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INSTRUMENTS
Vuu U SENSE;
sENsm“ 5 —GND GND— 8 —vDD
smsp— 2 7 —GND smsp— 7 —GND
mm— 3 e —PWM mm— s —PWM
mni— A 5 —fi mni— 5 —fi
LM3812, LM3813
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SNOS028D –JUNE 1999–REVISED APRIL 2013
LM3812/LM3813 Precision Current Gauge IC with Ultra Low Loss Sense Element and
PWM Output
Check for Samples: LM3812,LM3813
1FEATURES DESCRIPTION
The LM3812/LM3813 Current Gauges provide easy
2• No External Sense Element Required to use precision current measurement with virtually
• PWM Output Indicates the Current Magnitude zero insertion loss (typically 0.004Ω). The LM3812 is
and Direction used for high-side sensing and the LM3813 is used
• PWM Output can be Interfaced with for low-side sensing.
Microprocessors A Delta Sigma analog to digital converter is
• Precision ΔΣ Current-Sense Technique incorporated to precisely measure the current and to
provide a current averaging function. Current is
• Low Temperature Sensitivity averaged over 50 msec time periods in order to
• Internal Filtering Rejects False Trips provide immunity to current spikes. The ICs have a
• Internal Power-On-Reset (POR) pulse-width modulated (PWM) output which indicates
the current magnitude and direction. The shutdown
APPLICATIONS pin can be used to inhibit false triggering during start-
up, or to enter a low quiescent current mode.
• Battery Charge/Discharge Gauge The LM3812 and LM3813 are factory-set in two
• Motion Control Diagnostics different current options. The sense range is −1A to
• Power Supply Load Monitoring and +1A or −7A to +7A. The sampling interval for these
Management parts is 50ms. If faster sampling is desired, please
• Resettable Smart Fuse refer to the data sheets for the part numbers LM3814
and LM3815.
KEY SPECIFICATIONS
• Ultra Low Insertion Loss (Typically 0.004Ω)
• 2V to 5.25V Supply Range
• ±2% Accuracy at Room Temperature (Includes
Accuracy of the Internal Sense Element)
( LM3812-1.0, LM3813-1.0)
• Low Quiescent Current In Shutdown Mode
(Typically 2.5 µA)
• 50 msec Sampling Interval
Connection Diagrams
Figure 1. LM3812 (Top View) Figure 2. LM3813 (Top View)
for High-Side Sensing for Low-Side Sensing
1
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
2All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date. Copyright © 1999–2013, Texas Instruments Incorporated
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
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PIN DESCRIPTIONS (High-Side, LM3812)
Pin Name Function
1 SENSE+, VDD High side of internal current sense, also supply voltage.
2 SENSE−Low side of internal current sense.
3 FLTR+ Filter input — provides anti-aliasing for delta sigma modulator.
4 FLTR−Filter input.
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 PWM PWM output indicates the current magnitude and direction.
7 GND Ground
8 GND Ground
PIN DESCRIPTIONS (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.
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 PWM PWM output indicates the current magnitude and direction.
7 GND Ground
8 VDD VDD (supply)
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
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ABSOLUTE MAXIMUM RATINGS (1) (2)
Absolute Maximum Supply Voltage 5.5V
Power Dissipation See (3)
ESD Susceptibility (4) 1.5 kV
Sense Current (peak, for 200 msec) (5) 10A
Sink Current for PWM pin 1mA
Voltage on Pin 5 5.25V
Maximum Junction Temperature 150°C
Storage Temperature −65°C to +150°C
Lead Temperature (Soldering, 10 sec) 260°C
(1) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specification
(2) 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 ensure specific performance limits. For ensured specifications and test
conditions, see Electrical Characteristics. The ensured specifications apply only for the test conditions listed. Some performance
characteristics may degrade when the device is not operated under the listed test conditions.
(3) At elevated temperatures, devices must be derated based on package thermal resistance. The device in the surface-mount package
must be derated at θJA= 150°C/W (typically), junction-to-ambient.
(4) The human body model is a 100 pF capacitor discharged through a 1.5 kΩresistor into each pin.
(5) The absolute maximum peak and continuous currents specified are not tested. These specifications are dependent on the θJA, which is
150°C/W for the D0008A package.
OPERATING RATINGS (1)
Input Voltage 2.0V to 5.25V
Sense Current (continuous) (2) 7A
Junction Temperature Range −40°C to +125°C
(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 ensure specific performance limits. For ensured specifications and test
conditions, see Electrical Characteristics. The ensured specifications apply only for the test conditions listed. Some performance
characteristics may degrade when the device is not operated under the listed test conditions.
(2) The absolute maximum peak and continuous currents specified are not tested. These specifications are dependent on the θJA, which is
150°C/W for the D0008A package.
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 Typ(1) Limit(2) Units
IACC Average Current Accuracy (3) at 0.9A current 0.9 A
0.882 / 0.864 A (min)
0.918 / 0.936 A (max)
enEffective Output Noise (rms) 2 mA
(1) 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 type apply over full operating temperature ranges.
(2) Limits are 100% production tested at 25°C. Limits over the operating temperature range are ensured through correlation using Statistical
Quality Control (SQC) methods. The limits are used to calculate Averaging Outgoing Quality Level (AOQL).
(3) 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.
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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 Typ(1) Limit(2) Units
IACC Average Current Accuracy (3) at 2.5A current (4) 2.5 A
2.400 / 2.350 A (min)
2.600 / 2.650 A (max)
enEffective Output Noise (rms) 20 mA
(1) 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 type apply over full operating temperature ranges.
(2) Limits are 100% production tested at 25°C. Limits over the operating temperature range are ensured through correlation using Statistical
Quality Control (SQC) methods. The limits are used to calculate Averaging Outgoing Quality Level (AOQL).
(3) 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.
(4) 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.
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 Typ(1) Limit(2) Units
100 µA
IQ1 Quiescent Current Normal Mode, SD = high 160 µA (max)
2.5 µA
IQ2 Quiescent Current Shutdown Mode, SD = low 10 µA (max)
DRES PWM Resolution 0.1 %
52 ms
tSSampling Time 40 ms (min)
80 ms (max)
20 Hz
fPFrequency of PWM Waveform 12.5 Hz (min)
25 Hz (max)
1.2 V
VTH Threshold High Level for SD 1.8 V (min)
1.3 V
VTL Threshold Low Level for SD 0.7 V (max)
VDD −0.05 V
VOH Logic High Level for PWM Load current = 1 mA, 2V ≤VDD ≤5.25V VDD −0.2 V (min)
0.04 V
VOL Logic Low Level for PWM Sink current = 1 mA, 2V ≤VDD ≤5.25V 0.2 V (max)
PIInsertion Loss ISENSE = 1A (3) 0.004 Ω
(1) 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 type apply over full operating temperature ranges.
(2) Limits are 100% production tested at 25°C. Limits over the operating temperature range are ensured through correlation using Statistical
Quality Control (SQC) methods. The limits are used to calculate Averaging Outgoing Quality Level (AOQL).
(3) 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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INSTRUMENTS
‘sme (A)
‘sense (A)
7m
4'2 42
-|.2 -o,a -fl.t o u.‘ M 1.2 42407575472 D 2 4 e a MHZ
‘adusl (’1) ‘mm (A)
21.5
21
20.5
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PWM mquency (H1)
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-55 -25 5 35 55 95 125
Tampevaiuve (°c)
nu
ms
‘02
93
94
so
as
32
7a
74
7a
m (HA)
van (v) Van (V)
LM3812, LM3813
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SNOS028D –JUNE 1999–REVISED APRIL 2013
TYPICAL PERFORMANCE CHARACTERISTICS
Supply bypass capacitor is 0.1 µF and filter capacitor is 0.1 µF.
Measured Current vs Actual Current Measured Current vs Actual Current
(LM3812-1.0 and LM3813-1.0) (LM3812-7.0 and LM3813-7.0)
Figure 3. Figure 4.
PWM Frequency vs Supply Voltage PWM Frequency vs Temperature
Figure 5. Figure 6.
Operating Current vs Supply Voltage Shutdown Current vs Supply Voltage
Figure 7. Figure 8.
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H2
ma
WA
100
96
92
BE
34
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75
72
-55 -25 5 35
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Temperature
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245
2A
-50 -25 o 25 so 75 IUD ‘25
Temperamre (°c)
LM3812, LM3813
SNOS028D –JUNE 1999–REVISED APRIL 2013
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
Supply bypass capacitor is 0.1 µF and filter capacitor is 0.1 µF.
Operating Current vs Temperature Shutdown Current vs Temperature
Figure 9. Figure 10.
Current vs Duty Cycle Accuracy vs Supply Voltage
Figure 11. Figure 12.
Accuracy vs Temperature Accuracy vs Temperature
(LM3812-1.0 and LM3813-1.0) (LM3812-7.0 and LM3813-7.0)
Figure 13. Figure 14.
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INSTRUMENTS
{War (7”)
0.0‘
0.1
Currem (A)
Error (7,)
I
Currenl (A)
la
LM3812, LM3813
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SNOS028D –JUNE 1999–REVISED APRIL 2013
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
Supply bypass capacitor is 0.1 µF and filter capacitor is 0.1 µF.
Error vs Current Error vs Current
(LM3812-1.0 and LM3813-1.0) (LM3812-7.0 and LM3813-7.0)
Figure 15. Figure 16.
These curves represent a statistical average such that the noise is insignificant.
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‘SENSE
LOAD
0.1m
V I E
2 7
LM38 1 2 J _
2,0V‘ 5 25V
u f 3 $08 PWM
a “J
r— t 5 — sin
LOAD
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‘ o a Vm)
7
2 LM3813 :5 f—
505
0U”
W
LM3812, LM3813
SNOS028D –JUNE 1999–REVISED APRIL 2013
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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.
Figure 17. High Side Sense
Figure 18. Low Side Sense
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LEAD
LMEEVZ
2 av «n a m 508
Pw‘
LMSEIZ
SOB FWMZ
a‘,r
stwst
1w
LM3EV2
“>525; 505
my
LM3812, LM3813
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SNOS028D –JUNE 1999–REVISED APRIL 2013
ITOTAL = 2.2(D1−0.5)IMAX + 2.2(D2−0.5)IMAX
where D1is the duty cycle of PWM1 and
D2is the duty cycle of PWM2.
Figure 19. Paralleling LM3812 for Higher Load Current
Please refer to the PRODUCT OPERATION section for more information.
Figure 20. High Voltage Operation — VIN Greater Than 5.25V (High Side Sense)
(PWM output is referred to Pin 7)
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‘sms:
LOAD
1W5 25x
7
2 mes
1 508
um
LM3812, LM3813
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Figure 21. High Voltage Operation — VIN Greater Than 5.25V (Low Side Sense)
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DeHa S‘gms
Dlgxm
Mamamr
F‘LTERt
HLTER’
mm
|_
'—
Comp
mgnax Ramp
Generator
LM3812, LM3813
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SNOS028D –JUNE 1999–REVISED APRIL 2013
PRODUCT OPERATION
The current is sampled by the delta-sigma modulator, as illustrated in Figure 22. 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 IMAX (−IMAX). The IC can sense
currents from −IMAX to +IMAX. Options for IMAX are 1A or 10A. The sense current is given by:
ISENSE = 2.2 (D−0.5)(IMAX)
where
• D is the duty cycle of the PWM waveform.
• IMAX is the full scale current (1A or 10A). (1)
Similarly, the duty cycle is given by:
D = [ISENSE/(2.2 IMAX)] + 0.5 (2)
For quick reference, see the Conversion Tables in Table 1 and Table 2.
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 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.
Figure 22. Functional Block Diagram of LM3812 and LM3813
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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+½)/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 ½ 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.
Accuracy of 7A Versions
The graph of Figure 23 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 element and the die gets larger, and an error develops. Eventually the temperature difference
reaches steady state, which accounts for the under-damped exponential response.
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TMN COPPER TRACE
mxcx COPPER mm
twem (A)
o 20 to so an we
Number 0! Samu‘es
35W ”HA—ULFanH—
D 200 we sea 500 mm) mm Mao
Ume (ms)
LM3812, LM3813
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SNOS028D –JUNE 1999–REVISED APRIL 2013
Figure 23. Transient Response to 7 Amp Step Current
Accuracy Versus Noise
The graph shown in Figure 24 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
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 24). This value is very close to the actual value of 500 mA.
Moreover, the accuracy depends on the number of readings that are averaged.
Figure 24. 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 quantization error of ½ 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 Duty Cycle Sense Current Duty Cycle
(Amps)(1) (%) (Amps)(1) (%)
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
(1) 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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5v
PWM
Avg. Wm new 0 «u éflms
shown by 75% MW Eye‘s
Undefined
5:7”
n D:60%
9:23”
9:75»:
50
‘50
Tune (ms)
200
250
300
LM3812, LM3813
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Table 2. Duty Cycle to Current Conversion Table
Duty Cycle Sense Current Duty Cycle Sense Current
(%) (Amps)(1) (%) (Amps)(1)
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
(1) 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
Duty cycle of the PWM waveform during any sampling interval indicates the current magnitude (average) and
direction during the previous sampling interval.
Figure 25. Typical Timing Diagram for Mostly Positive Current
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REVISION HISTORY
Changes from Revision C (April 2013) to Revision D Page
• Changed layout of National Data Sheet to TI format .......................................................................................................... 15
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issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and
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