Datasheet

Output Buffer
ADC
TIME (ns)
V
OUT
(V)
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1.0
0 50 100 150 200 250 300
20 pF
10 pF
V
S
= 5V
LMP8602, LMP8602Q, LMP8603, LMP8603Q
www.ti.com SNOSB36D –JULY 2009–REVISED MARCH 2013
Figure 49. Capacitive Load Response at 5.0V
These figures can be used to estimate the disturbance that will be caused when driving a switched capacitive
load. To minimize the error signal introduced by the sampling that occurs on the ADC input, an additional RC
filter can be placed in between the LMP8602/LMP8602Q/LMP8603/LMP8603Q and the ADC as illustrated in
Figure 50.
Figure 50. Reduce Error When Driving ADCs
The external capacitor absorbs the charge that flows when the ADC sampling capacitor is connected. The
external capacitor 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 51 illustrates a low side current sensing application with a low side driver. The power transistor is pulse
width modulated to control the average current flowing through the inductive load which is connected to a
relatively high battery voltage. The current through the load is measured across a shunt resistor R
SENSE
in series
with the load. When the power transistor is on, current flows from the battery through the inductive 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 rejection 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
SENSE
, of 0.01 Ω and a current of 1A, then the output voltage at the input pins of
the LMP8602 is: R
SENSE
* I
LOAD
= 0.01 Ω * 1A = 0.01V
With the gain of 50 for the LMP8602 this will give an output of 0.5V. Or in other words, V
OUT
= 0.5V/A.
For the LMP8603 the calculation is similar, but with a gain of 100, giving an output of 1 V/A.
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