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PDF LM3812 Data sheet ( Hoja de datos )
| Número de pieza | LM3812 | |
| Descripción | Precision Current Gauge IC with Ultra Low Loss Sense Element and PWM Output | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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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 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.
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.
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
Connection Diagrams
DS100122-1
Top View
LM3812
for High-Side Sensing
DS100122-3
Top View
LM3813
for Low-Side Sensing
© 1999 National Semiconductor Corporation DS100122
www.national.com
1 page  
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
PWM Frequency vs Supply Voltage
PWM Frequency vs Temperature
DS100122-24
DS100122-13
Operating Current vs Supply Voltage
DS100122-16
Shutdown Current vs Supply Voltage
DS100122-18
5
DS100122-19
www.national.com
5 Page 
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+1⁄2)/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 1⁄2
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
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.
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-
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.
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 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-
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.
DS100122-23
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 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 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.
DS100122-26
FIGURE 8. Typical Response of LM3812-1.0/LM3813-1.0
11 www.national.com
11 Page | ||
| Páginas | Total 14 Páginas | |
| PDF Descargar | [ Datasheet LM3812.PDF ] | |
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