LM231, LM331

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SNOSBI2C – JUNE 1999 – REVISED SEPTEMBER 2015

8 Detailed Description

8.1 Overview

8.1.1 Detail of Operation, Functional Block Diagram

The Functional Block Diagram shows a band gap reference which provides a stable 1.9-VDC output. This 1.9 VDC

is well regulated over a VS range of 3.9 V to 40 V. It also has a flat, low temperature coefficient, and typically

changes less than ½% over a 100°C temperature change.

The current pump circuit forces the voltage at pin 2 to be at 1.9 V, and causes a current i = 1.90 V/RS to flow.

For RS=14 k, i=135 μA. The precision current reflector provides a current equal to i to the current switch. The

current switch switches the current to pin 1 or to ground, depending upon the state of the R-S flip-flop.

The timing function consists of an R-S flip-flop and a timer comparator connected to the external RtCt network.

When the input comparator detects a voltage at pin 7 higher than pin 6, it sets the R-S flip-flop which turns ON

the current switch and the output driver transistor. When the voltage at pin 5 rises to

⅔ VCC, the timer comparator

causes the R-S flip-flop to reset. The reset transistor is then turned ON and the current switch is turned OFF.

However, if the input comparator still detects the voltage on pin 7 as higher than pin 6 when pin 5 crosses

⅔

VCC, the flip-flop will not be reset, and the current at pin 1 will continue to flow, trying to make the voltage at pin 6

higher than pin 7. This condition will usually apply under start-up conditions or in the case of an overload voltage

at signal input. During this sort of overload the output frequency will be 0. As soon as the signal is restored to the

working range, the output frequency will be resumed.

8.2 Functional Block Diagram

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Product Folder Links: LM231 LM331