Page 1
June 1999
LM3812/LM3813
Precision Current Gauge IC with Ultra Low Loss Sense
Element and PWM Output
General Description
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.
A Delta Sigma analog to digital converter is incorporated to
precisely measure the current and to provide a current averaging function. Current is averaged over 50 msec time
periods 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.
The LM3812 and LM3813 are factory-set in two different
current options. The sense range is −1A to +1A or −7A to
+7A. The sampling interval for these parts is 50ms. If faster
sampling is desired, please refer to the data sheets for the
part numbers LM3814 and LM3815.
Key Specifications
n Ultra low insertion loss (typically 0.004Ω)
n 2V to 5.25V supply range
±
n
2% accuracy at room temperature (includes accuracy
of the internal sense element) (LM3812-1.0,
LM3813-1.0)
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
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
LM3812/LM3813 Precision Current Gauge IC with Ultra Low Loss Sense Element and PWM
Output
Connection Diagrams
Top View
10012201
LM3812
for High-Side Sensing
© 2004 National Semiconductor Corporation DS100122 www.national.com
Top View
LM3813
for Low-Side Sensing
10012203
Page 2
Ordering Information
Order No.
LM3812/LM3813
LM3812M-1.0
LM3812MX-1.0
LM3812M-7.0
LM3812MX-7.0
LM3813M-1.0
LM3813MX-1.0
LM3813M-7.0
LM3813MX-7.0
#
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.
#
Sense
Range
±
1A 50 ms High-side M08A SO-8 95 units in Rails
±
1A 50 ms High-side M08A SO-8 2.5k units on Tape and Reel
±
7A 50 ms High-side M08A SO-8 95 units in Rails
±
7A 50 ms High-side M08A SO-8 2.5k units on Tape and Reel
±
1A 50 ms Low-side M08A SO-8 95 units in Rails
±
1A 50 ms Low-side M08A SO-8 2.5k units on Tape and Reel
±
7A 50 ms Low-side M08A SO-8 95 units in Rails
±
7A 50 ms Low-side M08A SO-8 2.5k units on Tape and Reel
Sampling
Interval*
Sensing
Method
NS
Package
Number
‡
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 Name Function
1 SENSE+, V
2 SENSE− Low side of internal current sense.
3 FLTR+ Filter input — provides anti-aliasing for delta sigma modulator.
4 FLTR− Filter input.
5SD
6 PWM PWM output indicates the current magnitude and direction.
7 GND Ground
8 GND Ground
DD
High side of internal current sense, also supply voltage.
Shutdown pin. Connected to VDDthrough a pull up resistor for normal operation. When
low, the IC goes into a low current mode (typically 3 µA).
Package
‡
Type Supplied As:
Pin Description (Low-Side, LM3813)
Pin Name Function
1 SENSE+, GND High side of internal current sense, also ground.
2 SENSE− Low side of internal current sense.
3 FLTR+ Filter input – provides anti-aliasing for delta sigma modulator.
4 FLTR− Filter input.
5SD
6 PWM PWM output indicates the current magnitude and direction.
7 GND Ground
8V
DD
Shutdown pin. Connected to VDDthrough a pull up resistor for normal operation. When
low, the IC goes into a low current mode (typically 3 µA).
VDD(supply)
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LM3812/LM3813
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Absolute Maximum Supply Voltage 5.5V
Maximum Junction Temperature 150˚C
Storage Temperature −65˚C to +150˚C
Lead Temperature (Soldering, 10 sec) 260˚C
Operating Ratings (Note 1)
Power Dissipation (Note 2)
ESD Susceptibility (Note 3) 1.5 kV
Sense Current (peak, for 200 msec) (Note 4) 10A
Sink Current for PWM pin 1mA
Input Voltage 2.0V to 5.25V
Sense Current (continuous) (Note 4) 7A
Junction Temperature Range −40˚C to +125˚C
Voltage on Pin 5 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.
Symbol Parameter Conditions
I
ACC
Average Current Accuracy
at 0.9A current 0.9
(Note 7)
e
n
Effective Output Noise (rms) 2 mA
Typ
(Note 5)
0.882 / 0.864 A (min)
0.918 / 0.936 A (max)
Limit
(Note 6)
Units
A
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.
Symbol Parameter Conditions
I
ACC
Average Current Accuracy
at 2.5A current (Note 8) 2.5 A
(Note 7)
Typ
(Note 5)
Limit
(Note 6)
2.400 / 2.350 A (min)
2.600 / 2.650 A (max)
e
n
Effective Output Noise (rms) 20 mA
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.
Symbol Parameter Conditions
I
Q1
I
Q2
D
RES
t
S
f
P
V
TH
V
TL
Quiescent Current Normal Mode, SD = high 100 µA
Quiescent Current Shutdown Mode, SD = low 2.5 µA
PWM Resolution 0.1 %
Sampling Time 52 ms
Frequency of PWM Waveform 20 Hz
Threshold High Level for SD 1.2 V
Threshold Low Level for SD 1.3 V
Typ
(Note 5)
Limit
(Note 6)
160 µA (max)
10 µA (max)
40 ms (min)
80 ms (max)
12.5 Hz (min)
25 Hz (max)
1.8 V (min)
0.7 V (max)
Units
Units
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Electrical Characteristics (Continued)
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.
LM3812/LM3813
Symbol Parameter Conditions
V
OH
Logic High Level for PWM Load current = 1 mA, 2V ≤ VDD≤
Typ
(Note 5)
VDD− 0.05
5.25V
V
OL
Logic Low Level for PWM Sink current = 1 mA, 2V ≤ VDD≤
0.04 V
5.25V
P
I
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see Electrical Characteristics. The
guaranteed 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
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
S08 package.
Note 5: Typical numbers are at 25˚C and represent the most likely parametric norm. Specifications in standard type face are for T
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.
Insertion Loss I
= 150˚C/W (typically), junction-to-ambient.
= 1A (Note 9) 0.004 Ω
SENSE
Limit
(Note 6)
− 0.2
V
DD
0.2 V (max)
, which is 150˚C/W for the
JA
= 25˚C and those with boldface
J
Units
V
V (min)
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)
10012215 10012224
Measured Current vs Actual Current
(LM3812-7.0 and LM3813-7.0)
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Typical Performance Characteristics Supply bypass capacitor is 0.1 µF and filter capacitor is
0.1 µF. (Continued)
PWM Frequency vs Supply Voltage PWM Frequency vs Temperature
LM3812/LM3813
10012213
10012216
Operating Current vs Supply Voltage Shutdown Current vs Supply Voltage
10012218 10012219
Operating Current vs Temperature Shutdown Current vs Temperature
10012220 10012221
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Typical Performance Characteristics Supply bypass capacitor is 0.1 µF and filter capacitor is
0.1 µF. (Continued)
Current vs Duty Cycle Accuracy vs Supply Voltage
LM3812/LM3813
10012222
10012228
Accuracy vs Temperature (LM3812-1.0 and LM3813-1.0) Accuracy vs Temperature (LM3812-7.0 and LM3813-7.0)
10012229 10012230
Error vs Current (LM3812-1.0 and LM3813-1.0)
(Note 10)
Error vs Current (LM3812-7.0 and LM3813-7.0)
(Note 10)
10012227
Note 10: These curves represent a statistical average such that the noise is
insignificant.
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10012231
Page 7
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 V
resistor.
through a 10 kΩ
DD
FIGURE 1. High Side Sense
LM3812/LM3813
10012205
FIGURE 2. Low Side Sense
10012206
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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
VDDthrough a 10 kΩ resistor. (Continued)
LM3812/LM3813
10012207
FIGURE 3. Paralleling LM3812 for Higher Load Current
I
TOTAL
= 2.2(D1−0.5)I
+ 2.2(D2−0.5)I
MAX
MAX
where D1is the duty cycle of PWM1 and D2is the duty cycle of PWM2.
Please refer to the Product Operation section for more information.
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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
VDDthrough a 10 kΩ resistor. (Continued)
10012208
LM3812/LM3813
FIGURE 4. High Voltage Operation — VINGreater Than 5.25V (High Side Sense)
(PWM output is referred to Pin 7)
10012209
FIGURE 5. High Voltage Operation — VINGreater Than 5.25V (Low Side Sense)
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Product Operation
The current is sampled by the delta-sigma modulator, as
illustrated in Figure 6. The pulse density output of the deltasigma 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
LM3812/LM3813
is proportional to the amount of current flowing. A duty cycle
of 50% indicates zero current flow. If the current is flowing 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 I
sense currents from −I
MAX
to +I
MAX
or 10A. The sense current is given by:
= 2.2 (D−0.5)(I
I
SENSE
where D is the duty cycle of the PWM waveform, and I
the full scale current (1A or 10A). Similarly, the duty cycle is
given by:
SENSE
/(2.2 I
D=[I
For quick reference, see the Conversion Tables in Table 1
and Table 2.
(−I
MAX
. Options for I
MAX
)] + 0.5
MAX
MAX
)
). The IC can
are 1A
MAX
MAX
The user should note that, while the LM3812-7.0/
LM3813-7.0 will read 10A full scale, it is rated for 10A
operation 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 antialiasing 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.
When ordering, the user has to specify whether the part is
is
being used for low-side or high-side sense. The user also
needs to specify the full scale value. See the Ordering
Information table for details.
FIGURE 6. Functional block diagram of LM3812 and LM3813
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10012210
Page 11
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
output of the PWM circuitry. That is to say that instead of a
duty cycle of N/1024, the duty cycle actually is (N+
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.
The extra half cycle delay will show up as a DC offset of
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
In addition to quantization, the duty cycle will contain some
jitter. The jitter is quite small (for example, the standard
deviation of jitter is only 0.1% for the LM3812/13-1.0). Statistically the jitter can cause an error in a current sample.
Because 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.
The “crest factor” has often been used to define the maximum error caused by jitter. The crest factor defines a limit
within which 99.7% of the samples fall. The crest factor is
±
defined as
0.3% error in the duty cycle.
Since the jitter is a random variable, averaging multiple
outputs will reduce the effective jitter. Obeying statistical
laws, the jitter is reduced by the square root of the number of
readings 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.
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.
1
⁄2)/1024.
LM3812/LM3813
ment and the die gets larger, and an error develops. Eventually the temperature difference reaches steady state,
which accounts for the under-damped exponential response.
1
⁄
2
10012223
FIGURE 7. Transient Response to 7 Amp Step Current
Accuracy versus Noise
The graph shown in Figure 8 illustrates the typical response
±
1 Ampere current gauges. In this graph, the horizontal
of
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 difference between successive readings manifests itself
as jitter in the PWM output or noise in the current measurement (when duty cycle of the PWM output is converted to
current).
The accuracy of the measurement depends on the noise in
the current waveform. The accuracy can be improved by
averaging several outputs. Although there is variation in
successive readings, a very accurate measurement can be
obtained 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
accuracy depends on the number of readings that are averaged.
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.
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
current 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-
10012226
FIGURE 8. Typical Response of LM3812-1.0/LM3813-1.0
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Page 12
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
TABLE 1. Current to Duty Cycle Conversion Table
LM3812/LM3813
*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.
Sense Current
(Amps)*
1.00 95.5 -1.00 4.5
0.95 93.2 -0.95 6.8
0.90 90.9 -0.90 9.1
0.85 88.6 -0.85 11.4
0.80 86.4 -0.80 13.6
0.75 84.1 -0.75 15.9
0.70 81.8 -0.70 18.2
0.65 79.5 -0.65 20.5
0.60 77.3 -0.60 22.7
0.55 75.0 -0.55 25.0
0.50 72.7 -0.50 27.3
0.45 70.5 -0.45 29.5
0.40 68.2 -0.40 31.8
0.35 65.9 -0.35 34.1
0.30 63.6 -0.30 36.4
0.25 61.4 -0.25 38.6
0.20 59.1 -0.20 40.9
0.15 56.8 -0.15 43.2
0.10 54.5 -0.10 45.5
0.05 52.3 -0.05 47.7
0.00 50.0 -0.00 50.0
Duty Cycle
(%)
1
quantization error of
⁄2bit is not shown in these tables.
Please see the “PWM Output and Current Accuracy” section
for more details.
Sense Current
(Amps)*
Duty Cycle
(%)
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TABLE 2. Duty Cycle to Current Conversion Table
Duty Cycle
(%)
Sense Current
(Amps)
95.5 0.990 50.0 -0.000
92.5 0.935 47.5 -0.055
90.0 0.880 45.0 -0.110
87.5 0.825 42.5 -0.165
85.0 0.770 40.0 -0.220
82.5 0.715 37.5 -0.275
80.0 0.660 35.0 -0.330
77.5 0.605 32.5 -0.385
75.0 0.550 30.0 -0.440
72.5 0.495 27.5 -0.495
70.0 0.440 25.0 -0.550
67.5 0.385 22.5 -0.605
65.0 0.330 20.0 -0.660
62.5 0.275 17.5 -0.715
60.0 0.220 15.0 -0.770
57.5 0.165 12.5 -0.825
55.0 0.110 10.0 -0.880
52.5 0.055 7.5 -0.935
50.0 0.000 5.0 -0.990
*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.
Duty Cycle
(%)
LM3812/LM3813
Sense Current
(Amps)
Timing Diagram
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
10012211
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Page 14
Physical Dimensions inches (millimeters)
unless otherwise noted
Output
8-lead (0.150" Wide) Molded Small Outline Package
See Ordering Information table for Order Numbers
NS Package Number M08A
LM3812/LM3813 Precision Current Gauge IC with Ultra Low Loss Sense Element and PWM
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
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2. A critical component is any component of a life support
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expected to cause the failure of the life support device or
system, or to affect its safety or effectiveness.
provided in the labeling, can be reasonably expected to result
in a significant injury to the user.
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Substances’’ as defined in CSP-9-111S2.
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