VIN
LX
BOOT
ENB
TSET
GND
VBIAS
FB
+42 V
220 F
25 V
ESR
V
OUT
COUT
3.3 V
3 A
2.87 k7
R1
L1
D1
R2
910 7
0.22 F
0.01 F
CBOOT
CIN2 CIN1
0.22 F
CBYP
RTSET
54 k7
A8447
Efficiency vs. Output Current
70.0
72.0
74.0
76.0
78.0
80.0
82.0
84.0
86.0
88.0
90.0
0 500 1000 1500 2000 2500 3000
I
OUT
(mA)
Efficiency %
VOUT (V)
5
3.3
Data is for reference only. Efficiency data from circuit shown in left panel.
Approximate Scale 1:1
A8447
A8447-DS, Rev. 3
Features and Benefits
Wide input voltage range: 8 to 50 V
Integrated low RDS(on) DMOS switch
3 A continuous output current
Adjustable fixed off-time
Highly efficient
Adjustable output: 0.8 to 24 V
Small package with exposed thermal pad
High Voltage Step Down Regulator
Typical Application
Package: 8 pin SOIC with exposed thermal
pad (suffix LJ)
Description
The A8447 is a 3 A, high efficiency general-purpose buck
regulator designed for a wide variety of applications. The output
voltage is adjustable from 0.8 to 24 V, based on a resistor divider
and the 0.8 V ±2% reference. External components include an
external clamping diode, inductor, and filter capacitor. The off-
time is determined by an external resistor to ground. It operates
in both continuous and discontinuous modes to maintain light
load regulation. An internal blanking circuit is used to filter
out transients due to the reverse recovery of the external clamp
diode. Typical blanking time is 200 ns.
This new device is ideal for various end products including
applications with 8 to 50 V input voltage range and require up
to 3 A output current, such as uninterruptible power supplies,
point of sale (POS) applications, and industrial applications
with 24 or 36 V bus.
Applications include:
Printer power supplies
Office automation equipment
POS thermal, laser, photo, and inkjet printers
Tape drives
Industrial applications
High Voltage Step Down Regulator
A8447
2
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Absolute Maximum Ratings
Characteristic Symbol Conditions Min. Typ. Max. Units
VIN Supply Voltage VIN – – 50 V
VBIAS Input Voltage VBIAS –0.3 – 7 V
SW Switching Voltage VS–1 – – V
ENB Input Voltage Range VENB –0.3 – 7 V
Operating Ambient Temperature Range TA –20 – 85 °C
Junction Temperature TJ(max) – – 150 °C
Storage Temperature Tstg –55 – 150 °C
*Output current rating may be limited by duty cycle, ambient temperature, and heat sinking. Under any set of conditions, do not exceed
the specified current ratings, or a junction temperature, TJ, of 150°C.
Package Thermal Characteristics*
Package RθJA
(°C/W) PCB
LJ 35 4-layer
* Additional information is available on the Allegro website.
Ordering Information
Use the following complete part numbers when ordering:
Part Number Packing Description
A8447SLJTR-T 13-in. reel, 3000 pieces/reel
LJ package, SOIC surface mount with
exposed thermal pad; leadframe plating
100% matte tin.
High Voltage Step Down Regulator
A8447
3
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Functional Block Diagram
+
–
+
–
+–
Switch PWM Control
C
Soft Start
Ramp Generation
COMP
0.8 V
Boot Charge
VBIAS is connected to VOUT
when V
OUT
target is between
3.3 and 5 V
D1
L1
VIN
COUT
CBYP
VOUT
ESR
VBB UVLO
TSD
Switch
Disable
Bias Supply
I_Peak I_Demand
Clamp
Error
BOOT
ENB
TSET
GND
VIN
LX
FB
VBIAS
High Voltage Step Down Regulator
A8447
4
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Characteristics Symbol Test Conditions Min. Typ. Max. Units
VIN Quiescent Current3IVIN(Q)
VENB = LOW, IOUT = 0 mA, VIN = 42 V,
VBIAS = VOUT
– 0.90 1.35 mA
VENB = LOW, IOUT = 0 mA, VIN = 42 V,
VBIAS < 3 V – 4.4 6.35 mA
VENB = HIGH – – 100 A
VBIAS Input Current IBIAS VBIAS = VOUT – 3.5 5 mA
Buck Switch On Resistance RDS(on)
TA = 25°C, IOUT = 3 A – 450 – m
TA = 125°C, IOUT = 3 A – 650 – m
Fixed Off-Time Proportion toff Based on calculated value –15 – 15 %
Feedback Voltage VFB 0.784 0.8 0.816 V
Output Voltage Regulation VOUT IOUT = 0 mA to 3 A –3 – 3 %
Feedback Input Bias Current IFB –400 –100 100 nA
Soft Start Time tss 51015ms
Buck Switch Current Limit ICL
VFB > 0.4 V 3.5 – 5 A
VFB < 0.4 V 0.5 – 1.5 A
ENB Open Circuit Voltage VOC Output disabled 2.0 – 7 V
ENB Input Voltage Threshold VENB(0) LOW level input (Logic 0), output enabled – – 1.0 V
ENB Input Current IENB(0) VENB = 0 V –10 – –1 A
VIN Undervoltage Threshold VUVLO VIN rising 6.6 6.9 7.2 V
VIN Undervoltage Hysteresis VUVLO(hys) VIN falling 0.7 – 1.1 V
Thermal Shutdown Temperature TJTSD Temperature increasing – 165 – °C
Thermal Shutdown Hysteresis TJTSD(hys) Recovery = TJTSD – TJTSD(hys) –15–°C
1Negative current is defined as coming out of (sourcing) the specified device pin.
2Specifications over the junction temperature range of 0ºC to 125ºC are assured by design and characterization.
3VBIAS is connected to VOUT when the VOUT target is between 3.3 and 5 V.
ELECTRICAL CHARACTERISTICS1,2 at TA = 25°C, VIN = 8 to 50 V (unless noted otherwise)
High Voltage Step Down Regulator
A8447
5
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Functional Description
0
20
40
60
80
100
120
140
160
180
200
1 2 3 4 5 6 7 8 9 10 1112131415 16
Off-Time Setting versus Resistor Value
R
TSET
(kΩ)
t
OFF
(µs)
V
BIAS
= 5 V
V
BIAS
= 3.3 V
The A8447 is a fixed off-time, current mode controlled, buck
regulator. The regulator requires an external clamping diode,
inductor, and filter capacitor. It operates in both continuous and
discontinuous modes. An internal blanking circuit is used to filter
out transients resulting from the reverse recovery of the external
clamp diode. Typical blanking time is 200 ns.
The value of a resistor between the TSET and GND determines
the fixed off-time (see graph in the tOFF section).
VOUT. The output voltage is adjustable from 0.8 to 24 V, set by an
external resistor divider. The voltage can be calculated with the
following formula:
V
OUT = VFB × (1 + R1/R2) (1)
Light Load Regulation. To maintain voltage regulation during
light load conditions, the switching regulator enters a cycle-skip-
ping mode. As the output current decreases, there remains some
energy that is stored during the power switch minimum on-time.
In order to prevent the output voltage from rising, the regulator
skips cycles once it reaches the minimum on-time, effectively
making the off-time larger.
Soft Start. An internal ramp generator and counter allow the out-
put to slowly ramp up. This limits the maximum demand on the
external power supply by controlling the inrush current required
to charge the external capacitor and any dc load at startup.
Internally, the ramp is set to 10 ms nominal rise time. During soft
start, current limit is 3.5 A minimum.
The following conditions are required to trigger a soft start:
• VIN > 6 V
• ENB pin input falling edge
• Reset of a TSD (thermal shut down) event
VBIAS. To improve overall system efficiency, the regulator output,
VOUT, is connected to the VBIAS input to supply the operating
bias current during normal operating conditions. During startup
the circuitry is run off of the VIN supply. VBIAS should be con-
nected to VOUT when the VOUT target level is between 3.3 and
5 V. If the output voltage is less than 3.3 V, then the A8447 can
operate with an internal supply and pay a penalty in efficiency,
as the bias current will come from the high voltage supply, VIN.
VBIAS can also be supplied with an external voltage source. No
power-up sequencing is required for normal operation.
ON/OFF Control. The ENB pin is externally pulled to ground
to enable the device and begin the soft start sequence. When the
ENB is open circuited, the switcher is disabled and the output
decays to 0 V.
Protection. The buck switch will be disabled under one or more
of the following fault conditions:
• VIN < 6 V
• ENB pin = open circuit
• TSD fault
When the device comes out of a TSD fault, it will go into a soft
start to limit inrush current.
tOFF. The value of a resistor between the TSET pin and ground
determines the fixed off-time. The formula to calculate tOFF (s)
is:
,
tOFF RTSET
=
⎟
⎠
⎞
⎜
⎝
⎛
10.2 × 10
9
1– 0.03 × V
BIAS
(2)
where RTSET (k) is the value of the resistor. Results are shown
in the following graph:
The RTSET resistor should be not smaller than 7.65 k ±2% to
prevent very short off-times from violating the minimum on-time
of the switcher.
Shorted Load. If the voltage on the FB pin falls below 0.4 V, the
regulator will invoke a 1.5 A typical overcurrent limit to handle
the shorted load condition at the regulator output. For low output
voltages at power up and in the case of a shorted output, the off-
time is extended to prevent loss of control of the current limit due
to the minimum on-time of the switcher.
The extension of the off-time is based on the value of the TSET
multiplier and the FB voltage, as shown in the following table:
VFB (V) TSET Multiplier
< 0.16 8 × tOFF
< 0.32 4 × tOFF
< 0.5 2 × tOFF
> 0.5 tOFF
High Voltage Step Down Regulator
A8447
6
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Component Selection
L1. The inductor must be rated to handle the total load current.
The value should be chosen to keep the ripple current to a reason-
able value. The ripple current, IRIPPLE, can be calculated by:
IRIPPLE = VL(OFF) × tOFF / L (3)
V
L(OFF) = VOUT + Vf + IL(AV) × RL (4)
Example:
Given VOUT = 5 V, Vf = 0.55 V, VIN = 42 V, ILOAD = 0.5 A, power
inductor with L = 180 H and RL = 0.5 Rdc at 55°C, tOFF =
7 s, and RDS(on) = 1 .
Substituting into equation 4:
VL(OFF) = 5 V + 0.55 V+ 0.5 A × 0.5 = 5.8 V
Substituting into equation 3:
IRIPPLE = 5.8 V × 7 s / 180 H = 225 mA
The switching frequency, fSW, can then be estimated by:
f
SW = 1 / ( tON + tOFF ) (5)
t
ON = IRIPPLE × L / VL(ON) (6)
V
L(ON) = VIN – IL(AV) × RDS(on) – IL(AV) × RL– VOUT (7)
Substituting into equation 7:
VL(ON) = 42 V – 0.5 A × 1 – 0.5 A × 0.5 – 5 V = 36 V
Substituting into equation 6:
tON = 225 mA × 180 H / 36 V = 1.12 s
Substituting into equation 7:
fSW = 1 / (7 s +1.12 s) = 123 kHz
Higher inductor values can be chosen to lower the ripple cur-
rent. This may be an option if it is required to increase the total
maximum current available above that drawn from the switching
regulator. The maximum total current available, ILOAD(MAX) , is:
ILOAD(MAX) = ICL(min) – IRIPPLE / 2 (8)
where ICL(min) is 3.5 A, from the Electrical Characteristics table.
D1. The Schottky catch diode should be rated to handle 1.2 times
the maximum load current. The voltage rating should be higher
than the maximum input voltage expected during all operating
conditions. The duty cycle for high input voltages can be very
close to 100%.
COUT. The main consideration in selecting an output capacitor
is voltage ripple on the output. For electrolytic output capacitors,
a low-ESR type is recommended.
The peak-to-peak output voltage ripple is simply IRIPPLE ×
ESR. Note that increasing the inductor value can decrease the
ripple current. The minimum voltage rating of the capacitor is
10 V. However, because ESR decreases with voltage, the most
cost-effective choice may be rated higher in voltage. It is recom-
mended that the ESR be less than 100 m.
High Voltage Step Down Regulator
A8447
7
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
RTSET Selection. Correct selection of RTSET values will
ensure that minimum on time of the switcher is not violated
and prevent the switcher from cycle skipping. For a given VIN
to VOUT ratio, RTSET must be greater than or equal to the value
defined by the curve in the RTEST Value Selection graph below.
Note. The curve represents the minimum RTSET value. When
calculating RTSET , be sure to use VIN(max) / VOUT(min). Resistor
tolerance should also be considered, so that under all operating
conditions the resistance on the TSET pin remains as close to the
curve as possible.
The RTEST Selection table shows recommended RTSET values
based on common operating conditions. For other operating con-
ditions, refer to the RTSET Value Selection graph.
FB Resistor Selection. The impedance of the FB network
should be kept low to improve noise immunity. Large value resis-
tors can pick up noise generated by the inductor, which can affect
voltage regulation of the switcher.
RTSET Value Selection*
Selection Graph Recommended Common Values
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
0 15 30 45 60 75 90 105 120 135 150 165 180 195 210
R
TSET
(kΩ)
V
IN
/ V
OUT
VIN
(V)
VOUT
(V) VIN / VOUT
RTSET Value
(kΩ)
42 5 8.4 37.4
42 3.3 12.7 54.9
42 1.8 23.3 90.9
42 1.5 28 105
24 5 4.8 20.0
24 3.3 7.3 32.4
24 1.8 13.3 54.9
24 1.5 16 66.5
12 5 2.4 7.68
12 3.3 3.6 13.7
12 1.8 6.6 30.1
12 1.5 8 37.4
Recommended Components
Component Description Part Number
L1 Sumida 68 H RCH1216BNP-680K
D1 NIEC Schottky Barrier Diode 60 V TO-252AA NSQ03A06
CBYP Ceramic X7A 0.22 F 100 V Generic
CBOOT Ceramic X7A 0.01 F 100 V Generic
CIN Electrolytic 100 F 50 V; must be able to handle worst case ripple curent Generic
Ceramic X7A 0.22 F 50 V Generic
COUT
United Chemi-Con PXA 220 F 16 V Low ESR PXA16VC221MJ12TP
Rubycon ZL 220 F 25 V Low ESR (Option 1) 25ZL220M8x11.5
Panasonic FM 220 F 25 V Low ESR (Option 2) EEUFM1E221
VOUT
1.5 V 1.8 V 3.3 V 5 V
R1 1.3 k 2.55 k 2.87 k 6.3 k
R2 1.47 k 2 k 0.910 k 1.2 k
*The RTSET resistor should be not smaller than 7.65 k ±2% to prevent very short off-times from violating the minimum on-time of
the switcher.
High Voltage Step Down Regulator
A8447
8
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
R1
R2
D1
RTSET
COUT
CIN1
CIN2
L1
U1
VIN
VOUT
CBOOT
CBYP
GND
GND
GND
GND
Recommended PCB Layout
The large star ground area on the populated side of the PCB, shown in the diagram as the GND nodes, supports high current throughput, and allows
the VOUT node to be located as close as practical to the A8447 (U1). Thermal conduction from the A8447 is enhanced by direct contact of its
exposed thermal pad to the smaller ground area under the A8447. This area is connected by thermal vias to the large copper ground plane on the
unpopulated side of the PCB.
Star Ground
Exposed copper thermal
ground area on the
unpopulated side of the PCB
In order to minimize the effects of ground bounce and offset
issues, it is important to have a low impedance ground located
very close to the device. This grounding scheme is known as star
grounding. It is likely that a ground plane will be necessary to
meet thermal requirements. The recommended land pattern illus-
trates how to create a low impedance ground that will also assist
with removing thermal energy from the device.
The input capacitor must be placed as close as possible to the
VIN terminal because during the on cycle it is responsible for
supplying the current to the switcher. During the off cycle, the
current path is from the negative terminal of the COUT cap,
through the diode and inductor, and then to the load. As a result,
COUT and the rectifier diode must share the connection at the
negative terminal of the CIN capacitor in order to reduce ground
bounce when the diode is conducting.
The inductor should be connected as close as possible to the
switching node to minimize noise. Some applications may require
a shielded inductor due to EMI restrictions. This will depend on
the application and parameters defined by the system that will
host the regulator.
The high voltage-switching node could affect RTSET. If longer
off-times are used, the resistance on the RTSET pin can be quite
large. When designing the layout, try to keep RTSET away from
the inductor and switching node. It is also beneficial to keep the
trace as short as possible to reduce the effect of noise injection.
Because of this layout guideline, the TSET pin is located on the
other side of the device, away from the switching node.
The FB resistor network should have a lower impedance to avoid
interference from the switching node. Because the impedance
on the FB node can be controlled, it is not as critical to keep the
network isolated. It is important to keep the ground trace short so
that ground bounce cannot effect the output voltage regulation.
V
OUT
V
IN
R2
R1
L1 COUT
CIN1CIN2
CBYP
D1
RTSET
CBOOT
1
A8447
ENB LX
VBIAS
VIN
GND FB
TSET
BOOT
PAD
High Voltage Step Down Regulator
A8447
9
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Package LJ 8 Pin SOIC
3.30
2
1
8
Reference land pattern layout (reference IPC7351
SOIC127P600X175-9AM); all pads a minimum of 0.20 mm from all
adjacent pads; adjust as necessary to meet application process
requirements and PCB layout tolerances; when mounting on a multilayer
PCB, thermal vias at the exposed thermal pad land can improve thermal
dissipation (reference EIA/JEDEC Standard JESD51-5)
PCB Layout Reference View
C
1.27
5.602.41
1.75
0.65
2.41 NOM
3.30 NOM
C
SEATING
PLANE
1.27 BSC
GAUGE PLANE
SEATING PLANE
ATerminal #1 mark area
B
C
B
21
8
C
SEATING
PLANE
C0.10
8X
0.25 BSC
1.04 REF
1.70 MAX
For Reference Only; not for tooling use (reference MS-012BA)
Dimensions in millimeters
Dimensions exclusive of mold flash, gate burrs, and dambar protrusions
Exact case and lead configuration at supplier discretion within limits shown
4.90 ±0.10
3.90 ±0.10 6.00 ±0.20
0.51
0.31 0.15
0.00
0.25
0.17
1.27
0.40
8°
0°
Exposed thermal pad (bottom surface); dimensions may vary with device
A
Branded Face
Pin Out Diagram
1
2
3
4
8
7
6
5
BOOT
ENB
TSET
GND
VIN
LX
VBIAS
FB
Pad
Terminal List Table
Number Name Description
1 BOOT Gate drive boost node
2 ENB On/off control; logic input
3 TSET Off-time setting
4 GND Ground
5 FB Feedback for adjustable regulator
6 VBIAS Bias supply input
7 LX Buck switching node
8 VIN Supply input
Copyright ©2006-2009, Allegro MicroSystems, Inc.
The products described here are manufactured under one or more U.S. patents or U.S. patents pending.
Allegro MicroSystems, Inc. reserves the right to make, from time to time, such de par tures from the detail spec i fi ca tions as may be required to per-
mit improvements in the per for mance, 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 in for ma tion in clud ed herein is believed to be ac cu rate and reliable. How ev er, Allegro MicroSystems, Inc. assumes no re spon si bil i ty for its use;
nor for any in fringe ment of patents or other rights of third parties which may result from its use.
For the latest version of this document, visit our website:
www.allegromicro.com
