NJM4151
-
-
Ver.2012-10-24
The one-shot is made from a voltage comparator and an R-S latch, Transistors Q12-Q15 and Q18-Q20 form the
comparator, while Q8-Q11 and Q16-Q17 make up the R-S latch. One latch output, open-collector reset transistor Q16, is
connected to a comparator input and to the terminal, pin 5. Timing resistor R
O
is tied externally from pin 5 to V
+
and timing
capacitor C
O
is tied from pin 5 to ground. The other comparator input is tied to a voltage divider R
3
-R
5
which sets the
comparator threshold voltage at 0.667V
+
. One-shot operation is initiated when the collector of Q7 goes low and sets the
latch. This causes Q16 to turn off, releasing the voltage at pin 5 to charge exponentially towards V
+
through R
O
. As soon
as this voltage reaches 0.667 V
+
, comparator output Q20 will go high causing Q10 to reset the latch. When the latch is
reset, Q16 will discharge C
O
to ground. The one-shot has now completed its function of creating a pulse of period T=1.1
R
O
C
O
at the latch output, Q21. This pulse is buffered through Q23 to drive the open-collector logic circuit transistor Q32.
During the one-shot period the logic output will be in the low state. The one-shot output is also used to switch the
reference voltage by Q22 and Q24. The low T. C. reference voltage is derived from the combination of a 5.5V zener
diode with resistor and diode level shift networks. A stable 1.89 volts is developed at pin 2, the emitter of Q33.
Connecting the external current-setting resistor R
S
= 14.0Ω from pin 2 to ground gives 135µA from the collectors of Q33
and Q34. This current is reflected in the precision current mirror Q35-Q37 and produces the output current I
O
at pin 1.
When the R-S latch is reset, Q22 and Q24 will hold the reference voltage off, pin 2 will be at 0V, and the current will be off.
During the one-shot period T, the latch will be set, the voltage of pin 2 will go to 1.89V, and the output current will be
switched on.
■ TYPICAL APPLICATION
1. Single supply Voltage-to-Frequency Converter
Figure 2 shows the simplest type of VFC that can be made with the NJM4151. Input voltage range is from 0 to +10V,
and output frequency is from 0 to 10kHz. Full scale frequency can be tuned by adjusting R
s
, the output current set resistor.
This circuit has the advantage of being simple and low in cost, but it suffers from inaccuracy due to a number of error
sources. Linearity error is typically 1%. A frequency offset will also be introduced by the input comparator offset voltage.
Also, response time for this circuit is limited by the passive integration network R
B
C
B
. For the component values shown in
Figure 2, response time for a step change input from 0 to +10V will be 135msec. For applications which require fast
response time and high accuracy, use the circuits of Figure 3 and 4.
Figure 2. Single Supply Voltage-to-Frequency Converter
2. Precision VFC with Single Supply Voltage
For applications which require a VFC which will operate from a single positive supply with positive input voltage, the
circuit of Figure 3 will give greatly improved linearity, frequency offset, and response time. Here, an active integrator using
one section of the NJM3403A quad ground-sensing op-amp has replaced the R
B
-C
B
network in Figure 2. Linearity error
for this circuit is due only to the NJM4151 current source output conductance. Frequency offset is due only to the op-amp
input offset and can be nulled to zero by adjusting R
B
. This technique uses the op-amp bias current to develop the null
voltage, so an op-amp with stable bias current, like the NJM3403A, is required.
