TC9400_06 Datasheet, PDF (15/24 Pages) Microchip Technology – Voltage-to-Frequency/Frequency-to-Voltage Converters

TC9400/9401/9402
7.0 F/V CONVERTER DESIGN
INFORMATION
7.1 Input/Output Relationships
The output voltage is related to the input frequency
(F_IN) by the transfer equation:
EQUATION 7-1:
V_OUT = [V_REF C_REF R_INT] F_IN
The response time to a change in F_IN is equal to (R_INT
C_INT). The amount of ripple on V_OUT is inversely
proportional to C_INT and the input frequency.
C_INT 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,
V_REF 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 (V_DD + V_SS)/2 ±400 mV. Pin 11’s input voltage
range extends from V_DD 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 .
+5V
14
V_DD
Frequency
Input
+5V
0V
33k ^{0.01 μF}
TC9400
11 _DET
IN914 1.0M
GND V_SS
6
4
-5V
(a) ±5V Supply
FIGURE 7-1:
Frequency Input Level Shifter
Frequency
Input
+5V
0V
+8V to +5V
14
V_DD
10k
33k ^{0.01 μF}
IN914
TC9400
11 _DET
1.0M
0.1 μF 10k
V_SS
4
(b) Single Supply
© 2006 Microchip Technology Inc.
DS21483C-page 15