Download 1.2 MHz Step-up Converter for Display Bias

A8483
1.2 MHz Step-up Converter for Display Bias Supply
This device is in production, however, it has been deemed Pre-End
of Life. The product is approaching end of life. Within a minimum of
6 months, the device will enter its final, Last Time Buy, order phase.
Date of status change: June 3, 2013
Recommended Substitutions:
For existing customer transition, and for new customers or new applications, contact Allegro Sales.
NOTE: For detailed information on purchasing options, contact your
local Allegro field applications engineer or sales representative.
Allegro MicroSystems, LLC reserves the right to make, from time to time, revisions to the anticipated product life cycle plan
for a product to accommodate changes in production capabilities, alternative product availabilities, or market demand. The
information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, LLC assumes no responsibility for its use; nor for any infringements of patents or other rights of third parties which may result from its use.
A8483
1.2 MHz Step-up Converter
for Display Bias Supply
Features and Benefits
Description
• Output voltage up to 35 V
• 2.5 to 10 V input
• 0.9 to 18 V input with separate bias supply
• Delivers 15 V at 15 mA with 2.7 V input
• Constant 1.2 MHz switching frequency provides low noise
• 350 mA switch current limit
• 1 μA shutdown current
• Low-noise PWM/analog dimming
The A8483 is a current mode step-up dc-to-dc converter, available
in a 5-pin 3 mm × 3 mm thermally enhanced TDFN lead (Pb)
free package. Smaller external components and integrated 36 V
switch reduce component count and footprint for a variety of
applications.
Applications:
• OLED bias supply / WLED backlight
• Portable battery-powered equipment
• Cellular phones
• PDAs (Personal Digital Assistant)
• Camcorders, personal stereos, MP3 players, cameras
• Mobile GPS systems
Package: TDFN (suffix EK)
3 × 3 mm,
0.75 mm Nominal Height
Approximate Scale 1:1
VOUT
5
VIN
VSUPPLY
EN
4
Li-ion
2.5 V to
5.5 V
GND
2
L1
10 µH
5
VIN
R1
820 kΩ
A8483
C1
1 µF
IOUT1 + IOUT2 = 15 mA
D1
1
SW
FB
3
C2
1µ
µF
OLED
L1
10 µH
VSUPPLY
EN
4
Li-ion
2.5 V to
5.5 V
Enable
R1
732 kΩ
A8483
C1
1 µF
GND
2
FB
3
VOUT2
VSUPPLY C1
1 µF
R1
768 kΩ for
VOUT = 32 V
604 kΩ for
VOUT = 25 V
A8483
EN
4
5V
GND
2
Connect either VZ or VBIAS
D1
1
SW
FB
3
VSUPPLY
12 V
5
C1
1 µF
Enable
VIN
R1
768 kΩ for
VOUT = 32 V
604 kΩ for
VOUT = 25 V
GND
2
FB
3
R2
15 kΩ
C3
0.1 µF
Figure 3. LCD TV, Plasma TV Bias, FED Display, Varactor Diode
Bias. Using 5 V input.
Figure 4. LCD TV, Plasma TV Bias, FED Display, Varactor Diode
Bias. Using 12 V input.
VOUT
VOUT
VSUPPLY
Li-ion
2.5 V to
5.5 V
L1
10 µH
1
SW
A8483
C1
1 µF
EN
4
GND
2
R3
24 kΩ
FB
3
Enable
Cf
0.01 µF
R2
5 kΩ
L1
10 µH
5
VIN
D1
C2
0.22 µF
VC
Analog Voltage or
PWM
R1
47 Ω
VSUPPLY
Li-ion
2.5 V to
5.5 V
R1
560 kΩ
A8483
C1
1 µF
EN
4
GND
2
A8483-DS, Rev. 1
C2
1 µF
FB
3
Enable
R3
160 kΩ
Analog Voltage or
PWM 10 kHz
Figure 5. WLED Backlighting.
D1
1
SW
OLED
5
VIN
C2
1µ
µF / 50 V
Enable
5V
R2
15 kΩ
D1
1
SW
A8483
EN
4
VBIAS
32 V, 40 mA
25 V, 80 mA
VOUT
L1
22 µH
VZ
7.5 V
C2
1µ
µF / 50 V
C4
0.47 µF
Figure 2. Small LCD Bias Supply. Li+ battery to ±12 V.
32 V, 18 mA
25 V, 25 mA
VOUT
–12 V
D2
R2
39 kΩ
Figure 1. OLED Bias Supply. Use for cell phones, MP3 players,
DSCs, and PDAs.
5
VIN
C3
0.47 µF
C2
0.47 µF
Enable
R2
39 kΩ
L1
10 µH
VOUT1
+12 V
D1
1
SW
R2
47 kΩ
Cf
1 nF
Cf for PWM dimming only
VC
Figure 6. OLED Bias Supply. Use for low-noise PWM dimming, or
with analog voltage through the FB pin.
1.2 MHz Step-up Converter
for Display Bias Supply
A8483
Selection Guide Use the following complete part number when ordering:
Part Number
Packing*
Description
A8483EEKTR-T
7-in. reel, 1500 pieces/reel
Surface Mount
*Contact Allegro for additional packing options.
Functional Block Diagram
FB
SW
A1
VIN
VREF
1.25 V
615 mV
A2
RC
R
Q
Driver
S
CC
Σ
Ramp
Generator
EN
Enable
1.2 MHz
Oscillator
GND
Device Pin-out Diagram
Terminal List Table
Pin
Name
Function
1
SW
Internal power FET
2
GND
Ground
3
FB
Feedback input
4
EN
Enable input
5
VIN
Input supply
SW 1
5 VIN
GND 2
FB 3
4 EN
Absolute Maximum Ratings
Package Thermal Characteristics
SW Pin ..................................................................................–0.3 to 36 V
Remaining Pins .....................................................................–0.3 to 10 V
Operating Ambient Temperature , TA .................................... –40°C to 85°C
Junction Temperature, TJ(max)............................................................. 150°C
Storage Temperature, TS .................................................. –55°C to 150°C
RθJA = 50 °C/W, on a 4-layer board. Additional information is
available on the Allegro Web site.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
2
1.2 MHz Step-up Converter
for Display Bias Supply
A8483
ELECTRICAL CHARACTERISTICS at TA = 25°C, VIN = 3 V (unless otherwise noted)
Characteristics
Symbol
Test Conditions
Min.
Typ.
Max.
Units
2.5
–
10
V
Active: IOUT = 15 mA, VOUT = 12 V
–
2.5
3.5
mA
Shutdown (EN = 0 V)
–
0.1
1
μA
590
615
640
mV
–
Input Voltage Range
VIN
Supply Current
ISUP
Feedback Reference Voltage
VFB
–
IFB
VFB = 0.615 V
–
50
100
nA
ISWLIM
–
–
350
–
mA
FSW
–
0.8
1.2
1.6
MHz
D
–
85
90
–
%
Feedback Input Current
Switch Current Limit
Switch Frequency
Switch Maximum Duty Cycle
VCE(SAT)
ISW = 0.2 A
–
350
–
mV
ISL
VSW = 36 V
–
–
5
μA
Input Threshold Low
VIL
–
–
–
0.4
V
Input Threshold High
VIH
–
1.5
–
–
V
Leakage
IIL
–
–
1
μA
Switch Saturation voltage
Switch Leakage Current
Enable Input
Input Leakage
Operating Characteristics
(VIN = 3 V)
Feedback Bias Current versus Temperature
Efficiency versus Input Voltage
VOUT = 15 V; Toko 1098AS-100M Inductor
VFB = 0.615 V
65
90
85
62
75
IFB (nA)
Efficiency (%)
80
IOUT (mA)
70
25
15
2
65
60
59
56
53
55
50
50
2
3
4
5
6
7
8
9
10
–50
0
VIN (V)
100
150
Quiescent Supply Current versus Temperature
1.25
2.6
1.20
2.5
ISUPQ (mA)
FSW (MHz)
Switching Frequency versus Temperature
1.15
1.10
1.05
2.4
2.3
2.2
1.00
–50
50
Temperature (°C)
2.1
0
50
Temperature (°C)
100
150
–50
0
50
100
150
Temperature (°C)
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
3
1.2 MHz Step-up Converter
for Display Bias Supply
A8483
Functional Description
Pin Functions
VIN. Supply to the control circuit. A bypass capacitor, C1, must
be connected from close to this pin to GND.
GND. Ground reference connected directly to the ground plane.
The feedback resistor should have a separate connection directly
to this point.
EN. Voltage lower than 0.4 V disables the A8483 and puts the
control circuit into the low-power sleep mode. Voltage greater
than 1.5 V fully enables the A8483.
SW. Low-side switch connection between the inductor, L1, and
ground. Because rapid changes of current occur at this pin, the
traces on the PCB that are connected to this pin should be minimized. In addition, L1 and the diode, D1, should be connected as
close to this pin as possible.
FB. Feedback pin for OLED voltage control or WLED current
control. The reference voltage is 615 mV. Connect the feedback
resistor close to this pin to minimize noise.
Device Operation
The A8483 uses a 1.2 MHz constant switching frequency currentmode control scheme to regulate the output voltage or current
through the load.
A typical OLED bias supply is shown in figure 7. For driving
OLEDs, output voltage is sensed by the FB pin through a voltage
divider network. Output voltage, VOUT (V), is set according to the
following equation:
VOUT =
R1 + R2
R2
(1)
0.615 .
A typical WLED backlight supply is shown in figure 8. The load
current, ILOAD, is set by the selecting the external sense resistor,
R1, to produce 615 mV at the desired load, for example:
Voltage sensed across the FB pin is compared with the internal
615 mV reference to produce an error signal. The switch current is sensed by the internal sense resistor and compared to the
error signal for current mode PWM control. As the error signal
increases, ILOAD , increases to increase either output voltage,
VOUT, or current, IOUT , such that the FB pin voltage follows the
internal 615 mV reference voltage.
As ILOAD is reduced, the energy required in the inductor, L1, also
reduces, resulting in the inductor current dropping to zero for low
load current levels. This is known as Discontinuous mode operation and results in some low-frequency ripple. The average load
current will, however, remain regulated down to zero. In Discontinuous mode, when ILOAD drops to zero, the voltage at the SW
pin rings, due to the resonant LC circuit formed by L1 and the
switch and diode D1 capacitance. This ringing is low-frequency
and is not harmful. It can be damped with a resistor across the
inductor but this will reduce efficiency and is not recommended.
VOUT
VOUT
VSUPPLY
Li-ion
2.5 V to
5.5 V
L1
10 µH
1
SW
EN
4
GND
2
5
VIN
R1
820 kΩ
A8483
C1
1 µF
D1
FB
3
OLED
5
VIN
(2)
R1 = 0.615 ILOAD .
C2
1µ
µF
Enable
R2
39 kΩ
Figure 7. OLED Bias Supply. Use for cell phones, MP3 players, DSCs,
and PDAs. (Circuit also shown in figure 1.)
VSUPPLY
Li-ion
2.5 V to
5.5 V
L1
10 µH
1
SW
D1
A8483
C1
1 µF
EN
4
GND
2
R3
24 kΩ
FB
3
Enable
Cf
0.01 µF
R2
5 kΩ
C2
0.22 µF
VC
Analog Voltage or
PWM
R1
47 Ω
Figure 8. WLED Backlighting. (Circuit also shown in figure 5.)
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
4
1.2 MHz Step-up Converter
for Display Bias Supply
A8483
Applications Information
Display dimming can be achieved by controlling the output voltage, VOUT , using the FB pin. The circuitry behind the FB pin is
flexible, to accommodate a variety of schemes used for dimming:
• Adjust the duty cycle of the pulse train applied to the FB pin
through resistor R3, as shown in figure 11. Capacitor Cf is used
to generate the average voltage level. Variation of output voltage with PWM duty cycle is shown in figure 9.
• Apply a constant dc voltage to the FB pin through resistor R3,
as shown in figure 12. Variation of output voltage with dc voltage are shown in figure 10.
• Selection of the resistor connected between the FB pin and
GND, as shown in figure 12 for OLED applications, and in
figure 13 for WLED applications. The voltage drop across the
transistor should be negligible compared to the FB sense voltage.
Soft Start
Output voltage rise time at power-on can be extended by using a
soft start circuit, such as the one shown in figure 14 for an OLED
application, and in figure 16 for a WLED application. At poweron, the device duty cycle begins initially at a high level, drawing
a large current from the input supply, VIN. The soft-start circuits
shown can reduce the level of current flow by controlling the FB
pin.
When a signal is applied to the EN pin, capacitor C3 discharges,
pulling the FB pin high, and reducing VOUT to a minimum. When
the EN signal is removed, C3 recharges and as it does, the voltage
drop across R3 reduces, allowing the device duty cycle to
VOUT
L1
10 µH
5
VIN
VSUPPLY
R1
560 kΩ
A8483
C1
1 µF
EN
4
Li-ion
2.5 V to
5.5 V
D1
1
SW
GND
2
C2
1 µF
OLED
Dimming Control
FB
3
Enable
R3
160 kΩ
17
R2
47 kΩ
Cf
1 nF
15
Analog Voltage or
PWM 10 kHz
Cf for PWM dimming only
VC
Figure 11. OLED Bias Supply. Use for low-noise PWM dimming,
or with analog voltage through the FB pin.
11
9
VOUT
7
L1
10 µH
5
VIN
5
VSUPPLY
3
10
20
30
40
50
60
70
80
90
Duty Cycle (%)
D1
R1
560 kΩ
A8483
C1
1 µF
Li-ion
2.5 V to
5.5 V
1
SW
EN
4
GND
2
C2
10 µF
FB
3
Cf
1 nF
Enable
R3
47 kΩ
Figure 9. PWM Dimming Control of Duty Cycle. Performance of 5 V input
circuit shown in figure 3.
OLED
VOUT (V)
13
R2
47 kΩ
VC
Q1
20
Figure 12. OLED Dimming Control. Single-bit resolution using
external transistor.
18
16
VOUT (V)
14
VOUT
L1
10 µH
12
5
VIN
10
8
VSUPPLY
6
Li-ion
2.5 V to
5.5 V
4
2
1
SW
D1
A8483
C1
1 µF
EN
4
GND
2
FB
3
C2
0.22 µF
Enable
R2
80 Ω
0
0.2
0.4
0.6
VC (V)
0.8
1
Figure 10. DC Voltage Dimming Control of Duty Cycle. Performance of
circuit shown in figure 11.
R1
80 Ω
VC
Q1
Figure 13. WLED Dimming Control. Single-bit resolution using
external transistor.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
5
1.2 MHz Step-up Converter
for Display Bias Supply
A8483
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.
VOUT
VSUPPLY
L1
10 µH
D1
1
SW
R1
820 kΩ
A8483
C1
1 µF
EN
4
Li-ion
2.5 V to
5.5 V
GND
2
FB
3
OLED
5
VIN
C2
1 µF
D2
Enable
R4
47 kΩ
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, typically < 1 , 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 current 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,
Panasonic, 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
C3
10 nF
R3
24 kΩ
Figure 14. OLED Soft Start Circuit
EN
VOUT
IIN
Figure 15. OLED Soft Start Circuit. Performance of circuit shown in figure 14.
VOUT
5
VIN
VSUPPLY
Li-ion
2.5 V to
5.5 V
L1
22 µH
1
SW
D1
A8483
C1
1 µF
EN
4
GND
2
R2
5 kΩ
FB
3
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 advantage 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.
C2
0.22 µF
D2
Enable
Diode Selection (D1). The diode should have a low forward volt-
R2
39 kΩ
R4
47 kΩ
C3
0.01 µF
R3
24 kΩ
R1
4.75 Ω
Figure 16. WLED Soft Start Circuit
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.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
6
1.2 MHz Step-up Converter
for Display Bias Supply
A8483
IOUT1 + IOUT2 = 15 mA
L1
10 µH
5
VIN
C3
0.47 µF
C2
0.47 µF
13.0
12.8
12.6
VOUT1
12.4
12.2
EN
4
Li-ion
2.5 V to
5.5 V
R1
732 kΩ
A8483
C1
1 µF
+12 V
VOUT (V)
VSUPPLY
Line Regulation
VOUT1
D1
1
SW
GND
2
FB
3
12.0
VOUT2
11.8
Enable
D2
R2
39 kΩ
VOUT2
11.6
–12 V
11.4
C4
0.47 µF
11.2
11.0
2.5
3.5
4.5
5.5
VIN (V)
Figure 17. Line Regulation Perfomance for Small LCD Bias Supply, IOUT1 = IOUT2 = 7.5 mA (circuit also shown in figure 2)
32 V, 18 mA
25 V, 25 mA
VOUT
L1
10 µH
VSUPPLY C1
1 µF
32
R1
768 kΩ for
VOUT = 32 V
604 kΩ for
VOUT = 25 V
A8483
EN
4
5V
GND
2
Load Regulation
34
D1
1
SW
FB
3
VOUT = 32 V
30
C2
1µ
µF / 50 V
VOUT (V)
5
VIN
28
26
VOUT = 25 V
24
Enable
22
R2
15 kΩ
20
0
10
20
30
40
IOUT (mA)
Figure 18.Load Regulation for LCD TV, Plasma TV Bias, FED Display, Varactor Diode Bias; VSUPPLY = 5 V (circuit also shown in figure 3)
Connect either VZ or VBIAS
12 V
5
C1
1 µF
VIN
5V
1
SW
A8483
EN
4
VBIAS
GND
2
FB
3
D1
32
R1
768 kΩ for
VOUT = 32 V
604 kΩ for
VOUT = 25 V
Enable
C3
0.1 µF
Load Regulation
34
VOUT = 32 V
30
C2
1µ
µF / 50 V
VOUT (V)
L1
22 µH
VZ
7.5 V
VSUPPLY
32 V, 40 mA
25 V, 80 mA
VOUT
28
26
24
VOUT = 25 V
22
R2
15 kΩ
20
0
20
40
60
80
100
IOUT (mA)
Figure 19. Load Regulation for LCD TV, Plasma TV Bias, FED Display, Varactor Diode Bias; VSUPPLY = 12 V (circuit also shown in figure 4). The A8483
can operate with a VSUPPLY from 0.9 to 18 V with a separate bias supply to operate the A8483 in the normal VIN range. The bias voltage can be supplied
by an external power supply, such as 3.3 or 5 V, or by using a suitable Zener diode, VZ , for VSUPPLY > 10 V.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
7
1.2 MHz Step-up Converter
for Display Bias Supply
A8483
Leadframe plating 100% matte-tin.
Solder pad layout compatible with SOT23-5 (SOT95P280-5).
Package EK , TDFN
3.15 .124
2.85 .112
A
B
5
Preliminary dimensions, for reference only
(reference JEDEC MO-229 WEEA)
Dimensions in millimeters
U.S. Customary dimensions (in.) in brackets, for reference only
Dimensions exclusive of mold flash, gate burrs, and dambar protrusions
Exact case and lead configuration at supplier discretion within limits shown
3.15 .124
2.85 .112
A Terminal #1 mark area
A
B Exposed thermal pad (reference dimensions only,
terminal #1 identifier appearance at supplier discretion)
C Reference pad layout (reference IPC
SON95P300X310-7WEEAN); adjust as necessary
to meet application process requirements
1
2
5X
D Reference pad layout with contact pads only; adjust as
necessary to meet application process requirements
SEATING
PLANE
0.08 [.003] C
5X
0.45 .018
0.30 .012
C
0.80 .031
0.70 .028
0.10 [.004] M C A B
0.05 [.002] M C
0.95 .037
0.20 .008
REF
0.05 .002
0.00 .000
0.50 .020
0.30 .012
1
R0.20 .008
REF
2
1.00 .039
NOM
B
5
2.00 .079
NOM
0.85 .033
MIN
5
0.45 .018
MIN
0.85 .033
MIN
5
2.00 .079
MAX
0.45 .018
MIN
0.45 .018
MIN
2.10 .083
MIN
3.80 .150
MAX
1
D
0.95 .037
0.50 .020
MIN
1.00
MAX
.039
2.10 .083
MIN
3.80 .150
MAX
0.20 x 0.20 .008 x .008
REF
1
C
0.95 .037
0.50 .020
MIN
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
8
1.2 MHz Step-up Converter
for Display Bias Supply
A8483
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 products are not authorized for use as critical components in life-support devices or systems without
express written approval.
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© 2005-2013 Allegro MicroSystems, LLC
For the latest version of this document, visit our website:
www.allegromicro.com
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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