TC9401 datasheet(15/24 Pages) MICROCHIP

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DS21483C-page 15

TC9400/9401/9402

7.0

F/V CONVERTER DESIGN

INFORMATION

7.1

Input/Output Relationships

The output voltage is related to the input frequency

(FIN) by the transfer equation:

EQUATION 7-1:

The response time to a change in FIN is equal to (RINT

CINT). The amount of ripple on VOUT is inversely

proportional to CINT and the input frequency.

CINT can be increased to lower the ripple. Values of 1 μF

to 100

μF are perfectly acceptable for low frequencies.

When the TC9400 is used in the Single Supply mode,

VREF is defined as the voltage difference between Pin 7

and Pin 2.

7.2

Input Voltage Levels

The input frequency is applied to the Threshold Detec-

tor input (Pin 11). As discussed in the V/F circuit section

of this data sheet, the threshold of Pin 11 is approxi-

mately (VDD + VSS)/2 ±400 mV. Pin 11’s input voltage

range extends from VDD to about 2.5V below the thresh-

old. If the voltage on Pin 11 goes more than 2.5 volts

below the threshold, the V/F mode start-up comparator

will turn on and corrupt the output voltage. The Thresh-

old Detector input has about 200 mV of hysteresis.

In ±5V applications, the input voltage levels for the

TC9400 are ±400 mV, minimum. If the frequency

source being measured is unipolar, such as TTL or

CMOS operating from a +5V source, then an AC

coupled level shifter should be used. One such circuit

is shown in Figure 7-1(a).

The level shifter circuit in Figure 7-1(b) can be used in

single supply F/V applications. The resistor divider

ensures that the input threshold will track the supply

voltages. The diode clamp prevents the input from

going far enough in the negative direction to turn on the

start-up comparator. The diode’s forward voltage

decreases by 2.1mV/°C, so for high ambient tempera-

ture operation, two diodes in series are recommended;

also, see Figure .

FIGURE 7-1:

Frequency Input Level Shifter

VOUT = [VREF CREF RINT] FIN

+5V

-5V

VDD

1.0M

33k

IN914

VSS

DET

TC9400

(a) ±5V Supply

(b) Single Supply

0.01

μF

Frequency

Input

GND

+8V to +5V

10k

+5V

VDD

1.0M

33k

IN914

VSS

DET

TC9400

0.01

μF

Frequency

Input

0.1

μF

10k

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