LM3824MM-1.0/NOPB - Texas Instruments

LM3822

Precision Current Gauge IC with Internal Zero Ohm

Sense Element and PWM Output

General Description

The LM3822 Current Gauge provides easy to use precision

current measurement with virtually zero insertion loss (typi-

cally 0.003Ω). The LM3822 is used for high-side sensing.

A Delta Sigma analog to digital converter is incorporated to

precisely measure the current and to provide a current av-

eraging 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 indi-

cates 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 LM3822 is factory-set in two different current options.

The sense range is −1.0A to +1.0A or −2.0A to +2.0A. The

sampling interval for this part is 50ms. If faster sampling is

desired, please refer to the data sheet for the part number

LM3824.

Key Specifications

nUltra low insertion loss (typically 0.003Ω)

n2V to 5.5V supply range

n±2% accuracy at room temperature for the 1A device

(includes accuracy of the internal sense element)

nLow quiescent current in shutdown mode (typically

1.8 µA)

n50 msec sampling interval

nIn MSOP-8 Package

Features

nNo external sense element required

nPWM output indicates the current magnitude and

direction

nPWM output is easily interfaced with microprocessors

and controllers

nPrecision ∆Σ current-sense technique

nLow temperature sensitivity

nInternal filtering rejects false trips

nInternal Power-On-Reset (POR)

nDC Offset is less than 1 mA for 1A part

Applications

nBattery charge/discharge gauge

nMotion control diagnostics

nPower supply load monitoring and management

nResettable smart fuse

Connection Diagram

10124901

Top View

LM3822 for High-Side Sensing

November 2000

LM3822 Precision Current Gauge IC with Internal Zero Ohm Sense Element and PWM Output

Ordering Information

Order No.

Sense

Range

Sampling

Interval*

Sensing

Method

Package

Number

Package

Type

Supplied As:

LM3822MM-1.0 ±1.0A 50 ms High-side MUA08A MSOP-8 Tape and Reel

(1000 units/reel)

LM3822MMX-1.0 ±1.0A 50 ms High-side MUA08A MSOP-8 Tape and Reel

(3500 units/reel)

LM3822MM-2.0 ±2.0A 50 ms High-side MUA08A MSOP-8 Tape and Reel

(1000 units/reel)

LM3822MMX-2.0 ±2.0A 50 ms High-side MUA08A MSOP-8 Tape and Reel

(3500 units/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.

Pin Description (High-Side, LM3822)

Pin Name Function

1 SENSE+, V

High side of internal current sense, also supply voltage.

2 GND Supply Ground.

3 FLTR+ Filter input — provides anti-aliasing for delta sigma modulator.

4 FLTR− Filter input.

5SD Shutdown input. Connected to V

through a pull-up resistor for normal operation.

When low, the LM3822 is put into a low current mode.

6 TEST Connect to GND for normal operation.

7 PWM Digital output indicates the current magnitude and direction.

8 SENSE− Low side of internal current sense.

LM3822

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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

Sink Current for PWM pin 1mA

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) 5A

Junction Temperature Range −40˚C to +85˚C

Electrical Characteristics 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 type apply over full operating temperature

ranges.

LM3822-1.0

SENSE+V

= 3.6V for the following specifications. Supply bypass capacitor is 1 µF and filter capacitor is 0.1 µF.

Symbol Parameter Conditions Typ

(Note 5)

Limit

(Note 6) Units

ACC

Average Current Accuracy

(Note 7)

1.0A current 1.0 A

0.98 / 0.96 A (min)

1.02 / 1.04 A (max)

Effective Output Noise (rms) 2 mA

LM3822-2.0

SENSE+V

= 3.6V for the following specifications. Supply bypass capacitor is 1 µF and filter capacitor is 0.1 µF.

Symbol Parameter Conditions Typ

(Note 5)

Limit

(Note 6) Units

ACC

Average Current Accuracy

(Note 7)

2.0A current (Note 8) 2.0 A

1.94 / 1.90 A (min)

2.06 / 2.10 A (max)

Effective Output Noise (rms) 6 mA

Common Device Parameters

Unless otherwise specified, V

= 3.6V for the following specifications. Supply bypass capacitor is 1 µF and filter capacitor is

0.1 µF.

Symbol Parameter Conditions Typ

(Note 5)

Limit

(Note 6) Units

Quiescent Current Normal Mode, SD = high 95 µA

150 µA (max)

Quiescent Current Shutdown Mode, SD = low 1.8 µA

10 µA (max)

RES

PWM Resolution 0.1 %

Sampling Time 50 ms

40 ms (min)

80 ms (max)

Frequency of PWM Waveform 20 Hz

12.5 Hz (min)

25 Hz (max)

Threshold High Level for SD 1.3 V

1.8 V (min)

LM3822

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Electrical Characteristics 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 type apply over full operating temperature

ranges. (Continued)

Common Device Parameters (Continued)

Unless otherwise specified, V

= 3.6V for the following specifications. Supply bypass capacitor is 1 µF and filter capacitor is

0.1 µF.

Symbol Parameter Conditions Typ

(Note 5)

Limit

(Note 6) Units

Threshold Low Level for SD 1.2 V

0.7 V (max)

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

≤

5.25V

− 0.05

− 0.2

V (min)

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

≤

5.25V

0.04 V

0.2 V (max)

Insertion Loss I

SENSE

= 1A (Note 9) 0.003 Ω

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

θJA = 220˚C/W (typically), junction-to-ambient.

Note 3: The human body model is a 100 pF capacitor discharged through a 1.5 kΩresistor into each pin.

Note 4: The absolute maximum peak and continuous currents specified are not tested. These specifications are dependent on the θJA, which is 220˚C/W for the

MSOP-8 package.

Note 5: Typical numbers are at 25˚C and represent the most likely parametric norm. Specifications in standard type face are for TJ= 25˚C and those with boldface

typeapply 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 Average 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: This parameter is production tested at 1A and guaranteed by design at 2A.

Note 9: The tolerance of the internal lead frame resistor is corrected internally. The temperature coefficient of this resistor is 2600 ppm/˚C.

LM3822

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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

(LM3822-1.0)

Measured Current vs Actual Current

(LM3822-2.0)

10124924 10124925

PWM Frequency vs Supply Voltage PWM Frequency vs Temperature

10124933 10124923

Operating Current vs Supply Voltage Shutdown Current vs Supply Voltage

10124918 10124920

LM3822

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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

10124919 10124921

Current vs PWM Duty Cycle Accuracy vs Supply Voltage

10124922

10124928

Accuracy vs Temperature (LM3822-1.0) Accuracy vs Temperature (LM3822-2.0)

10124936 10124935

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

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Typical Application Circuits In the application circuits, the 0.1 µF ceramic capacitor between pins 1 and 2

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

through a 10 kΩresistor.

10124905

FIGURE 1. High Side Sense

10124907

FIGURE 2. Paralleling LM3822 for Higher Load Current

TOTAL

= 2.2(D

−0.5)I

MAX

+ 2.2(D

−0.5)I

MAX

where D

is the duty cycle of PWM1 and D

is the duty cycle of PWM2.

Please refer to the Product Operation section for more information.

LM3822

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Typical Application Circuits In the application circuits, the 0.1 µF ceramic capacitor between pins 1 and 2

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 kΩresistor. (Continued)

10124908

FIGURE 3. High Voltage Operation — V

Greater Than 5.5V (High Side Sense)

(PWM output is referred to Pin 6)

LM3822

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Product Operation

The current is sampled by the delta-sigma modulator, as

illustrated in Figure 4. 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 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

MAX

(−I

MAX

). The IC can

sense currents from −I

MAX

to +I

MAX

. Options for I

MAX

are

1.0A or 2.0A. The sense current is given by:

SENSE

= 2.2 (D−0.5)(I

MAX

)

where D is the duty cycle of the PWM waveform, and I

MAX

the full scale current (1.00A or 2.00A). Similarly, the duty

cycle is given by:

D=[I

SENSE

/(2.2 I

MAX

)] + 0.5

For quick reference, see the Conversion Table in Table 1.

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.

The user also needs to specify the full scale value. See the

Ordering Information table for details.

10124910

FIGURE 4. Functional block diagram of LM3822

LM3822

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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+

⁄

)/1024.

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 1.0 mA for

1.0 Amp parts, and 10 mA for 2.0 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 LM3822-1.0). 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.

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

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.

ACCURACY VERSUS NOISE

The graph shown in Figure 5 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 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).

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 aver-

aging the readings shown in this example, the average

current measurement is 502.3 mA (Figure 5). This value is

very close to the actual value of 500 mA. Moreover, the

accuracy depends on the number of readings that are aver-

aged.

LOW CURRENT MEASUREMENTS

The DC offset of the LM3822-1.0 is typically under 1 mA.

This low offset allows accurate low current measurements.

Even currents in the 10 mA range can be measured with

accuracies typically better than ±5%.

10124926

FIGURE 5. Typical Response of LM3822

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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

quantization error of

⁄

bit is not shown in these tables.

Please see the “PWM Output and Current Accuracy” section

for more details.

TABLE 1. Current to Duty Cycle Conversion Table

Sense Current

(Imax = 1.0A)

Sense Current

(Imax = 2.0A)

Duty Cycle (%)

1 2 95.5

0.95 1.90 93.2

0.90 1.80 90.9

0.85 1.70 88.6

0.80 1.60 86.4

0.75 1.50 84.1

0.70 1.40 81.8

0.65 1.30 79.5

0.60 1.20 77.3

0.55 1.10 75.0

0.50 1 72.7

0.45 0.90 70.5

0.40 0.80 68.2

0.35 0.70 65.9

0.30 0.60 63.6

0.25 0.50 61.4

0.20 0.40 59.1

0.15 0.30 56.8

0.10 0.20 54.5

0.05 0.10 52.3

0.00 0.00 50

−0.05 −0.10 47.7

−0.10 −0.20 45.5

−0.15 −0.30 43.2

−0.20 −0.40 40.9

−0.25 −0.50 38.6

−0.30 −0.60 36.4

−0.35 −0.70 34.1

−0.40 −0.80 31.8

−0.45 −0.90 29.5

−0.50 −1 27.3

−0.55 −1.10 25

−0.60 −1.20 22.7

−0.65 −1.30 20.5

−0.70 −1.40 18.2

−0.75 −1.50 15.9

−0.80 −1.60 13.6

−0.85 −1.70 11.4

−0.90 −1.80 9.1

−0.95 −1.90 6.8

−1 −2 4.5

LM3822

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Timing Diagram

10124911

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

FIGURE 6. Typical Timing Diagram for Mostly Positive Current

LM3822

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Physical Dimensions inches (millimeters) unless otherwise noted

8-Lead MSOP

See Ordering Information table for Order Numbers

NS Package Number MUA08A

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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WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR

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(b) support or sustain life, and whose failure to perform when

properly used in 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 the life support device or

system, or to affect its safety or effectiveness.

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LM3822 Precision Current Gauge IC with Internal Zero Ohm Sense Element and PWM Output