RC4152

PRODUCT SPECIFICATION

5

Principles of Operation

The RC4152 contains the following components: an open

loop comparator, a precision one-shot timer, a switched volt-

age reference, a switched current source, and an open collec-

tor logic output transistor. These functional blocks are

internally interconnected. Thus, by adding some external

resistors and capacitors, a designer can create a complete

voltage-to-frequency converter.

The comparator’s output controls the one-shot (monostable

timer). The one-shot in turn controls the switched voltage

reference, the switched current source and the open collector

output transistor. The functional block diagram shows the

components and their interconnection.

To detail, if the voltage at pin 7 is greater than the voltage at

pin 6, the comparator switches and triggers the one-shot.

When the one-shot is triggered, two things happen. First, the

one-shot begins its timing period. Second, the one-shot’s

output turns on the switched voltage reference, the switched

current source and the open collector output transistor.

The one-shot creates its timing period much like the popular

555 timer does, by charging a capacitor from a resistor tied

to +VS. The one-shot senses the voltage on the capacitor

(pin 5) and ends the timing period when the voltage reaches

2/3 of the supply voltage. At the end of the timing period, the

capacitor is discharged by a transistor similar to the open

collector output transistor.

Meanwhile, during the timing period of the one-shot, the

switched current source, the switched voltage reference, and

the open collector output transistor all will be switched on.

The switched current source (pin 1) will deliver a current

proportional to both the reference and an external resistor,

RS. The switched reference (pin 2) will supply an output

voltage equal to the internal reference voltage (2.25V). The

open collector output transistor we be turned on, forcing the

logic output (pin 3) to a low state. At the end of the timing

period all of these outputs will turn off. The switched voltage

reference has produced an off-on-off voltage pulse, the

switched current source has emitted a quanta of charge, and

the open collector output has transmitted a logic pulse.

To summarize, the purpose of the circuit is to produce a cur-

rent pulse, well-defined in amplitude and duration, and to

simultaneously produce an output pulse which is compatible

with most logic families. The circuit's outputs show a pulse

waveform in response to a voltage difference between the

comparators inputs.

Figure 1. Single Supply VFC

4152-04

4152

1

2

3

4

5

6

7

8

Switched

Current

Source

Switched

Voltage

Reference

Voltage

Reference

Precision

One Shot

Open Collector

Logic Output

Transistor

Open Loop

Comparator

Ground

LOAD

C

O

0.01 F

m

0 to +10V

C

Integrator

B

R

B

Current Setting Resistor

R

R

I

OUT

F

Open Collector Output

OUT

One Shot

Timing

100K