Applications Information
Low-Cost Single-Inductor Triple-Output TFT LCD Supply
By the Power Management Business Unit Staff
This application note describes an easy method of
conversion from a single power supply to three distinct
voltage levels. This method eliminates all but one inductor,
thereby reducing matching effort during design, as well as
limiting final application size and cost. The Allegro™ A8483
device is examined as an integral component of this solution.
The vast majority of portable devices today feature thin-film
transistor (TFT) LCD displays. Such displays need to operate
from a low-voltage input supply or a battery source, in the
range of +3 to +5 volts. A typical TFT LCD bias supply
requires a main supply, also referred to as AVDD (typically
in the range of +8 V to +12 V), a positive gate-drive bias
(typically in the range of +12 V to +18 V), and a negative
gate-drive bias (typically in the range of –5 V to –10 V).
Because portable electronics applications demand small size
and low profile, the supply must operate at sufficiently high
frequency to reduce the size of external filter components.
Additionally, a single dc-to-dc controller generating three
outputs is highly desirable, because it limits the cost of
the solution and minimizes the PCB area required. The
circuit shown in the figure below uses a combination boost
regulator and charge pump circuitry to accomplish this task.
Boost Regulator
The circuit employs a single dc-to-dc controller (the A8483,
a current-mode–controlled boost regulator) to generate a
+8.75 V main supply, capable of delivering up to 50 mA.
(Selecting an appropriate feedback voltage divider can
change the output voltage.) The 1.2 MHz operating
D5
BAT54S
C8
1 μF
25 V
C5 D3
1 μF
16 V
BAT54S
C3 D2
1 μF
16 V
BAT54S
L1
10 μH
4
C1
1 μF
6.3 V
D1
RB421D
1
SW
VIN
EN
A8483
GND
2
Enable
C9
2.2 μF
25 V
R4
470 Ω
D6
15 V
C6
2.2 μF
16 V
D4
7.5 V
C10
2.2 μF
25 V
R7
15 kΩ
VOUT2
–7.5 V
C7
2.2 μF
16 V
R5
7.5 kΩ
C4
1 μF
16 V
VSW
3.3 V
5
VOUT1
+15 V
R6
1 kΩ
R1
39 kΩ
FB
VOUT3
8.75 V
50 mA
R2
512 kΩ
3
R3
39 kΩ
C2
2.2 μF
16 V
This circuit delivers three output voltages from a single supply, by combining regulator topologies.
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frequency reduces the inductor size and allows operation with
tiny ceramic capacitors.
Internal compensation simplifies circuit design and reduces
external component count. The low-profile (0.75 mm nominal
height) MLP/TDFN package allows the boost regulator to be
mounted on either side of the PCB to optimize footprint and
minimize height.
Positive and Negative Charge Pumps
When the internal switch of the A8483 is on, the SW node
is near 0 V, with low impedance to circuit ground. When the
internal switch of the A8483 switches off in order to provide
boost action, the SW node goes up to approximately 9.05 V. This
operation allows for an easy implementation of the charge pump
with diodes and capacitors, to generate gate-drive supplies at
+15 V and -7.5 V with 1 mA output-current capability.
The circuit diagram shows optional Zener diodes across gatedriver outputs. These can help to provide good regulation.
Additional sequencing features can be added to this solution as
needed, by providing pass elements in series with the outputs.
Allegro MicroSystems, LLC reserves the right to make, from time to time, such departures from the detail specifications as may be required to
permit improvements in the performance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that
the information being relied upon is current.
Allegro’s products are not to be used in life support devices or systems, if a failure of an Allegro product can reasonably be expected to cause the
failure of that life support device or system, or to affect the safety or effectiveness of that device or system. The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, LLC assumes no responsibility for its use; nor for any infringement of patents or
other rights of third parties which may result from its use.
Copyright ©2007-2013, Allegro MicroSystems, LLC
For the latest version of this document, visit our website:
www.allegromicro.com
AN295043
Allegro MicroSystems, LLC
www.allegromicro.com
(508) 853-5000
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036
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