LM3813 [NSC]
Precision Current Gauge IC with Ultra Low Loss Sense Element and PWM Output; 精密电流计IC，提供超低损耗检测元件和PWM输出
| 型号： | LM3813 |
| 厂家： | 
National Semiconductor |
| 描述： | Precision Current Gauge IC with Ultra Low Loss Sense Element and PWM Output |
| 文件： | 总14页 (文件大小：261K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
June 1999
LM3812/LM3813
Precision Current Gauge IC with Ultra Low Loss Sense
Element and PWM Output
General Description
Key Specifications
n Ultra low insertion loss (typically 0.004Ω)
n 2V to 5.25V supply range
The LM3812/LM3813 Current Gauges provide easy to use
precision current measurement with virtually zero insertion
loss (typically 0.004Ω). The LM3812 is used for high-side
sensing and the LM3813 is used for low-side sensing.
±
n
2% accuracy at room temperature (includes accuracy
of the internal sense element) (LM3812-1.0,
LM3813-1.0)
A Delta Sigma analog to digital converter is incorporated to
precisely measure the current and to provide a current aver-
aging function. Current is averaged over 50 msec time peri-
ods in order to provide immunity to current spikes. The ICs
have a pulse-width modulated (PWM) output which indicates
the current magnitude and direction. The shutdown pin can
be used to inhibit false triggering during start-up, or to enter
a low quiescent current mode.
n Low quiescent current in shutdown mode (typically
2.5 µA)
n 50 msec sampling interval
Features
n No external sense element required
n PWM output indicates the current magnitude and
direction
n PWM output can be interfaced with microprocessors
n Precision ∆Σ current-sense technique
n Low temperature sensitivity
The LM3812 and LM3813 are factory-set in two different cur-
rent options. The sense range is −1A to +1A or −7A to +7A.
The sampling interval for these parts is 50ms. If faster sam-
pling is desired, please refer to the data sheets for the part
numbers LM3814 and LM3815.
n Internal filtering rejects false trips
n Internal Power-On-Reset (POR)
Applications
n Battery charge/discharge gauge
n Motion control diagnostics
n Power supply load monitoring and management
n Resettable smart fuse
Connection Diagrams
DS100122-1
DS100122-3
Top View
LM3812
for High-Side Sensing
Top View
LM3813
for Low-Side Sensing
© 1999 National Semiconductor Corporation
DS100122
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Ordering Information
NS
Package
Number‡
M08A
Order No.#
Sense
Range
Sampling
Sensing
Method
Package
Type
SO-8
SO-8
SO-8
SO-8
SO-8
SO-8
SO-8
SO-8
*
Interval
Supplied As:
95 units in Rails
±
±
±
±
±
±
±
±
LM3812M-1.0
LM3812MX-1.0
LM3812M-7.0
LM3812MX-7.0
LM3813M-1.0
LM3813MX-1.0
LM3813M-7.0
LM3813MX-7.0
1A
1A
7A
7A
1A
1A
7A
7A
50 ms
50 ms
50 ms
50 ms
50 ms
50 ms
50 ms
50 ms
High-side
High-side
High-side
High-side
Low-side
Low-side
Low-side
Low-side
M08A
2.5k units on Tape and Reel
95 units in Rails
M08A
M08A
2.5k units on Tape and Reel
95 units in Rails
M08A
M08A
2.5k units on Tape and Reel
95 units in Rails
M08A
M08A
2.5k units on Tape and Reel
#
Suffix M indicates that the part is available in Surface Mount package. Suffix X indicates that the part is available in 2.5k units
on Tape and Reel.
*
Current is sampled over a fixed interval. The average current during this interval is indicated by the duty cycle of the PWM output
during next interval.
‡
The Package code M08A is internal to National Semiconductor and indicates an 8-lead surface mount package, SO-8.
Pin Description (High-Side, LM3812)
Pin
1
Name
SENSE+, VDD
Function
High side of internal current sense, also supply voltage.
Low side of internal current sense.
2
SENSE−
FLTR+
FLTR−
SD
3
Filter input — provides anti-aliasing for delta sigma modulator.
Filter input.
4
5
Shutdown pin. Connected to VDD through a pull up resistor for normal operation.
When low, the IC goes into a low current mode (typically 3 µA).
6
7
8
PWM
GND
GND
PWM output indicates the current magnitude and direction.
Ground
Ground
Pin Description (Low-Side, LM3813)
Pin
1
Name
SENSE+, GND
SENSE−
FLTR+
Function
High side of internal current sense, also ground.
Low side of internal current sense.
2
3
Filter input – provides anti-aliasing for delta sigma modulator.
Filter input.
4
FLTR−
5
SD
Shutdown pin. Connected to VDD through a pull up resistor for normal operation.
When low, the IC goes into a low current mode (typically 3 µA).
6
7
8
PWM
GND
VDD
PWM output indicates the current magnitude and direction.
Ground
VDD (supply)
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2
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Maximum Junction Temperature
Storage Temperature
150˚C
−65˚C to +150˚C
260˚C
Lead Temperature (Soldering, 10 sec)
Operating Ratings (Note 1)
Absolute Maximum Supply Voltage
Power Dissipation
5.5V
(Note 2)
1.5 kV
10A
Input Voltage
2.0V to 5.25V
7A
ESD Susceptibility (Note 3)
Sense Current (peak, for 200 msec) (Note 4)
Sink Current for PWM pin
Voltage on Pin 5
Sense Current (continuous) (Note 4)
Junction Temperature Range
−40˚C to +125˚C
1mA
5.25V
Electrical Characteristics
LM3812-1.0, LM3813-1.0
=
VDD 5.0V for the following specifications. Supply bypass capacitor is 1 µF and filter capacitor is 0.1 µF.
Typ
(Note 5)
Limit
(Note 6)
Symbol
Parameter
Conditions
at 0.9A current
Units
IACC
Average Current Accuracy
(Note 7)
0.9
A
0.882 / 0.864
0.918 / 0.936
A (min)
A (max)
mA
en
Effective Output Noise (rms)
2
LM3812-7.0, LM3813-7.0
=
VDD 5.0V for the following specifications. Supply bypass capacitor is 1 µF and filter capacitor is 0.1 µF.
Typ
(Note 5)
Limit
(Note 6)
Symbol
Parameter
Conditions
Units
IACC
Average Current Accuracy
(Note 7)
at 2.5A current (Note 8)
2.5
A
2.400 / 2.350
2.600 / 2.650
A (min)
A (max)
mA
en
Effective Output Noise (rms)
20
Common Device Parameters
=
Unless otherwise specified, VDD 5.0V for the following specifications. Supply bypass capacitor is 1 µF and filter capacitor is
0.1 µF.
Typ
(Note 5)
Limit
(Note 6)
Symbol
Parameter
Conditions
Units
=
IQ1
Quiescent Current
Normal Mode, SD high
100
µA
µA (max)
µA
160
10
=
IQ2
Quiescent Current
Shutdown Mode, SD low
2.5
µA (max)
%
DRES
tS
PWM Resolution
Sampling Time
0.1
52
ms
40
80
ms (min)
ms (max)
Hz
fP
Frequency of PWM Waveform
20
12.5
25
Hz (min)
Hz (max)
V
VTH
VTL
Threshold High Level for SD
Threshold Low Level for SD
1.2
1.3
1.8
0.7
V (min)
V
V (max)
3
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Common Device Parameters (Continued)
=
Unless otherwise specified, VDD 5.0V for the following specifications. Supply bypass capacitor is 1 µF and filter capacitor is
0.1 µF.
Typ
(Note 5)
Limit
(Note 6)
Symbol
Parameter
Conditions
Units
=
VOH
Logic High Level for PWM
Load current 1 mA, 2V ≤ VDD
5.25V
≤
VDD − 0.05
0.04
V
VDD − 0.2
0.2
V (min)
=
VOL
Logic Low Level for PWM
Sink current 1 mA, 2V ≤ VDD
≤
V
V (max)
Ω
5.25V
=
PI
Insertion Loss
ISENSE 1A (Note 9)
0.004
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is in-
tended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see Electrical Characteristics. The guar-
anteed specifications apply only for the test conditions listed. Some performance characteristics may degrade when the device is not operated under the listed test
conditions.
Note 2: At elevated temperatures, devices must be derated based on package thermal resistance. The device in the surface-mount package must be derated at
=
θ
150˚C/W (typically), junction-to-ambient.
JA
Note 3: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin.
Note 4: The absolute maximum peak and continuous currents specified are not tested. These specifications are dependent on the θ , which is 150˚C/W for the S08
JA
package.
=
Note 5: Typical numbers are at 25˚C and represent the most likely parametric norm. Specifications in standard type face are for T 25˚C and those with boldface
J
type apply over full operating temperature ranges.
Note 6: Limits are 100% production tested at 25˚C. Limits over the operating temperature range are guaranteed through correlation using Statistical Quality Control
(SQC) methods. The limits are used to calculate National’s Averaging Outgoing Quality Level (AOQL).
Note 7: There is a variation in accuracy over time due to thermal effects. Please refer to the “PWM Output and Current Accuracy” section for more information.
Note 8: The PWM accuracy for LM3812-7.0 and LM3813-7.0 depends on the amount of copper area under pins 1 and 2, and the layout. Please refer to the “PWM
Output and Current Accuracy” section for more information.
Note 9: The tolerance of the internal lead frame resistor is corrected internally. The temperature coefficient of this resistor is 2600 ppm/˚C.
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4
Typical Performance Characteristics Supply bypass capacitor is 0.1 µF and filter capacitor is 0.1 µF.
Measured Current vs Actual Current
(LM3812-1.0 and LM3813-1.0)
Measured Current vs Actual Current
(LM3812-7.0 and LM3813-7.0)
DS100122-15
DS100122-24
PWM Frequency vs Supply Voltage
PWM Frequency vs Temperature
DS100122-16
DS100122-13
Operating Current vs Supply Voltage
Shutdown Current vs Supply Voltage
DS100122-18
DS100122-19
5
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Typical Performance Characteristics Supply bypass capacitor is 0.1 µF and filter capacitor is
0.1 µF. (Continued)
Operating Current vs Temperature
Shutdown Current vs Temperature
DS100122-20
DS100122-21
Current vs Duty Cycle
Accuracy vs Supply Voltage
DS100122-22
DS100122-28
Accuracy vs Temperature (LM3812-1.0 and LM3813-1.0) Accuracy vs Temperature (LM3812-7.0 and LM3813-7.0)
DS100122-29
DS100122-30
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6
Typical Performance Characteristics Supply bypass capacitor is 0.1 µF and filter capacitor is
0.1 µF. (Continued)
Error vs Current (LM3812-1.0 and LM3813-1.0)
(Note 10)
Error vs Current (LM3812-7.0 and LM3813-7.0)
(Note 10)
DS100122-27
DS100122-31
Note 10: These curves represent a statistical average such that the noise is insignificant.
Typical Application Circuits In the application circuits, the 0.1 µF ceramic capacitor between pins 1 and 8
is used for bypassing, and the 0.1 µF ceramic capacitor between pins 3 and 4 is used for filtering. Shutdown (SD) is tied to
VDD through a 10 kΩ resistor.
DS100122-5
FIGURE 1. High Side Sense
DS100122-6
FIGURE 2. Low Side Sense
7
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Typical Application Circuits In the application circuits, the 0.1 µF ceramic capacitor between pins 1 and 8
is used for bypassing, and the 0.1 µF ceramic capacitor between pins 3 and 4 is used for filtering. Shutdown (SD) is tied to
VDD through a 10 kΩ resistor. (Continued)
DS100122-7
FIGURE 3. Paralleling LM3812 for Higher Load Current
=
ITOTAL 2.2(D1−0.5)IMAX + 2.2(D2−0.5)IMAX
where D1 is the duty cycle of PWM1 and D2 is the duty cycle of PWM2.
Please refer to the Product Operation section for more information.
DS100122-8
FIGURE 4. High Voltage Operation — VIN Greater Than 5.25V (High Side Sense)
(PWM output is referred to Pin 7)
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8
Typical Application Circuits In the application circuits, the 0.1 µF ceramic capacitor between pins 1 and 8
is used for bypassing, and the 0.1 µF ceramic capacitor between pins 3 and 4 is used for filtering. Shutdown (SD) is tied to
VDD through a 10 kΩ resistor. (Continued)
DS100122-9
FIGURE 5. High Voltage Operation — VIN Greater Than 5.25V (Low Side Sense)
9
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Product Operation
The current is sampled by the delta-sigma modulator, as il-
lustrated in Figure 6. The pulse density output of the
delta-sigma modulator is digitally filtered. The digital output
is then compared to the output of a digital ramp generator.
This produces a PWM output. The duty cycle of the PWM
output is proportional to the amount of current flowing. A duty
cycle of 50% indicates zero current flow. If the current is flow-
ing in positive direction, the duty cycle will be greater than
50%. Conversely, the duty cycle will be less than 50% for
currents flowing in the negative direction. A duty cycle of
95.5% (4.5%) indicates the current is at IMAX (−IMAX). The IC
can sense currents from −IMAX to +IMAX. Options for IMAX are
1A or 10A. The sense current is given by:
The user should note that, while the LM3812-7.0/
LM3813-7.0 will read 10A full scale, it is rated for 10A op-
eration for a duration of no more than 200 msec, and 7A
operation continuously.
In this IC, the current is averaged over 50 msec time slots.
Hence, momentary current surges of less than 50 msec are
tolerated.
This is
a sampled data system which requires an
anti-aliasing filter, provided by the filter capacitor.
The delta-sigma modulator converts the sensed current to
the digital domain. This allows digital filtering, and provides
immunity to current and noise spikes. This type of filtering
would be difficult or impossible to accomplish on an IC with
analog components.
=
ISENSE 2.2 (D−0.5)(IMAX
)
where D is the duty cycle of the PWM waveform, and IMAX is
the full scale current (1A or 10A). Similarly, the duty cycle is
given by:
When ordering, the user has to specify whether the part is
being used for low-side or high-side sense. The user also
needs to specify the full scale value. See the Ordering Infor-
mation table for details.
=
D
[ISENSE/(2.2 IMAX)] + 0.5
For quick reference, see the Conversion Tables in Table 1
and Table 2.
DS100122-10
FIGURE 6. Functional block diagram of LM3812 and LM3813
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10
ment and the die gets larger, and an error develops. Eventu-
ally the temperature difference reaches steady state, which
accounts for the under-damped exponential response.
PWM Output and Current
Accuracy
Offset
The PWM output is quantized to 1024 levels. Therefore, the
duty cycle can change only in increments of 1/1024.
There is a one-half (0.5) quantization cycle delay in the out-
put of the PWM circuitry. That is to say that instead of a duty
cycle of N/1024, the duty cycle actually is (N+1⁄
)/1024.
2
The quantization error can be corrected for if a more precise
result is desired. To correct for this error, simply subtract
1/2048 from the measured duty cycle.
1
The extra half cycle delay will show up as a DC offset of
⁄
2
bit if it is not corrected for. This is approximately 1.1 mA for 1
Amp parts, and 11 mA for 7 Amp parts.
Jitter
DS100122-23
In addition to quantization, the duty cycle will contain some
jitter. The jitter is quite small (for example, the standard de-
viation of jitter is only 0.1% for the LM3812/13-1.0). Statisti-
cally the jitter can cause an error in a current sample. Be-
cause the jitter is a random variable, the mean and standard
deviation are used. The mean, or average value, of the jitter
is zero. The standard deviation (0.1%) can be used to define
the peak error caused from jitter.
FIGURE 7. Transient Response to 7 Amp Step Current
Accuracy versus Noise
The graph shown in Figure 8 illustrates the typical response
±
of 1 Ampere current gauges. In this graph, the horizontal
axis indicates time, and the vertical axis indicates measured
current (the PWM duty cycle has been converted to current).
The graph was generated for an actual current of 500 mA.
The “crest factor” has often been used to define the maxi-
mum error caused by jitter. The crest factor defines a limit
within which 99.7% of the samples fall. The crest factor is de-
The difference between successive readings manifests itself
as jitter in the PWM output or noise in the current measure-
ment (when duty cycle of the PWM output is converted to
current).
±
fined as 0.3% error in the duty cycle.
Since the jitter is a random variable, averaging multiple out-
puts will reduce the effective jitter. Obeying statistical laws,
the jitter is reduced by the square root of the number of read-
ings that are averaged. For example, if four readings of the
duty cycle are averaged, the resulting jitter (and crest factor)
are reduced by a factor of two.
The accuracy of the measurement depends on the noise in
the current waveform. The accuracy can be improved by av-
eraging several outputs. Although there is variation in suc-
cessive readings, a very accurate measurement can be ob-
tained by averaging the readings. For example, on
averaging the readings shown in this example, the average
current measurement is 502.3 mA (Figure 8). This value is
very close to the actual value of 500 mA. Moreover, the ac-
curacy depends on the number of readings that are
averaged.
Jitter and Noise
Jitter in the PWM output appears as noise in the current
measurement. The Electrical Characteristics show noise
measured in current RMS (root mean square). Arbitrarily one
could specify PWM jitter, as opposed to noise. In either case
the effect results in a random error in an individual current
measurement.
Noise, just like jitter, can be reduced by averaging many
readings. The RMS value of the noise corresponds to one
standard deviation. The “crest factor” can be calculated in
±
terms of current, and is equal to 3 sigma (RMS value of the
noise).
Noise will also be reduced by averaging multiple readings,
and follows the statistical laws of a random variable.
Accuracy of 7A Versions
The graph of Figure 7 shows two possible responses to a 7A
current step. The flat response shows basically a 7A level
with some noise. This is what is possible with a good thick
trace and a good thermal connection to the IC on the sense
pins.
DS100122-26
FIGURE 8. Typical Response of LM3812-1.0/LM3813-1.0
The second trace that asymptotically approaches a higher
value shows what can happen under extremely poor thermal
conditions. Here a very small wire connects the IC to the cur-
rent source. The very small wire does not allow heat in the
sense resistor to dissipate. Hence, as the sense resistor
heats up, a temperature difference between the sense ele-
11
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1
quantization error of
⁄2 bit is not shown in these tables.
Look-Up Tables
The following tables show how to convert the duty cycle of
the PWM output to a current value, and vice versa. The
Please see the “PWM Output and Current Accuracy” section
for more details.
TABLE 1. Current to Duty Cycle Conversion Table
Duty Cycle Sense Current
Sense Current
Duty Cycle
*
%
*
(Amps)
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
(
)
(Amps)
-1.00
-0.95
-0.90
-0.85
-0.80
-0.75
-0.70
-0.65
-0.60
-0.55
-0.50
-0.45
-0.40
-0.35
-0.30
-0.25
-0.20
-0.15
-0.10
-0.05
-0.00
( )
%
95.5
93.2
90.9
88.6
86.4
84.1
81.8
79.5
77.3
75.0
72.7
70.5
68.2
65.9
63.6
61.4
59.1
56.8
54.5
52.3
50.0
4.5
6.8
9.1
11.4
13.6
15.9
18.2
20.5
22.7
25.0
27.3
29.5
31.8
34.1
36.4
38.6
40.9
43.2
45.5
47.7
50.0
=
*
Maximum Sense Current 1.0 Amps for LM3812-1.0 and LM3813-1.0.
The sense current should be multiplied by 10 for LM3812-7.0 and LM3813-7.0.
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12
Look-Up Tables (Continued)
TABLE 2. Duty Cycle to Current Conversion Table
Duty Cycle
(%)
Sense Current
(Amps)
0.990
Duty Cycle
(%)
Sense Current
(Amps)
-0.000
-0.055
-0.110
-0.165
-0.220
-0.275
-0.330
-0.385
-0.440
-0.495
-0.550
-0.605
-0.660
-0.715
-0.770
-0.825
-0.880
-0.935
-0.990
95.5
92.5
90.0
87.5
85.0
82.5
80.0
77.5
75.0
72.5
70.0
67.5
65.0
62.5
60.0
57.5
55.0
52.5
50.0
50.0
47.5
45.0
42.5
40.0
37.5
35.0
32.5
30.0
27.5
25.0
22.5
20.0
17.5
15.0
12.5
10.0
7.5
0.935
0.880
0.825
0.770
0.715
0.660
0.605
0.550
0.495
0.440
0.385
0.330
0.275
0.220
0.165
0.110
0.055
0.000
5.0
=
*
Maximum Sense Current 1.0 Amps for LM3812-1.0 and LM3813-1.0.
The sense current should be multiplied by 10 for LM3812-7.0 and LM3813-7.0.
Timing Diagram
DS100122-11
Duty cycle of the PWM waveform during any sampling interval indicates the current magnitude (average) and direction during the previous sampling interval.
FIGURE 9. Typical Timing Diagram for Mostly Positive Current
13
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Physical Dimensions inches (millimeters) unless otherwise noted
8-lead (0.150" Wide) Molded Small Outline Package
See Ordering Information table for Order Numbers
NS Package Number M08A
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COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
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systems which, (a) are intended for surgical implant
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accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
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