Datasheet LM3813M-7.0, LM3813M-1.0, LM3812MX-7.0, LM3812M-7.0, LM3812M-1.0 Datasheet (NSC)

June 1999

LM3812/LM3813
Precision Current Gauge IC with Ultra Low Loss Sense
Element and PWM Output

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-widthmodulated (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.

Connection Diagrams

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

DS100122-1

Top View

LM3812

for High-Side Sensing

© 1999 National Semiconductor Corporation DS100122 www.national.com

Top View

LM3813

for Low-Side Sensing

DS100122-3

Ordering Information

Order No.

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.

‡

The Package code M08A is internal to National Semiconductor and indicates an 8-lead surface mount package, SO-8.

#

Sense
Range

±
±
±
±
±
±
±
±

Sampling

Interval

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

Sensing

*

Method

NS
Package
Number

Package

‡

Type Supplied As:

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).

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

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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)

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
Power Dissipation (Note 2)
ESD Susceptibility (Note 3) 1.5 kV
Sense Current (peak, for 200 msec) (Note 4) 10A

Maximum Junction Temperature 150˚C
Storage Temperature −65˚C to +150˚C
Lead Temperature (Soldering, 10 sec) 260˚C

Operating Ratings (Note 1)

Input Voltage 2.0V to 5.25V
Sense Current (continuous) (Note 4) 7A
Junction Temperature Range −40˚C to +125˚C

Sink Current for PWM pin 1mA
Voltage on Pin 5 5.25V

Electrical Characteristics

LM3812-1.0, LM3813-1.0

=

V

5.0V for the following specifications. Supply bypass capacitor is 1 µF and filter capacitor is 0.1 µF.

DD

Symbol Parameter Conditions

I

e

ACC

Average Current Accuracy
(Note 7)

n

Effective Output Noise (rms) 2 mA

at 0.9A current 0.9

(Note 5)

LM3812-7.0, LM3813-7.0

=

V

5.0V for the following specifications. Supply bypass capacitor is 1 µF and filter capacitor is 0.1 µF.

DD

Symbol Parameter Conditions

I

e

ACC

Average Current Accuracy
(Note 7)

n

Effective Output Noise (rms) 20 mA

at 2.5A current (Note 8) 2.5 A

(Note 5)

Typ

Typ

Limit

(Note 6)

Units

A

0.882 / 0.864 A (min)

0.918 / 0.936 A (max)

Limit

(Note 6)

Units

2.400 / 2.350 A (min)

2.600 / 2.650 A (max)

Common Device Parameters

Unless otherwise specified, V

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

=

5.0V for the following specifications. Supply bypass capacitor is 1 µF and filter capacitor is

DD

Typ

(Note 5)

Limit

(Note 6)

Units

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)

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%

Common Device Parameters (Continued)

Unless otherwise specified, V

0.1 µF.

Symbol Parameter Conditions

V

OH

V

OL

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

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

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.

Logic High Level for PWM Load current=1 mA, 2V ≤ VDD≤

Logic Low Level for PWM Sink current=1 mA, 2V ≤ VDD≤

Insertion Loss I

=

150˚C/W (typically), junction-to-ambient.

JA

=

5.0V for the following specifications. Supply bypass capacitor is 1 µF and filter capacitor is

DD

Typ

(Note 5)

Limit

(Note 6)

VDD− 0.05

5.25V

− 0.2

V

DD

0.04 V

5.25V

SENSE

=

1A (Note 9) 0.004 Ω

0.2 V (max)

, which is 150˚C/W for the S08

JA

=

25˚C and those with boldface

J

Units

V (min)

V

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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)

PWM Frequency vs Supply Voltage

DS100122-15

Measured Current vs Actual Current
(LM3812-7.0 and LM3813-7.0)

DS100122-24

PWM Frequency vs Temperature

Operating Current vs Supply Voltage

DS100122-13

DS100122-18

DS100122-16

Shutdown Current vs Supply Voltage

DS100122-19

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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

Current vs Duty Cycle

DS100122-20

Shutdown Current vs Temperature

DS100122-21

Accuracy vs Supply Voltage

DS100122-22

Accuracy vs Temperature (LM3812-1.0 and LM3813-1.0)

DS100122-29

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DS100122-28

Accuracy vs Temperature (LM3812-7.0 and LM3813-7.0)

DS100122-30

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)

DS100122-27

Note 10: These curves represent a statistical average such that the noise is insignificant.

Error vs Current (LM3812-7.0 and LM3813-7.0)

(Note 10)

DS100122-31

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.

FIGURE 1. High Side Sense

FIGURE 2. Low Side Sense

DS100122-5

DS100122-6

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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)

DS100122-7

FIGURE 3. Paralleling LM3812 for Higher Load Current

where D1is the duty cycle of PWM1 and D2is the duty cycle of PWM2.

Please refer to the Product Operation section for more information.

I

TOTAL

=

2.2(D

−0.5)I

1

+ 2.2(D2−0.5)I

MAX

MAX

FIGURE 4. High Voltage Operation — VINGreater Than 5.25V (High Side Sense)

(PWM output is referred to Pin 7)

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DS100122-8

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
VDDthrough a 10 kΩ resistor. (Continued)

DS100122-9

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
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.Aduty
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
can sense currents from −I
1A or 10A. The sense current is given by:

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:

For quick reference, see the Conversion Tables in
and

Table 2

Figure 6

I

SENSE

D=[I

.

. The pulse density output of the

(−I

MAX

=

2.2 (D−0.5)(I

SENSE

/(2.2 I

to +I

MAX

MAX

)] + 0.5

MAX

. Options for I

MAX

). The IC

MAX

MAX

)

MAX

Table 1

are

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
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

is

analog components.
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 Infor­mation table for details.

FIGURE 6. Functional block diagram of LM3812 and LM3813

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DS100122-10

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 de-

±

fined 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.

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-

1

⁄2)/1024.

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

DS100122-23

FIGURE 7. Transient Response to 7 Amp Step Current

Accuracy versus Noise

The graph shown in

±

of

1 Ampere current gauges. In this graph, the horizontal

Figure 8

illustrates the typical response

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.

DS100122-26

FIGURE 8. Typical Response of LM3812-1.0/LM3813-1.0

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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

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

*

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

(%)

quantization error of

1

⁄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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Look-Up Tables (Continued)

TABLE 2. Duty Cycle to Current Conversion Table

Duty Cycle

(%)

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.

Timing Diagram

Sense Current

(Amps)

Duty Cycle

(%)

Sense Current

(Amps)

Duty cycle of the PWM waveform during any sampling interval indicates the current magnitude (average) and direction during the previous sampling interval.

DS100122-11

FIGURE 9. Typical Timing Diagram for Mostly Positive Current

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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

LIFE SUPPORT POLICY

NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the

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
the life support device or system, or to affect its
safety or effectiveness.

labeling, can be reasonably expected to result in a
significant injury to the user.

National Semiconductor

LM3812/LM3813 Precision Current Gauge IC with Ultra Low Loss Sense Element and PWM

Corporation

Americas
Tel: [phone]
Fax: [phone]
Email: [e-mail]

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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.

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