The L292 is a monolithic LSI circuit in 15-lead
Multiwatt ® package. It is intended for use, together
with L290 and L291, as a complete 3-chip motor
positioning system for applications such as car-
riage/daisy-wheel position control in type-writes.
The L290/1/2 system can be directly controlled by
as a 3-chip microprocessor controlled positioning
system. The device may be used separately - par-
ticularly the L292 motor driver - but since they will
usually be used together, a description of a typical
L290/1/2 system follows.
speed, high-accurancy positioning.
Speed commands for the system originate in the
microprocessor. It is continuosly updated on the
motor position by means of pulses from the L290
tachometer chip, whitch in tur gets its information
from the optical encoder. From this basic input, the
microprocessor computes a 5-bit control word that
sets the system speed dependent on the distance
When the motor is stopped and the microprocessor
orders it to a new positio, the system operates
initially in an open-loop configuration as there is no
feedback from the tachometer generator. A maxi-
mum speed is reached, the tachometer chip output
backs off the processor signal thus reducing accel-
ering torque. The motor continues to run at rop
speed but under closed-loop control.
As the target position is approached, the microproc-
essor lowers the value of the speed-demand word;
this reduces the voltage at the main summing point,
in effect braking the motor. The braking is applied
progressively until the motor is running at minimum
the position mode, (strobe signal at pin 8 of L291)
and within 3 to 4 ms the L292 drives the motor to
a null position, where it is held by electronic "de-
The mechanical/electrical interface consists of an
speed of rotation. The optical encoder also provides
an output at one position on the disk which is used
to set the initial position.
The opto encoder signals, FTA and FTB are filtered
by the networks R
The main function on the L290 is to implement the
the rotation speed and its polarity indicates the
direction of rotation.
The above function is performed by amplifying the
input signals in A
sign of the first input of the other multiplier before
differentiation, these are obtained using the compa-
TACHO can be found from the following expression:
This signals are used by the microprocessor to
determine the position by counting the pulses.
The L2910 internal reference voltage is also derived
L291 to compensate for variations in input levels,
temperature changes and ageing.
The "one pulse per rotation" opto encoder output
is connected to pin 12 of the L290 (FTF) where it is
squared to give the STF logic output for the micro-
The TACHO signal and V
system where TACHO and the D/A converter output
The input to D/A converter consists of 5 bit word
plus a sign bit supplied by the microprocessor. The
sign bit represets the direction of motor rotation.
The (analogue) output of the D/A conveter -
DAC/OUT - is compared with the TACHO signal and
the risulting error signal is amplified by the error
amplifier, and subsequently appears on pin 1.
The ERRV sognal (from pin 1 , L291) is fed to pin
6 of the final chip, the L292 H-bridge motor-driver.
This input signals is bidirectional so it must be
converted to a positive signal bacause the L292
uses a single supply voltage. This is accomplished
by the first stage - the level shifter, which uses an
internally generated 8 V reference.
This same reference voltage supplies the triangle
wave oscillator whose frequency is fixed by the
of the output stage and should be in the range 1 to
Motor current is regulated by an internal loop in the
L292 which is performed by the resistors R
put of which is filtered by an external RC network
and fed back to the error amplifier.
The choise of the external components in these RC
network (pins 5, 7, 9) is determined by the motor
type and the bandwidth requirements. The values
shown in the diagram are for a 5
The error signal obtained by the addition of the input
and the current feedback signals (pin 7) is used to
pulse width modulate the oscillator signal by means
of the comparator. The pulse width modulated sig-
nal controls the duty cycle of the Hbridge to give an
output current corresponding to the L292 input
The interval between one side of the bridge switch-
ing off and the other switching on,
using the network shown in fig. 2.
Finally, two enable inputs are provited on the L292
(pins 12 and 13-active low and high respectively).
Thus the output stage may be inhibited by taking
pin 12 high or by taking pin 13 low. The output will
also be inhibited if the supply voltage falls below 18
because they are intended to be driven directly by
a microprocessor. Currently available microproces-
sors may generates spikes as high as 1.5 V during
of applications such as motor inhibit during reset of
the logical system and power-on reset (see fig. 3).