Specifications
there must be some current flowing in the resistor above the
3 mA circuit current to operate the regulator. As an exam-
ple, if the raw supply varies from 9V to 16V, a resistance of
470X will minimize these zener voltage variations to 160
mV. If the resistor goes under 400X or over 600X the zener
variation quickly rises above 200 mV for the same input vari-
ation. Take care also that the power dissipation of the IC is
not exceeded at higher supply voltages.
Figure 4
shows
suitable dropping resistor values.
TL/H/7451–7
FIGURE 4. Zener Regular Bias Resistor Range
Input Interface Circuits
The ground referenced input capability of the LM2907-8 al-
lows direct coupling to transformer inputs, or variable reluc-
tance pickups.
Figure 5(a)
illustrates this connection. In
many cases, the frequency signal must be obtained from
another circuit whose output may not go below ground. This
may be remedied by using ac coupling to the input of the
LM2907 as illustrated in
Figure 5(b).
This approach is very
suitable for use with phototransistors for optical pickups.
Noisy signal sources may be coupled as shown in
Figure
5(c).
The signal is bandpass filtered. This can be used, for
example, for tachometers operating from breakerpoints on a
conventional Kettering ignition system. Remember that the
minimum input signal required by the LM2907 is only 30
mVp-p, but this signal must be able to swing at least 15 mV
on either side of the inverting input. The maximum signal
which can be applied to the LM2907 input, is
g
28V. The
input bias current is a typically 100 nA. A path to ground
must be provided for this current through the source or by
other means as illustrated. With 14-pin package versions of
LM2907, LM2917, it is possible to bias the inverting input to
the tachometer as illustrated in
Figure 5(d).
This enables the
circuit to operate with input signals that do not go to ground,
but are referenced at higher voltages. Alternatively, this
method increases the noise immunity where large signal
TL/H/7451–8
(a) Ground Referenced Inputs
TL/H/7451–9
(b) AC Coupled Input
TL/H/7451–10
(c) Bandpass Filtered Input
Reduces Noise
TL/H/7451–11
(d) Above Ground Sensing
TL/H/7451–12
(e) High Common-Mode Rejection Input Circuit
FIGURE 5. Tachometer Input Configurations
5