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 oc-

curs 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 8.

30083461

FIGURE 8. Reduce Error When Driving ADCs

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

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, V

For the LMP8603 the calculation is similar, but with a gain of

100, giving an output of 1 V/A.

30083452

FIGURE 9. Low Side Current Sensing Application with Large Common Mode Transients

19

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