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The second input to each multipler consists of the

sign of the first input of the other multiplier before

differentiation, these are obtained using the compa-

signal TACHO. The peak-topeak ripple signal of the

TACHO can be found from the following expression:

π

4

(

The max value of TACHO is:

=

π

4

and STB.

This signals are used by the microprocessor to

determine the position by counting the pulses.

The L2910 internal reference voltage is also derived

|

This reference is used by the D/A converter in the

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-

processor.

Pin 12 of this chip is the main summing point of the

system where TACHO and the D/A converter output

are compared.

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

where:

=

1

2RC

)

The oscillator determines the switching frequency

of the output stage and should be in the range 1 to

30 KHz.

Motor current is regulated by an internal loop in the

and the differential current sense amplifier, the out-

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

Ω, 5 MH motor.

(See L292 Transfer Function Calculation in Appli-

cation Information).

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

signal.

The interval between one side of the bridge switch-

ing off and the other switching on,

τ, is programmed

τ.

This can be foud from:

τ

Since R

τ is approximately 1.5 KΩ and the recom-

mended

τ to avoid simultaneous conduction is 2.5

high precision types (maximum tolerance

± 2 %)

and the recommended value is given by:

≤ 0.44 V

It is possible to synchronize two L292 ’s, if desired,

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

V.

L292