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CIRCUIT OPERATION

The L294 work as a trasconductance amplifier: it

can supply an output current directly proportional

complete switching control of the output current

waveform (see fig. 1).

The following explanation refers to the Block Dia-

gram, to fig. 1 and to the typical application circuit

of fig. 2.

signal (TTL compatible): it is active low and enables

the output stages "source" and "sink". At the end of

and D2, allowing fast current turn-off.

voltage signal by means of the external sensing

amplified by the op amp and converted by the

transconductance amplifier OTA, charges the ex-

ternal RC network at pin 8 (R1, C1). The voltage at

this pin is sensed by the inverting input of a com-

parator. The voltage on the non-inverting input of

stage "source" is switched off. The comaprator

output is confirmed by the voltage on the non-in-

verting input, which decreases of a costant fraction

current in the load now flow through D1.

Two Cases are possible: the time constant of the

recirculation phase is higher than R1.C1; the time

constant is lower than R1.C1. In the first case, the

voltage sensed in the non-inverting input of the

the second case, when the current decreases too

quickly, the comparator senses the voltage signal

stored in the R1 C1 network.

teristics, while in the second case it depends only

on the value of R1. C1.

In the other words, R1. C1 fixes the minimum value

≥ 1/10 R1.C1. Note that C1 should be

chosen in the range 2.7 to 10 nF for stability rea-

sons of the OTA).

is confirmed by the voltage on the non-inverting

in the typical transfer function of fig.3.

device is not operating.

cluded between 450 and 600 mV, the operation is

not guaranteed.

The order parts of the device have protection and

diagnostic functions. At pin 3 is connected an ex-

ternal capacitor C2, charged at costant current

when the Enable is low.

After a time interval equal to K C2 (K is defined in

the table of Electrical Characteristics and has the

dimensions of ohms) the output stages are

switched off independently by the Input signal.

This avoids the load being driven in conduction for

an excessive period of time (overdriving protec-

tion). The action of this protection is shown in fig.

1b. Note that the voltage ramp at pin 3 starts

whenever the Enable signal becomes active (low

state), regardless of the Input signal. To reset pin 3

and to restore the normal conditions, pin 9 must

return high.

This protection can be disabled by grounding pin 3.

The thermal protection included in the L294 has a

hysteresis.

It switches off the output stages whenever the

junction temperature increases too much. After a

fall of about 20

°C, the circuit starts again.

Finally, the device is protected against any type of

short circuit at the outputs: to ground, to supply and

across the load.

When the source stage current is higher than 5A

and/or when the pin 10 voltage is higher then 1V

stages are switched off and the device is inhibited.

This condition is indicated at the open-collector

output DIAG (pin 5); the internal flip-flop F/F

changes and forces the output transistor into satu-

ration. The F/F must be supplied independently

the output stages are still operative by switching the

device on again. After that, two cases are possible:

the reason for the "bad operation" is still present

and the protection acts again; the reason has been

removed and the device starts to work properly.

L294