MORE NOISE FILTERING

The current flowing through the sense resistor and certain

loads can sometimes be very noisy particularly when the

load is a DC motor or switching supply Large amounts of

noise on the supply line can also cause problems when

threshold voltages are set to very small values In these

cases while the average current level may remain well be-

low the threshold trip point noise peaks may exceed it A

LED display could then flicker or appear dimly lit or exces-

sive software routines and processor time may be required

for a mP to disregard such noise Often such noise must be

filtered directly at the inputs using the input resistors R1

and R2 and a capacitor Care must be taken however that

such a filter will not cause an erroneous output state upon

power-up or whenever switch S1 is closed The most effec-

tive general methodology to achieve this is to split the resis-

tor in the positive input lead into two resistor values and

connect a capacitor from here to the negative input For

example the 12k resistor R2 of

Figure 10 could be re-

placed with 39k and 12k resistors as shown in

Figure 16a

(R1 increasing from 62k to 10k to compensate) The value

of capacitor C2 depends upon the degree of filtering re-

quired the amount of noise present and the response times

desired The choice of values for the new resistors is almost

arbitrary Generally the larger value is attached to the sense

resistor for better decoupling The smaller value must be

large enough so that the DC voltage across it upon power-

up exceeds the maximum offset voltage expected of the

that guarantees that the output will not be in an erroneous

high state upon power-up or whenever S1 is closed (Should

this feature be unnecessary to a particular application cir-

cuit the methodology described can be replaced with a sim-

ple capacitor across the comparator input pins)

For extremely severe cases additional filter stages can be

cascaded at the inputs (see

Figure 17 ) Since the input bias

currents of the comparator are equal at the input threshold

level the voltage drops across the 1k resistors cancel and

do not affect the DC operation of the circuit (ignoring resis-

tor match tolerance and Ios) If an application circuit is noisy

enough to require such an elaborate filter then ferrite

beads shown here as L1 and L2 will also probably help

TLH8707 – 16

a Open-Circuit Detector

TLH8707 – 17

b Over-Current Limit Detector

FIGURE 16 Input Noise Filters for

Various Application Circuits

TLH8707 – 18

FIGURE 17 Additional Noise Filters

10