A3250 and A3251: Field Programmable, Chopper-Stabilized, Unipolar Hall-Effect Switches

A3250 and A3251
Field-Programmable, Chopper-Stabilized
Unipolar Hall-Effect Switches
Discontinued Product
This device is no longer in production. The device should not be
purchased for new design applications. Samples are no longer available.
Date of status change: October 31, 2011
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, Inc. 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, Inc. assumes no responsibility for its use; nor for any infringements of patents or other rights of third parties which may result from its use.
A3250 and A3251
Field-Programmable, Chopper-Stabilized
Unipolar Hall-Effect Switches
Features and Benefits
Description
▪ Chopper stabilization for stable switchpoints throughout
operating temperature range
▪ Externally programmable operate point (through VCC pin)
▪ On-board voltage regulator for 4.2 V to 24 V operation
▪ On-chip protection against:
▫ Supply transients
▫ Output short-circuits
▫ Reverse-battery condition
The A3250 and A3251 are field-programmable, chopperstabilized, unipolar Hall-effect switches designed for use in
high-temperature applications. These devices use a chopper-stabilization technique to eliminate offset inherent in
single-element devices.
Package: 3-pin SOT89 (suffix LT) and
3-pin SIP (suffix UA)
The A3250 and A3251 are externally programmable
devices. The devices have a wide range of programmability of the magnetic operate point (BOP) while the hysteresis
remains fixed. This advanced feature allows for optimization of the device switchpoint and can drastically reduce
the effects of variations found in a production environment,
such as magnet and device placement tolerances.
These devices provide on-chip transient protection. A Zener
clamp on the power supply protects against overvoltage
conditions on the supply line. These devices also include
short-circuit protection on the output.
The output of the A3250 switches LOW when subjected
to a south-polarity magnetic field with a flux density that
exceeds the threshold for BOP , and switches HIGH when the
field drops below the magnetic release point, BRP . The output of the A3251 has the opposite polarity, switching HIGH
in a south-polarity magnetic field that BOP , and switching
LOW when the field drops below BRP .
Continued on the next page…
Not to scale
Functional Block Diagram
VCC
Program/Lock
Programming
Logic
Regulator
Amp
Sample and Hold
Dynamic Offset
Cancellation
Offset Adjust
VOUT
Current Limit
Low-Pass
Filter
GND
3250-DS Rev. 11
A3250 and
A3251
Field-Programmable, Chopper-Stabilized,
Unipolar Hall-Effect Switches
Description (continued)
The other differences in the devices are the power-on state. The
A3250 powers-on in the HIGH state, while the A3251 powers-on
in the LOW state.
Three package styles provide a magnetically optimized package
for most applications. Type LT is a miniature SOT89/TO-243AA
surface mount package that is thermally enhanced with an
exposed ground tab, and type UA is a three-lead ultramini SIP
for through-hole mounting. The packages are lead (Pb) free, with
100% matte tin plated leadframes (suffix, –T).
Selection Guide
A3250LLTTR-T
VOUT
Packing1
Package
TA
(ºC)
Power-On
Running2
7-in. reel, 1000 pieces/reel
Surface mount
–40 to 150
High
Low
Part Number
1Contact Allegro for additional packing options.
2In south polarity magnetic field of sufficient strength.
Absolute Maximum Ratings
Characteristic
Symbol
Notes
Rating
Units
Supply Voltage
VCC
26.5
V
Reverse Supply Voltage
VRCC
–18
V
VZ
30
V
IOUT
20
mA
Zener Overvoltage
Output Current
Magnetic Flux Density
B
Unlimited
G
Range E
–40 to 85
ºC
Range L
Operating Ambient Temperature
TA
–40 to 150
ºC
Maximum Junction Temperature
TJ(max)
165
ºC
Tstg
–65 to 170
ºC
Storage Temperature
Pin-out Diagrams
1
2
Terminal List
Number
Name
Function
1
VCC
Connects power supply to chip
2
GND
Ground
3
VOUT
Device output
3
LT
1
2
3
UA
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
2
A3250 and
A3251
Field-Programmable, Chopper-Stabilized,
Unipolar Hall-Effect Switches
OPERATING CHARACTERISTICS valid over operating TA and VCC, unless otherwise specified
Characteristic
Symbol
Test Conditions
Min.
Typ.
Max.
Units
4.2
–
24
V
ELECTRICAL CHARACTERISTICS
Supply Voltage1
Output Saturation Voltage
Output Leakage Current
VCC
Running mode
VOUT(sat)
IOUT = 20 mA; Switch state = ON
–
175
400
mV
IOFF
VOUT = 24 V; Switch state = OFF
–
–
10
μA
A3250; B < BRP; VOUT = HIGH
–
4.0
7.0
mA
A3251; B > BOP; VOUT = HIGH
–
4.0
7.0
mA
A3250; B > BOP; VOUT = LOW
–
6.0
10.0
mA
A3251; B < BRP; VOUT = LOW
–
6.0
10.0
mA
ICC(off)
Supply Current
ICC(on)
Output Rise Time
tr
RLOAD = 820 Ω, CLOAD = 10 pF
–
–
5.0
μs
Output Fall Time
tf
RLOAD = 820 Ω, CLOAD = 10 pF
–
–
5.0
μs
Chopping Frequency
fC
–
340
–
kHz
Power-Up Time
ton
VOUT = HIGH
–
20
50
μs
Short-circuit protection
60
90
120
mA
A3250; B < BRP, t > ton
–
HIGH
–
mV
A3251; B < BRP, t > ton
–
LOW
–
mV
–20
13
50
G
BOP ≤ 500 gauss
–35
–
35
G
Package TA range = J
5.0
18
35
G
Package TA range = L
5.0
13
35
G
50
–
≥350
G
Switchpoint set
–
6
–
Bit
Programming lock
–
1
–
Bit
–
7.0
–
G
28
–
–
V
VCC = 28 V
–
–
13
mA
VRCC = –18 V, TJ < TJ(max)
–
–
–5.0
mA
Output Current
Limit1,2
Power-On State
IOUT(lim)
POS
MAGNETIC CHARACTERISTICS
Initial Operate Point
BOP
Temperature Drift of BOP
ΔBOP
Hysteresis (BOP – BRP)
Bhys
PROGRAMMING CHARACTERISTICS
Programmable BOP Values3
Number of Programming Bits
Resolution
BOP(prog)
–
BRES
TRANSIENT PROTECTION CHARACTERISTICS
Supply Zener Voltage
VZ
Supply Zener Current
IZ
Reverse Battery Current
IRCC
1
Do not exceed TJ(max): Additional information on power derating is provided in the applications section.
Short-circuit protection is not intended for continuous operation; permanent damage may result.
Device can be used below 50 G but is not guaranteed to be a unipolar switch. It is the responsibility of the programmer to verify that the desired
switchpoint has been achieved.
2
3
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
3
A3250 and
A3251
Field-Programmable, Chopper-Stabilized,
Unipolar Hall-Effect Switches
Typical Characterization Data
All data are taken with A3250 devices, the average of 3 lots, 30 pieces per lot
Average BRP vs. TA
Program Code: 1, VCC = 12 V
30
10
25
5
Average BRP (G)
Average BOP (G)
Average BOP vs. TA
Program Code: 1, VCC = 12 V
20
15
10
5
0
-5
-50
-20
10
40
70
TA (°C)
100
130
0
-5
-10
-15
-20
-50
160
-20
Average BOP vs. TA
Program Code: 8, VCC = 12 V
40
70
TA (°C)
100
130
160
130
160
130
160
Average BRP vs. TA
Program Code: 8, VCC = 12 V
60
75
70
Average BRP (G)
Average BOP (G)
10
65
60
55
50
50
40
30
45
40
-50
-20
10
40
70
TA (°C)
100
130
20
-50
160
-20
130
110
125
105
120
115
110
105
100
-50
-20
10
40
70
TA (°C)
100
40
70
TA (°C)
100
Average BRP vs. TA
Program Code: 16, VCC = 12 V
Average BRP (G)
Average BOP (G)
Average BOP vs. TA
Program Code: 16, VCC = 12 V
10
130
160
100
95
90
85
80
-50
-20
10
40
70
TA (°C)
100
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
4
A3250 and
A3251
Field-Programmable, Chopper-Stabilized,
Unipolar Hall-Effect Switches
Typical Characterization Data
All data are taken with A3250 devices, the average of 3 lots, 30 pieces per lot
Average B hys vs. Temperature
Program Code: 16, VCC = 12 V
35
35
30
30
Average BHYS (G)
Average BHYS (G)
Average B hys vs. Temperature
Program Code: 1, VCC = 12 V
25
20
15
10
5
25
20
15
10
5
0
0
-50
-20
10
40
70
100
130
160
-50
-20
10
TA (°C)
Average B hys vs. Temperature
Program Code: 8, VCC = 12 V
40
70
TA (°C)
100
130
160
Average BOP vs. Temperature
-40°C to 25°C and 150°C to 25°C
30
Average BOP (G)
Average BHYS (G)
35
30
25
20
15
10
20
10
0
Code 1
-10
5
Code 8
-20
0
-50
-20
10
40
TA (°C)
70
100
130
160
-30
Code 16
-40°C to 25°C
150°C to 25°C
TA (°C)
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
5
A3250 and
A3251
Field-Programmable, Chopper-Stabilized,
Unipolar Hall-Effect Switches
Typical Characterization Data
All data are taken with A3250 devices, the average of 3 lots, 30 pieces per lot
Average ICC(on) vs. Temperature
Average ICC(off) vs. Temperature
10
10
ICC(off) @ 3.8 V
ICC(off) (mA)
8
6
4
ICC(on) @ 3.8 V
2
ICC(on) @ 12.0 V
ICC(off) @ 12.0 V
ICC(off) @ 26.5 V
6
4
2
ICC(on) @ 26.5 V
0
-50
-20
10
40
70
100
130
0
-50
160
-20
10
40
70
100
130
160
TA (°C)
TA (°C)
Average VOUT(SAT) vs. Temperature
VCC = 3.8 V, Iout = 20 mA
280
VOUT(SAT) (mV)
ICC(on) (mA)
8
260
240
220
200
180
160
140
-50
-20
10
40
70
100
130
160
TA (°C)
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
6
A3250 and
A3251
Field-Programmable, Chopper-Stabilized,
Unipolar Hall-Effect Switches
THERMAL CHARACTERISTICS may require derating at maximum conditions, see application information
Characteristic
Symbol
RθJA
Package Thermal Resistance
Test Conditions
Value
Units
Package UA, 1-layer PCB with copper limited to solder pads
165
ºC/W
Package LT, 1-layer PCB with copper limited to solder pads
180
ºC/W
Package LT, 2-layer PCB with 0.94 in2 copper each side
78
ºC/W
Power Dissipation
Power Dissipation, PD (m W)
VCC(max)
1-layer PCB, Package LT
(RθJA = 180 ºC/W)
1-layer PCB, Package UA
(RθJA = 165 ºC/W)
2-layer PCB, Package LT
(RθJA = 78 ºC/W)
VCC(min)
40
60
80
100
120
140
160
180
1900
1800
1700
1600
1500
1400
1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
0
2
(R -lay
θJ
A
er
= PC
78 B
ºC , Pa
/W ck
) ag
1-la
(R yer P
CB
θJA =
165 , Pac
1-la
ºC/ kage
y
e
W)
rP
(R
UA
CB
θJA =
,
180 Pac
ºC/ kage
W)
LT
20
40
TA (ºC)
60
e
LT
80
100
120
Temperature (°C)
140
160
180
Hysteresis Curves
A3251
A3250
V+
Hysteresis of ΔVOUT
Switching Due to ΔB
V+
Hysteresis of ΔVOUT
Switching Due to ΔB
VOUT(off)
Switch to High
VOUT
Switch to Low
Switch to Low
Switch to High
VOUT(off)
VOUT(on)(sat)
BRP
BOP
BHYS
B+
BOP
VOUT(on)(sat)
BRP
20
VOUT
Maximum Allowable VCC (V)
Power Derating Curve
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
B+
BHYS
Output voltage in relation to impinging magnetic flux density in a south polarity magnetic
field of sufficient strength. Transition through BOP must precede transition through BRP.
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
7
A3250 and
A3251
Field-Programmable, Chopper-Stabilized,
Unipolar Hall-Effect Switches
Functional Description
Chopper-Stabilized Technique
The Hall circuit is based on a Hall element, a small sheet of
semiconductor material in which a constant bias current flows
when a constant voltage source is applied. The output takes the
form of a voltage measured across the width of the Hall element,
and has negligible value in the absence of a magnetic field.
When a magnetic field is applied with flux lines at right angles
to the current in the Hall element, a small signal voltage directly
proportional to the strength of the magnetic field occurs at the
output of the Hall element.
This small signal voltage is disproportionally small relative to
the offset produced at the input of the device. This makes it very
difficult to process the signal and maintain an accurate, reliable
output over the specified temperature and voltage range. Therefore, it is important to reduce any distortion of the signal that
could be amplified when the signal is processed.
Chopper stabilization is a unique approach used to minimize
input offset on the Hall IC. This technique removes a key
source of output drift due to temperature and mechanical stress,
and produces a 3X reduction in offset in comparison to other,
conventional methods.
This offset reduction chopping technique is based on a signal modulation-demodulation process. The undesired offset
signal is separated from the magnetically-induced signal in the
frequency domain. The offset (and any low-frequency noise)
component of the signal can be seen as signal distortion added
after the signal modulation process has taken place. Therefore,
the DC offset is not modulated and remains a low-frequency
component. Consequently, the signal demodulation process acts
as a modulation process for the offset, causing the magneticallyinduced signal to recover its original spectrum at baseband while
the DC offset becomes a high-frequency signal. Then, the signal
passes using a low-pass filter, while the modulated DC offset is
suppressed.
The advantage of this approach is significant offset reduction,
which desensitizes the Hall IC against the effects of temperature
and mechanical stress. The disadvantage is that this technique
features a demodulator that uses a sample-and-hold block to
store and recover the signal. This sampling process can slightly
degrade the SNR (signal-to-noise ratio) by producing replicas of
the noise spectrum at the baseband. This degradation is a function
of the ratio between the white noise spectrum and the sampling
frequency. The effect of the degradation of the SNR is higher
jitter, also known as signal repeatability. However, the jitter in a
continuous-time device can be 5X that of the A3250/A3251.
Amp
Sample and
Hold / LPF
Regulator
Chopper stabilization circuit (dynamic quadrature offset cancellation)
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
8
A3250 and
A3251
Field-Programmable, Chopper-Stabilized,
Unipolar Hall-Effect Switches
V+
Programming Protocol
VPH
The operate switchpoint, BOP , can be field-programmed. To do
so, a coded series of voltage pulses through the VCC pin is used
to set bitfields in onboard registers. The effect on the device
output can be monitored, and the registers can be cleared and
set repeatedly until the required BOP is achieved. To make the
setting permanent, bitfield-level solid state fuses are blown, and
finally, a device-level fuse is blown, blocking any further coding. It is not necessary to program the release switchpoint, BRP ,
because the difference between BOP and BRP , referred to as the
hysteresis, BHYS , is fixed.
The range of values between BOP(min) and BOP(max) is scaled to
64 increments. The actual change in magnetic flux (G) represented by each increment is indicated by BRES (see the Operating
Characteristics table; however, testing is the only method for
verifying the resulting BOP). For programming, the 64 increments are individually identified using 6 data bits, which are
physically represented by 6 bitfields in the onboard registers.
By setting these bitfields, the corresponding calibration value is
programmed into the device.
Three voltage levels are used in programming the device: a low
voltage, VPL , a minimum required to sustain register settings; a
mid-level voltage, VPM , used to increment the address counter
in the device; and a high voltage, VPH , used to separate sets of
VPM pulses (when short in duration) and to blow fuses (when
long in duration). A fourth voltage level, essentially 0 V, is used
to clear the registers between pulse sequences. The pulse values
are shown in the Programming Protocol Characteristics table and
in figure 1.
VPM
VPL
Td(P)
0
Td(0)
Td(1)
t
Figure 1. Pulse amplitudes and durations
Additional information on device programming and programming products is available on www. allegromicro.com. Programming hardware is available for purchase, and programming
software is available free of charge.
Code Programming. Each bitfield must be individually set. To
do so, a pulse sequence must be transmitted for each bitfield that
is being set to 1. If more than one bitfield is being set to 1, all
pulse sequences must be sent, one after the other, without allowing VCC to fall to zero (which clears the registers).
The same pulse sequence is used to provisionally set bitfields as
is used to permanently set bitfield-level fuses. The only difference is that when provisionally setting bitfields, no fuse-blowing
pulse is sent at the end of the pulse sequence.
PROGRAMMING PROTOCOL CHARACTERISTICS, TA = 25ºC, unless otherwise noted
Characteristic
Symbol
Min.
Typ.
Max.
Units
4.5
5.0
5.5
V
VPM
10
11
12
V
VPH
23
25
26
V
VPL
Programming Voltage1
Programming Current2
Pulse Width
Test Conditions
Minimum voltage range during programming
IPP
Maximum supply current during programming
–
500
–
mA
td(0)
OFF time between programming bits
20
–
–
μs
td(1)
Pulse duration (ON time) for enable, address, fuse
blowing or lock bits
20
–
–
μs
td(P)
Pulse duration (ON time) for fuse blowing
100
300
–
μs
Pulse Rise Time
tr
VPL to VPM; VPL to VPH
11
–
–
μs
Pulse Fall Time
tf
VPM to VPL; VPH to VPL
5
–
–
μs
1Programming
voltages are measured at the VCC pin.
capacitor with a minimum capacitance of 0.1 μF must be connected from VCC to the GND pin of the device in order to provide
the current necessary to blow the fuse.
2A bypass
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
9
A3250 and
A3251
Field-Programmable, Chopper-Stabilized,
Unipolar Hall-Effect Switches
The pulse sequences consist of the following groups of pulses:
1. An enable sequence.
2. A bitfield address sequence.
3. When permanently setting the bitfield, a long VPH fuse-blowing pulse. (Note: Blown bit fuses cannot be reset.)
4. When permanently setting the bitfield, the level of VCC must
be allowed to drop to zero between each pulse sequence, in
order to clear all registers. However, when provisionally setting bitfields, VCC must be maintained at VPL between pulse
sequences, in order to maintain the prior bitfield settings while
preparing to set additional bitfields.
Bitfields that are not set are evaluated as zeros. The bitfield-level
fuses for 0 value bitfields are never blown. This prevents inad-
vertently setting the bitfield to 1. Instead, blowing the devicelevel fuse protects the 0 bitfields from being accidentally set in
the future.
When provisionally trying the calibration value, one pulse
sequence is used, using decimal values. The sequence for setting
the value 510 is shown in figure 2.
When permanently setting values, the bitfields must be set individually, and 510 must be programmed as binary 101. Bit 3 is
set to 1 (0001002, which is 410), then bit 1 is set to 1 (0000012,
which is 110). Bit 2 is ignored, and so remains 0.Two pulse
sequences for permanently setting the calibration value 5 are
shown in figure 3. The final VPH pulse is maintained for a longer
period, enough to blow the corresponding bitfield-level fuse.
V+
VPH
VPM
VPL
0
Enable
Address
Try 510
Optional
Monitoring
Clear
t
Figure 2. Pulse sequence to provisionally try calibration value 5.
V+
VPH
VPM
VPL
Address
0
Enable
Address
Encode 001002 (410)
Blow
Enable
Blow
Encode 000012 (110)
Figure 3. Pulse sequence to permanently encode calibration value 5 (101 binary, or
bitfield address 3 and bitfield address 1).
t
10
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
A3250 and
A3251
Field-Programmable, Chopper-Stabilized,
Unipolar Hall-Effect Switches
V+
VPH
Enabling Addressing Mode. The first segment of code is a
keying sequence used to enable the bitfield addressing mode. As
shown in figure 4, this segment consists of one short VPH pulse,
seven or more VPM pulses, and one short VPH pulse, with no
supply interruptions. This sequence is designed to prevent the
device from being programmed accidentally, such as by noise on
the supply line.
VPM
VPL
Minimum 7 pulses
0
t
Figure 4. Addressing mode enable pulse sequence
V+
Address Selection. After addressing mode is enabled, the
target bitfield address, is indicated by a series of VPM pulses,
as shown in figure 3. When provisionally trying a value, this
sequence is followed by a short VPH pulse, which serves to
delimit the address and set the corresponding bitfield. When
permanently setting a bitfield, the VPH pulse is continued for a
longer period of time, suffienct to not only set the bitfield to 1,
but also to blow the bitfield fuse.
VPH
Address 1
Address 2
Address n ( ≤ 63)
VPM
VPL
0
t
Figure 5. Pulse sequence to select addresses
V+
Falling edge of final BOP address digit
VPH
Lock Bit Programming. After the desired BOP calibration value
is programmed, and all of the corresponding bitfield-level fuses
are blown, the device-level fuse should be blown. To do so, the
lock bit (bitfield address 65) should be encoded as 1 and have
its fuse blown. This is done in the same manner as permanently
setting the other bitfields, as shown in figure 6.
VPM
VPL
7 pulses
65 pulses
0
Enable
Address
Blow
Encode Lock Bit
Figure 6. Pulse sequence to encode lock bit
t
11
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
A3250 and
A3251
Field-Programmable, Chopper-Stabilized,
Unipolar Hall-Effect Switches
Application Information
For additional general application information, visit the Allegro
MicroSystems Web site at www. allegromicro.com.
Typical Application Circuit
RS
100 Ω
It is strongly recommended that an external ceramic bypass
capacitor, CBYP, in the range of 0.01 μF to 0.1 μF be connected
between the VCC pin and the supply and GND pin to reduce
both external noise and noise generated by the chopper-stabilization technique. (The diagram at the right shows CBYP at 0.1 μF.)
CBYP should be installed so that the traces that connect it to the
A3250/A3251 are no greater than 5 mm in length. (For programming the device, the capacitor may be further away from the
device, including mounting on the board used for programming
the device.)
The series resistor RS, in combination with CBYP creates a filter
for EMI pulses. (Additional information on EMC is provided
on the Allegro MicroSystems Web site.) RS will have a drop
of approximately 800 mV. This must be taken into consideration when determining the minimum VCC requirement for the
A3250/A3251. The pull-up resistor, RL, should be chosen to
limit the current through the output transistor; do not exceed the
maximum continuous output current of the device.
5V
A
VSupply
CBYP
0.1 μF
VCC
A3250/A3251
RL
1.2 kΩ
VOUT
GND
A
A
Maximum separation 5 mm
from CBYP to device
Typical application circuit
12
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
A3250 and
A3251
Field-Programmable, Chopper-Stabilized,
Unipolar Hall-Effect Switches
Power Derating
The device must be operated below the maximum junction
temperature of the device, TJ(max). Under certain combinations of
peak conditions, reliable operation may require derating supplied power or improving the heat dissipation properties of the
application. This section presents a procedure for correlating
factors affecting operating TJ. (Thermal data is also available on
the Allegro MicroSystems Web site.)
The Package Thermal Resistance, RJA, is a figure of merit summarizing the ability of the application and the device to dissipate
heat from the junction (die), through all paths to the ambient air.
Its primary component is the Effective Thermal Conductivity,
K, of the printed circuit board, including adjacent devices and
traces. Radiation from the die through the device case, RJC, is
relatively small component of RJA. Ambient air temperature,
TA, and air motion are significant external factors, damped by
overmolding.
The effect of varying power levels (Power Dissipation, PD), can
be estimated. The following formulas represent the fundamental
relationships used to estimate TJ, at PD.
PD = VIN × IIN

(1)
T = PD × RJA (2)
TJ = TA + ΔT
Example: Reliability for VCC at TA = 150°C, package UA, using
minimum-K PCB.
Observe the worst-case ratings for the device, specifically:
RJA = 165°C/W, TJ(max) = 165°C, VCC(max) = 24 V, and
ICC(max) = 10 mA.
Calculate the maximum allowable power level, PD(max). First,
invert equation 3:
Tmax = TJ(max) – TA = 165 °C – 150 °C = 15 °C
This provides the allowable increase to TJ resulting from internal
power dissipation. Then, invert equation 2:
PD(max) = Tmax ÷ RJA = 15°C ÷ 165 °C/W = 91 mW
Finally, invert equation 1 with respect to voltage:
VCC(est) = PD(max) ÷ ICC(max) = 91 mW ÷ 10 mA = 9 V
The result indicates that, at TA, the application and device can
dissipate adequate amounts of heat at voltages ≤VCC(est).
Compare VCC(est) to VCC(max). If VCC(est) ≤ VCC(max), then reliable operation between VCC(est) and VCC(max) requires enhanced
RJA. If VCC(est) ≥ VCC(max), then operation between VCC(est) and
VCC(max) is reliable under these conditions.
(3)
For example, given common conditions such as: TA= 25°C,
VCC = 12 V, ICC = 4 mA, and RJA = 165 °C/W, then:
PD = VCC × ICC = 12 V × 4 mA = 48 mW

T = PD × RJA = 48 mW × 165 °C/W = 8°C
TJ = TA + T = 25°C + 8°C = 33°C
A worst-case estimate, PD(max), represents the maximum allowable power level (VCC(max), ICC(max)), without exceeding TJ(max),
at a selected RJA and TA.
13
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
A3250 and
A3251
Field-Programmable, Chopper-Stabilized,
Unipolar Hall-Effect Switches
Package LT, 3-Pin SOT89
4.50±0.10
+0.10
1.73 –0.11
2.50
2.00
A
2.24
0.80
1.14
+0.15
4.10 –0.16
+0.15
2.45 –0.16
+0.09
2.20 –0.07
2.60
+0.20
1.00 –0.11
1
2
3
4.60
Parting
Line
1.00
+0.04
0.40 –0.05
0.70
1.50±0.10
B
1.50
PCB Layout Reference View
Basic pads for low-stress, not self-aligning
Additional pad for low-stress, self-aligning
Additional area for IPC reference layout
0.42±0.06
0.50±0.06
2X 1.50 BSC
For Reference Only; not for tooling use (reference JEDEC. TO-243AA)
Dimensions in millimeters
Dimensions exclusive of mold flash, gate burrs, and dambar protrusions
Exact case and lead configuration at supplier discretion within limits shown
A
Active Area Depth, 0.78 mm REF
B
C
Reference land pattern layout (reference IPC7351 SOT89N);
All pads a minimum of 0.20 mm from all adjacent pads;
adjust as necessary to meet application process
requirements and PCB layout tolerances
Branding scale and appearance at supplier discretion
D
Hall element, not to scale
NNT
1
C
Standard Branding Reference View
= Supplier emblem
N = Last two digits of device part number
T = Temperature code
14
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
A3250 and
A3251
Field-Programmable, Chopper-Stabilized,
Unipolar Hall-Effect Switches
Package UA, 3-Pin SIP
+0.08
4.09 –0.05
45°
B
C
E
2.06
1.52 ±0.05
1.45 E
Mold Ejector
Pin Indent
+0.08
3.02 –0.05
E
Branded
Face
45°
1
2.16
MAX
D Standard Branding Reference View
= Supplier emblem
N = Last two digits of device part number
T = Temperature code
0.79 REF
A
0.51
REF
NNT
1
2
3
+0.03
0.41 –0.06
15.75 ±0.51
For Reference Only; not for tooling use (reference DWG-9049)
Dimensions in millimeters
Dimensions exclusive of mold flash, gate burrs, and dambar protrusions
Exact case and lead configuration at supplier discretion within limits shown
A
Dambar removal protrusion (6X)
B Gate burr area
C Active Area Depth, 0.50 mm REF
+0.05
0.43 –0.07
D
Branding scale and appearance at supplier discretion
E
Hall element, not to scale
1.27 NOM
Copyright ©2004-2010, Allegro MicroSystems, Inc.
Allegro MicroSystems, Inc. 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, Inc. assumes no responsibility for its use;
nor for any infringement 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
15
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
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