VFC121 Datasheet, PDF (6/8 Pages) Burr-Brown (TI) – Precision Single Power Supply VOLTAGE-TO-FREQUENCY CONVERTER

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V_IN = 0 to +2V
11
R_BIAS = 8kΩ
(Optional)
R_IN = 8kΩ
R_TRIM
R_IN
C_INT = 2700pF
10
Integrator
+5V
+V_S
9
13
Comparator
V_REF
One
Shot
I_REF
V_REF
Ground
(Optional)
2
4
6
3
5
C_OS = 1200pF
2.6V
V_T
FIGURE 3. 2V Full Scale Input, 100kHz Full Scale Output.
V_{PULL UP}
R_{PULL UP}
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f_OUT =
0 to 100kHz
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the integrating op amp to see a constant current, reducing
errors that might occur if the load were unbalanced.) In this
state, the output of the integrator resumes a positive ramp,
restarting the cycle.
The output frequency is regulated by the balance of current
(or charge) between the current V_IN/R_IN and the time-
averaged reset current. The size of the integrating capacitor,
C_INT, determines the slew rate of the integrator, and thus
how far down the integrator ramps during the one-shot
period, but has no effect on the output frequency of the
VFC121.
The reference voltage used internally is generated from a
bandgap reference, which is actively trimmed to achieve the
low drift characteristics of the VFC121. To maximize flexi-
bility of designs using the VFC121, both the bandgap
reference voltage and a thermometer voltage are available
externally.
INSTALLATION AND
OPERATING INSTRUCTIONS
BASIC OPERATION
The VFC121 allows users a wide range of input voltages and
supply voltages, and easy control of the full scale output
frequency. The basic connections are shown in Figure 3,
with components that generate a 100kHz output with a 2V
full scale input.
For other input and output ranges, the full scale input
voltages and full scale output frequencies can be calculated
as follows:
f_FS
=
V_FS
2(R_IN)(C_OS
+
60)
The full scale input current of 250µA was chosen to provide
a 25% duty cycle in the output frequency. The VFC121 is
designed to give optimum linearity under these conditions,
but other current levels can be used without significantly
degrading linearity. By reducing R_IN, the integrating current
is increased, increasing the positive ramp rate of the integra-
tor output. Since the one-shot period is unchanged, the duty
cycle of the output increases.
Stray capacitance at the C_OS pin typically adds about 60pF
to the capacitance of the external C_OS, which accounts for
the adjustment in the above equation. This usually becomes
negligible as the required output frequency is reduced, and
C_OS is increased.
R_BIAS is included in the circuit in Figure 3 to compensate for
the effects of bias currents at the input of the integrating op
amp. It is optional in most applications, but when needed,
R_BIAS should equal R_IN.
Table 1 indicates standard external component values for
common input voltage ranges and output frequency ranges.
COMPONENT SELECTION
Selection of the external resistor and capacitor type is
important. Temperature drift of the external input resistor
and one-shot capacitor will affect temperature stability of
the output frequency. NPO ceramic capacitors will normally
produce the best results. Silver-mica types will result in
slightly higher drift, but may be adequate in many applica-
tions. A low temperature coefficient film resistor should be
used for R_IN.
The integrator capacitor, C_INT, serves as a “charge bucket,”
where charge accumulation is induced by the input, V_IN, and
FULL SCALE INPUT RANGE (V)
R_IN + R_TRIM (kΩ)
2
8
5
20
10
40
FULL SCALE OUTPUT FREQUENCY (kHz) C_OS (pF)
C_INT (pF)
1500
1000
500
250
125
25
22
68
180
470
1000
4700
150
270
470
1000
2200
10,000
NOTE: Higher output frequencies can be achieved by reducing R_IN.
TABLE 1. Standard External Component Values
®
VFC121
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