Datasheet
VFC320
5
SBVS017A
(8)
In the time t
1
+ t
2
the integrator capacitor C
2
charges and
discharges but the net voltage change is zero.
Thus ∆Q = 0 = I
IN
t
1
+ (I
IN
– I
A
) t
2
So that I
IN
(t
1
+ t
2
) = I
A
t
2
But since t
1
+ t
2
= and I
IN
=
f
OUT
=
In the time t
1
, I
B
charges the one-shot capacitor C
1
until its
voltage reaches –7.5V and trips comparator B.
Thus t
2
=
Using in yield f
V
RC
I
I
OUT
IN
B
A
() ()
.
76
75
11
=•
Since I
A
= I
B
the result is
f
OUT
=
Since the integrating capacitor, C
2
, affects both the rising
and falling segments of the ramp voltage, its tolerance and
temperature coefficient do not affect the output frequency. It
should, however, have a leakage current that is small com-
pared to I
IN
, since this parameter will add directly to the gain
error of the VFC. C
1
, which controls the one-shot period,
should be very precise since its tolerance and temperature
coefficient add directly to the errors in the transfer function.
begins to ramp down again before the input amplifier has a
chance to saturate. In effect the comparators and flip-flop
form a one-shot whose period is determined by the internal
reference and a 1mA current sink plus the external capacitor,
C
1
. After the one-shot resets, f
OUT
changes back to logic 0
and the cycle begins again.
The transfer function for the VFC320 is derived for the
circuit shown in Figure 4. Detailed waveforms are shown in
Figure 5.
f
OUT
=
1
t
1
+ t
2
0V
–7.5V
∆V
OUT
t
1
t
2
VFC Output
f
OUT
One-shot
V
C1
Integrator Output
V
OUT
FIGURE 5. Integrator and VFC Output Timing.
(1)
FIGURE 4. Functional Block Diagram of the VFC320.
f
OUT
V
IN
1
R
1
I
A
R
2
R
2
V
IN
C
IN
7.5
I
B
7.5 R
1
C
1
V
IN
(3)
(2)
(4), (5)
(6)
(7)
(9)
Comparators
Flip-
flop
Common
f
OUT
f
IN
One-shot
V
OUT
1
5411
I
A
I
IN
e
1
e
2
Switch
C
1
One-shot
Capacitor
–V
CC
12
+V
CC
7
Pull-up
Resitor
R
2
1013
Input
Amp
Constant
Current Sinks
(1mA)
–7.5V
Ref
B
Integrating
Capacitor
R
1
Input Resistor
Q
1
C
2
I
B
A
For Postive Input Voltages use e
1
, short e
2
.
For Negative Input Voltages use e
2
, short e
1
.
For Differental Input Voltages use e
1 and
e
2
.
V
IN:
f
OUT
=
V
IN
7.5 R
1
C
1
+V
PULL-UP
(V
PU
)
(5V to 15V Typically)
14