Page 171 - Op Amps Design, Applications, and Troubleshooting

154 VOLTAGE COMPARATORS

We will select a standard value of 240 kilohms.

Calculate V RfF. A simple way to calculate the required value of V REF is to apply
Equation (3.18).

where V LT and V H are the lower threshold and hysteresis voltages, respectively In
our present case, we can determine V REF as shown:

For purposes of illustration, let us derive this reference voltage from a zener diode
network across the -15-volt supply. As long as the equivalent (Thevenin) resis-
tance of the zener circuit is small compared to R F and Kj (the usual case), it will not
affect our previous selection of components.
Appendix 5 shows the data sheet for a family of zener diodes. One of the
listed devices is the 1N5233B, which is a 6.0-volt, V 2-watt zener diode. The maxi-
mum zener current can be estimated by applying the power formula:
I
; ;
I z(max) = £• (3.19)

where P z and V z are the power and voltage ratings of the zener. In the case of the
1N5233B, the maximum current is

05 W
J z(max) = —— = 83.3 mA
6V
This establishes the upper limit of zener current, and even this must be derated for
temperatures above 25°C The zener test current (I ZT) is listed as 20 milliarnps,
which is generally a good quiescent current choice.
The series current limiting resistor for the zener regulator can be computed
as follows:

In the present case, we have