LM7341MF datasheet(16/20 Pages) NSC

16 / 20 page

INVERTING COMPARATOR WITH HYSTERESIS

The inverting comparator with hysteresis requires a three re-

sistor network that is referenced to the supply voltage V

CC of

the comparator, as shown in Figure 3. When V

IN at the in-

verting input is less than V

A, the voltage at the non-inverting

node of the comparator (V

IN < VA), the output voltage is high

(for simplicity assume V

OUT switches as high as VCC). The

three network resistors can be represented as R

1||R3 in series

with R

2. The lower input trip voltage VA1 is defined as

A1 = VCCR2 / ((R1||R3) + R2)

When V

IN is greater than VA (VIN > VA), the output voltage is

low, very close to ground. In this case the three network re-

sistors can be presented as R

2||R3 in series with R1. The

upper trip voltage V

A2 is defined as

A2 = VCC (R2||R3) / ((R1+ (R2||R3)

The total hysteresis provided by the network is defined as

Delta V

A = VA1- VA2

For example to achieve 50 mV of hysteresis when V

CC = 30V

set R

1 = 4.02 kΩ, R2 = 4.02 kΩ, and R3 = 1.21 MΩ. With these

resistors selected the error due to input bias current is ap-

proximately 1 mV. To minimize this error it is best to use low

resistor values on the inputs.

20206056

FIGURE 3. Inverting Comparator with Hysteresis

LM7341MF Datasheet(PDF) 16 Page - National Semiconductor (TI)