selection
The output diode conducts the inductor current to the output
capacitor and load while the switch is off. For high efficiency,
minimize the forward voltage drop of the diode. Schottky
diodes are recommended. However, for high voltage, high
temperature applications, where the Schottky diode reverse
leakage current becomes significant and can degrade efficiency,
use an ultrafast junction diode.
The diode must be rated to handle the average output load
current. Many diode manufacturers derate the current
capability of the diode as a function of the duty cycle. Verify
that the output diode is rated to handle the average output load
current with the minimum duty cycle. The minimum duty cycle
of the ADD8754 is
where V
For example, D
Table 11. Schottky Diode Manufacturers
Vendor
ON Semiconductor
Diodes, Inc.
Central Semiconductor Corp. www.centralsemi.com
Sanyo
Loop Compensation
Use of external components to compensate the regulator loop
allows optimization of the loop dynamics for a given application.
A step-up converter produces an undesirable right-half plane
zero in the regulation feedback loop. This requires compensat-
ing the regulator such that the crossover frequency occurs well
below the frequency of the right-half plane zero. The right-half
plane zero is determined by the following equation:
where:
F
R
divided by the load current.
Z
LOAD
(RHP) is the right-half plane zero.
D
F
is the equivalent load resistance, or the output voltage
Z
MIN
(
IN_MAX
RHP
=
V
)
is the maximum input voltage.
OUT
MIN
=
⎛
⎜
⎜
⎝
V
= 0.45 when V
V
V
−
OUT
OUT
V
IN
IN
⎞
⎟
⎟
⎠
_
2
MAX
×
2
R
π
LOAD
Web Address
www.onsemi.com
www.diodes.com
www.sanyovideo.com
×
OUT
L
= 10 V and V
IN_MAX
ADD8754
= 5.5 V
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