Datasheet

TC9400/9401/9402
DS21483D-page 8 © 2007 Microchip Technology Inc.
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
easily understood by referring to Figure 3-1. The input
voltage (V
IN
) is converted to a current (I
IN
) 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 = C
REF
x V
REF
), causing
the op amp output to step up a finite amount.
At the end of the charging period, C
REF
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
balance 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 C
REF
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: 10 Hz to 10 kHz V/F Converter.
–
+
+5V
+
5V
14
V
DD
+
5V
R
L
10 kΩ
R
L
10 kΩ
8
10
9
F
OUT
F
OUT
/2
11
3ms
Delay
Self-
Start
12
5
20 kΩ
60 pF
Op Amp
C
INT
820 pF
C
REF
180 pF
12 pF
R
IN
1MΩ
V
IN
+5V
-5V
50 kΩ
510 kΩ
10 kΩ
3
1
Offset
Adjust
I
IN
Zero Adjust
0V –10V
I
BIAS
V
SS
4
-5V
2
Output
Common
V
REF
OUT
R
BIAS
100 kΩ
AMP OUT
TC9400
TC9401
TC9402
GND
6
Threshold
Detector
Threshold
Detect
Reference Voltage
(Typically -5V)
÷2
V
REF
7
-3V
INPUT