TC9400 Просмотр технического описания (PDF) - Microchip Technology

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TC9400

Voltage-to-Frequency/Frequency-to-Voltage Converters

Microchip Technology 

TC9400/9401/9402

3.0 DETAILED DESCRIPTION

3.1 Voltage-to-Frequency (V/F) Circuit

Description

The TC9400 V/F converter operates on the principal of

charge balancing. The operation of the TC9400 is eas-

ily understood by referring to Figure 3-1. The input volt-

age (VIN) is converted to a current (IIN) by the input

resistor. This current is then converted to a charge on

the integrating capacitor and shows up as a linearly

decreasing voltage at the output of the Op Amp. The

lower limit of the output swing is set by the threshold

detector, which causes the reference voltage to be

applied to the reference capacitor for a time period long

enough to charge the capacitor to the reference volt-

age. This action reduces the charge on the integrating

capacitor by a fixed amount (q = CREF x VREF), causing

the Op Amp output to step up a finite amount.

At the end of the charging period, CREF is shorted out.

This dissipates the charge stored on the reference

capacitor, so that when the output again crosses zero,

the system is ready to recycle. In this manner, the con-

tinued discharging of the integrating capacitor by the

input is balanced out by fixed charges from the refer-

ence voltage. As the input voltage is increased, the

number of reference pulses required to maintain bal-

ance increases, which causes the output frequency to

also increase. Since each charge increment is fixed, the

increase in frequency with voltage is linear. In addition,

the accuracy of the output pulse width does not directly

affect the linearity of the V/F. The pulse must simply be

long enough for full charge transfer to take place.

The TC9400 contains a "self-start" circuit to ensure the

V/F converter always operates properly when power is

first applied. In the event that, during power-on, the Op

Amp output is below the threshold and CREF is already

charged, a positive voltage step will not occur. The Op

Amp output will continue to decrease until it crosses the

-3.0V threshold of the "self-start" comparator. When

this happens, an internal resistor is connected to the

Op Amp input, which forces the output to go positive

until the TC9400 is in its Normal Operating mode.

The TC9400 utilizes low power CMOS processing for

low input bias and offset currents, with very low power

dissipation. The open drain N-channel output FETs

provide high voltage and high current sink capability.

FIGURE 3-1:

10Hz TO 10kHz V/F CONVERTER

Threshold

11 Detect

Threshold

Detector

+5V

14

VDD

3µsec

Delay

-3V

12 AMP OUT

Self-

Start

+5V

FOUT 8

RL

10kΩ

+5V

FOUT/2 10

RL

10kΩ

÷2

9

Output

Common

5 VREF OUT

INPUT

VIN

CINT

820pF

RIN

1MΩ

CREF

180pF

12pF

3 IIN

0V –10V

+5V

510kΩ

50kΩ

Zero Adjust

2

-5V

Offset

Adjust

10kΩ

20kΩ

60pF

–

Op Amp

+

IBIAS

1

RBIAS

100kΩ

TC9400

TC9401

TC9402

VSS

4

VREF

7

GND

6

Reference Voltage

(Typically -5V)

-5V

© 2002 Microchip Technology Inc.

DS21483B-page 7

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