Datasheet
TC9400/9401/9402
DS21483D-page 14 © 2007 Microchip Technology Inc.
6.0 F/V CONVERTER DESIGN
INFORMATION
6.1 Input/Output Relationships
The output voltage is related to the input frequency
(F
IN
) by the transfer equation:
EQUATION 6-1:
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 frequen-
cies.
When the TC9400 is used in the Single Supply mode,
V
REF
is defined as the voltage difference between Pin 7
and Pin 2.
6.2 Input Voltage Levels
The input frequency is applied to the Threshold
Detector input (Pin 11). As discussed in the V/F circuit
section of this data sheet, the threshold of Pin 11 is
approximately (V
DD
+ V
SS
)/2 ±400 mV. Pin 11’s input
voltage range extends from V
DD
to about 2.5V below
the threshold. 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 Threshold 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 6-1(a).
The level shifter circuit in Figure 6-1(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 2.1 mV/°C, so for high ambient
temperature operation, two diodes in series are
recommended.
FIGURE 6-1: Frequency Input Level Shifter.
V
OUT
= [V
REF
C
REF
R
INT
] F
IN
+5V
14
64
+5V
-5V
V
DD
1.0
11
33 kΩ
IN914
V
SS
DET
TC9400
(a) ±5V Supply
(b) Single Supply
0.01 µF
Frequency
Input
0V
GND
+8V to +15V
14
10 kΩ
4
+5V
V
DD
1.0
11
33 kΩ
IN914
V
SS
DET
TC9400
0.01 µF
Frequency
Input
0V
0.1 µF
10 kΩ
MΩ
MΩ