TC9400_06 Datasheet, PDF (7/24 Pages) Microchip Technology – Voltage-to-Frequency/Frequency-to-Voltage Converters

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 (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-
Threshold
11 Detect
Threshold
Detector
-3V
12 AMP OUT
Self-
Start
5 ^{VREF OUT}
C_INT
820 pF
INPUT
V_IN
R_IN
1MΩ
C_REF
180 pF
12 pF
3 ^IIN
20 kΩ
60 pF
–
+5V
0V –10V
510 kΩ
50 kΩ
Zero Adjust
2
Op Amp
+
-5V
Offset
Adjust
10 kΩ
I_BIAS
1
R_BIAS
100 kΩ
FIGURE 3-1:
10 Hz to 10 kHz V/F Converter
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 addi-
tion, 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.
+5V
14
V_DD
3μsec
Delay
+5V
F_{OUT 8}
R_L
10 kΩ
+5V
F_OUT/2 ₁₀
R_L
10 kΩ
÷2
9
Output
Common
TC9400
TC9401
TC9402
V_SS
4
V_REF
7
GND
6
Reference Voltage
(Typically -5V)
-5V
© 2006 Microchip Technology Inc.
DS21483C-page 7