Output Buffer

ADC

TIME (ns)

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

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

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:

For the LMP8603 the calculation is similar, but with a gain of 100, giving an output of 1 V/A.

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