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RC4153
PRODUCT SPECIFICATION
9
Voltage
Reference
7.3V
Switched
Current
Source
R
IN
V
IN
0 to 10V
A
Io
UT
C
(Timing)
o
C
B
I
C
Trigger
+V
S
Ext
Load
E
Open
Collector
Output
D
One
Shot
Integrator
4153-07
Principles of Operation
The 4153 consists of several functional blocks which provide
either voltage-to-frequency or frequency-to-voltage conver-
sion, depending on how they are connected. The operation is
best understood by examining the block diagram as it is pow-
ered in a voltage-to-frequency mode (Figure 4).
When power is first applied, all capacitors are discharged.
The input current, V
IN
/R
IN
, causes C
I
to charge, and point C
will try to ramp down. The trigger threshold of the one-shot
is approximately +1.3V, and if the integrator output is less
than +1.3V, the one-shot will fire and pulse the open collec-
tor output E and the switched current source A (see Figures 4
and 5). Because the point C is less than +1.3V, the one-shot
fires, and the switched current source delivers a negative
current pulse to the integrator. This causes C
IN
to charge in
the opposite direction, and point C will ramp up until the end
of the one-shot pulse. At that time, the positive current
V
IN
/R
IN
will again make point C ramp down until the trigger
threshold is reached.
When power is applied, the one-shot will continuously fire
until the integrator output exceeds the trigger threshold.
Once this is reached, the one-shot will fire as needed to keep
the integrator output above the trigger threshold. If V
IN
is
increased, the slope of the downward ramp increases, and the
one-shot will fire more often in order to keep the integrator
output high. Since the one-shot firing frequency is the same
as the open collector output frequency, any increase in V
IN
will cause an increase in F
OUT
. This relationship is very
linear because the amount of charge in each I
OUT
pulse is
carefully defined, both in magnitude and duration. The dura-
tion of the pulse is set by the timing capacitor C
O
(point D).
This feedback system is called a charge-balanced loop.
The scale factor K (the number of pulses per second or a
specified V
IN
), is adjusted by changing either R
IN
and there-
fore I
IN
, or by changing the amount of charge in each I
OUT
pulse. Since the magnitude of I
OUT
is fixed at 1 milliamp, the
way to change the amount of charge is by adjusting the one-
shot duration set by C
O
(I
OUT
may be adjusted by changing
V
REF
). The accuracy of the relationship between V
IN
and
F
OUT
is affected by three major sources of error: temperature
drift, nonlinearity and offset.
The total temperature drift is the sum of the individual drift
of the components that make up the system. The greatest
source of drift in a typical application is in the timing
capacitor, C
O
. Low temperature coefficient capacitors, such
as silver mica and polystyrene, should be measured for drift
using a capacitance meter. Experimentation has shown that
the lowest tempco’s are achieved by wiring a parallel capaci-
tor composed of 70% silver mica and 30% polystyrene.
The reference on the chip can be replaced by an external
reference with much tighter drift specifications, such as an
LM199. The 199’s 6.9V output is close to the 4153’s 7.3V
output, and has less than 10 ppm/
°
C drift.
Nonlinearity is primarily caused by changes in the
precise amount of charge in each I
OUT
pulse. As frequency
increases, internal stray capacitances and switching
problems change the width and amplitude of the I
OUT
pulses,
causing a nonlinear relationship between V
IN
and F
OUT
.
Figure 4. Voltage-to-Frequency Block Diagram
Figure 5. Voltage-to-Frequency Timing Waveforms
V
OUT
+5V
+10V
0
A
-I
B
C
IN
D
E
Switched Current
Source Output
-1.0 mA
Switched Current Source
Logic (Internal)
Integrator Output
V +1.3V
-0.65V
One Shot Timing (C )
T = 1.5 x 10 C
-4.1V
-V
S
Logic Output
-0.2V
F =
O
V
2V R C
IN
O
O
O
4153-08