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Rev. B.2

LM393/LM339

36V Low Power Dual/Quad Differential Comparators

When Vi is low, the output is also low. For the output to switch from low to high, Vi must rise up to Vtr. When Vi is

high, the output is also high. In order for the comparator to switch back to a low state, Vi must equal Vtf before the

non-inverting input V+ is again equal to Vr.

tr

V

R2

R1

R2

r

V





tf

V

2

1

1

)

tf

V

DD

(V

r

V









R

R

R

r

V

R2

2

1

tr

V

R

R 



DD

V

R2

1

r

V

R2

2

1

tf

V

R

R

R







DD

V

R2

1

tf

V

tr

V

hyst

V

R







Inverting Comparator with Hysteresis

The inverting comparator with hysteresis requires a three-resistor network that is referenced to the comparator supply

voltage (VDD), as shown in Figure 4.

Vo

LM393

LM393

LM393

Figure 4. Inverting Configuration with Hysteresis

When Vi is greater than V+, the output voltage is low. In this case, the three network resistors can be presented as

paralleled resistor R2 || R3 in series with R1. When Vi at the inverting input is less than V+, the output voltage is high.

The three network resistors can be represented as R1 ||R3 in series with R2.

DD

V

2

3

||

1

2

tr

V

R

R

R

R





DD

V

1

3

||

2

3

||

2

tf

V

R

R

R

R

R





DD

V

3

2

||

1

2

||

1

tf

V

tr

V

hyst

V

R

R

R

R

R









Low Input Bias Current

The LM393/339 family is a CMOS comparator family and features very low input bias current in pA range. The low

input bias current allows the comparators to be used in applications with high resistance sources. Care must be taken

to minimize PCB Surface Leakage. See below section on “PCB Surface Leakage” for more details.