lmp8602mmx National Semiconductor Corporation, lmp8602mmx Datasheet - Page 19

• Manufacturer Part Number: lmp8602mmx
• Description: 60v Common Mode, Fixed Gain, Bidirectional Precision Current Sensing Amplifier
• Manufacturer: National Semiconductor Corporation

in between the LMP8602/LMP8603 and the ADC as illustrat-
ed in Figure 8.
The external capacitor absorbs the charge that flows when
the ADC sampling capacitor is connected. The external ca-
pacitor should be much larger than the sample and hold
capacitor at the input of the ADC and the RC time constant of
the external filter should be such that the speed of the system
is not affected.
LOW SIDE CURRENT SENSING APPLICATION WITH
LARGE COMMON MODE TRANSIENTS
Figure 9 illustrates a low side current sensing application with
a low side driver. The power transistor is pulse width modu-
lated to control the average current flowing through the in-
ductive load which is connected to a relatively high battery
FIGURE 8. Reduce Error When Driving ADCs
FIGURE 9. Low Side Current Sensing Application with Large Common Mode Transients
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voltage. The current through the load is measured across a
shunt resistor R
transistor is on, current flows from the battery through the in-
ductive load, the shunt resistor and the power transistor to
ground. In this case, the common mode voltage on the shunt
is close to ground. When the power transistor is off, current
flows through the inductive load, through the shunt resistor
and through the freewheeling diode. In this case the common
mode voltage on the shunt is at least one diode voltage drop
above the battery voltage. Therefore, in this application the
common mode voltage on the shunt is varying between a
large positive voltage and a relatively low voltage. Because
the large common mode voltage range of the LMP8602/
LMP8603 and because of the high AC common mode rejec-
tion ratio, the LMP8602/LMP8603 is very well suited for this
application.
For this application the following example can be used for the
calculation of the output signal:
When using a sense resistor, R
of 1A, then the output voltage at the input pins of the LMP8602
is: R
With the gain of 50 for the LMP8602 this will give an output of
0.5V. Or in other words, VOUT = 0.5V/A.
For the LMP8603 the calculation is similar, but with a gain of
100, giving an output of 1 V/A.
SENSE
* I
LOAD
SENSE
= 0.01 Ω * 1A = 0.01V
in series with the load. When the power
SENSE
, of 0.01 Ω and a current
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