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A8483 Fiches technique(PDF) 6 Page - Allegro MicroSystems

No de pièce A8483
Description  1.2 MHz Step-up Converter for Display Bias Supply
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Fabricant  ALLEGRO [Allegro MicroSystems]
Site Internet  http://www.allegromicro.com
Logo ALLEGRO - Allegro MicroSystems

A8483 Fiches technique(HTML) 6 Page - Allegro MicroSystems

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A8483-DS, Rev. 1
Worcester, Massachusetts 01615-0036 (508) 853-5000
115 Northeast Cutoff, Box 15036
www.allegromicro.com
Allegro MicroSystems, Inc.
1.2 MHz Step-up Converter for Display Bias Supply
A8483
6
increase gradually. When the voltage drop across R3 is reduced
to less than about 0.8 V, the feedback from the sense resistor, R1,
takes over full control of the output voltage.
The length of the soft start delay depends on the combined effect
of R2, R3, R4, C3 and the amplitude of the EN signal. The delay
can be adjusted by the selection of these values.
Component Selection
The component values shown in figure 1 are sufficient for most
applications. To reduce output ripple, the value of the output
inductor, L1, may be increased, but in most cases this results in
excessive board area and additional cost.
Inductor Selection (L1).
With an internal PWM frequency of
1.2 MHz, the optimum inductor value for most cases would be
10 μH. The inductor should have low winding resistance, typi-
cally < 1
Ω, and the core should have low losses at 1.2 MHz.
For worst-case conditions of high output voltage and current, and
low input voltage, the inductor should be rated at the switch cur-
rent limit of 350 mA.
If high temperature operation is required, derating should be
considered. In some cases where lower inductor currents are
expected, the current rating can be decreased.
Several inductor manufacturers, including Coilcraft, Murata, Pan-
asonic, Sumida, Taiyo Yuden, and TDK, have and are developing
suitably small-size inductors. Two recommended inductors are:
• TDK: NLCV32T-100K-PF, 10 μH
• Toko: 1098AS-100M, 10 μH
Diode Selection (D1).
The diode should have a low forward volt-
age to reduce conduction losses, and a low capacitance to reduce
switching losses. Schottky diodes can provide both these features
if carefully selected. The forward voltage drop is a natural advan-
tage for Schottky diodes, and it reduces as the current rating of
the component increases.
However, as the current rating increases, the diode capacitance
also increases, so the optimum selection is usually the lowest
current rating above the circuit maximum. In this application, an
average current rating of 100 to 200 mA is usually sufficient.
Capacitor Selection.
Because the values recommended for the
capacitors are low, ceramic capacitors are the best choice for this
application. To reduce performance variation over temperature
changes, low drift types such as X7R and X5R should be used.
C1. A 1.0 μF capacitor on the VIN pin is suitable for most
applications. In cases where large inductor currents are switched,
a larger capacitor may be needed.
C2. The output capacitor can be as small as 0.22 μF for most
applications and most VIN / VOUT combinations. Increasing this
capacitor value aids in reducing ripple and increasing efficiency
in low input voltage / high output voltage conditions.
Suitable capacitors are available from: TDK, Taiyo Yuden,
Murata, Kemet, and AVX.
Figure 14. OLED Soft Start Circuit
A8483
VIN
SW
EN
GND
FB
C1
1 µF
1
23
5
4
L1
10 µH
VOUT
C2
1 µF
R4
47 k
Ω
R1
D1
820 k
Ω
R3
24 k
Ω
R2
39 k
Ω
Enable
D2
C3
10 nF
VSUPPLY
Li-ion
2.5 V to
5.5 V
A8483
VIN
SW
EN
GND
FB
C1
1 µF
1
23
5
4
L1
22 µH
VOUT
C2
0.22 µF
R4
47 k
Ω
R2
5 k
Ω
D1
R3
24 k
Ω
R1
4.75
Ω
D2
C3
0.01 µF
Li-ion
2.5 V to
5.5 V
VSUPPLY
Enable
Figure 16. WLED Soft Start Circuit
VOUT
EN
IIN
Figure 15. OLED Soft Start Circuit. Performance of circuit shown in figure 14.


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