RC4152N Ver la hoja de datos (PDF) - Fairchild Semiconductor

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componentes Descripción

Fabricante

RC4152N

Voltage-to-Frequency Converters

Fairchild Semiconductor 

PRODUCT SPECIFICATION

RC4152

Applications

Single Supply VFC

The stand-alone voltage-to-frequency converter is one of the

simplest applications for the RC4152. This application uses

only passive external components to create the least expen-

sive VFC circuit (see Figure 1).

The positive input voltage VIN is applied to the input com-

parator through a low pass filter. The one-shot will fire repet-

itively and the switched current source will pump out current

pulses of amplitude VREF/RS and duration 1.1 ROCO into the

integrator. Because the integrator is tied back to the inverting

comparator input, a feedback loop is created. The pulse repe-

tition rate will increase until the average voltage on the inte-

grator is equal to the DC input voltage at pin 7. The average

voltage at pin 6 is proportional to the output frequency

because the amount of charge in each current pulse is

precisely controlled.

Because the one-shot firing frequency is the same as the

open collector output frequency, the output frequency is

directly proportional to VIN.

The external passive components set the scale factor. For

best linearity, RS should be limited to a range of 12 kW to

20 kW

The reference voltage is nominally 2.25V for the RC4152.

Recommended values for different operating frequencies are

shown in the table below.

Operating

Range

DC to 1.0 kHz

DC to 10 kHz

DC to 100 kHz

RO

6.8 kW

6.8 kW

6.8 kW

CO

0.1 mF

0.01 mF

0.001 mF

RB

100 kW

100 kW

100 kW

CB

10 mF

10 mF

10 mF

The single supply VFC is recommended for uses where

dynamic range of the input is limited, and the input does not

reach 0V. With 10 kHz values, nonlinearity will be less than

1.0% for a 10 mV to 10V input range, and response time will

be about 135 ms.

Precision Current Sourced VFC

This circuit operates similarly to the single supply VFC,

except that the passive R-C integrator has been replaced by

an active op amp integrator. This increases the dynamic

range down to 0V, improves the response time, and

eliminates the nonlinearity error introduced by the limited

compliance of the switched current source output.

The integrator algebraically sums the positive current pulses

from the switched current source with the current VIN/RB.

To operate correctly, the input voltage must be negative, so

that when the circuit is balanced, the two currents cancel.

T = ------1-------

FOUT

--V-----I-N----

RB

=

IOUT

T----P-

T

where TP = 1.1 ROCO

IOUT

=

V-----R----E---F-

RS

By rearranging and substituting,

FOUT

=

--V-----I--N---- -R----S- -----------1-----------

VREF RB 1.1ROCO

Recommended component values for different operating

frequencies are shown in the table below.

Range

Output

Scale

Input VIN

FO

Factor RO

CO

0 to -10V 0 to 1.0 kHz 0.1 KHz/V 6.8 kW 0.1 mF

CI

RB

0.05 mF 100 kW

0 to -10V 0 to 10 kHz 1.0 KHz/V 6.8 kW 0.01 mF 0.005 mF 100 kW

0 to -10V 0 to 100 kHz 10 KHz/V 6.8 kW 0.001 mF 500 pF 100 kW

The graphs shown under Typical Performance Characteris-

tics show nonlinearity versus input voltage for the precision

current sourced VFC. The best linearity is achieved by using

an op amp having greater than 1.0 V/ms slew rate, but any op

amp can be used.

Precision Voltage Sourced VFC

This circuit is identical to the current sourced VFC, except

that the current pulses into the integrator are derived directly

from the switched voltage reference. This improves tempera-

ture drift at the expense of high frequency linearity.

The switched current source (pin 1) output has been tied to

ground, and RS has been put in series between the switched

voltage reference (pin 2) and the summing node of the op

amp. This eliminates temperature drift associated with the

switched current source. The graphs under the Typical

Performance Characteristics show that the nonlinearity error

is worse at high frequency, when compared with the current

sourced circuit.

Single Supply FVC

A frequency-to-voltage converter performs the exact oppo-

site of the VFCs function; it converts an input pulse train into

an average output voltage. Incoming pulses trigger the input

comparator and fire the one-shot. The one-shot then dumps a

charge into the output integrator. The voltage on the integra-

tor becomes a varying DC voltage proportional to the

frequency of the input signal. Figure 4 shows a complete

single supply FVC.

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