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CIRCUIT OPERATION
The L294 work as a trasconductance amplifier: it
can supply an output current directly proportional
to an input voltage level (V
i
). Fyrthermore, it allows
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.
The t
on
time is fixed by the width of the Enable input
signal (TTL compatible): it is active low and enables
the output stages "source" and "sink". At the end of
t
on
, the load current I
load
recirculates through D1
and D2, allowing fast current turn-off.
The rise time t
r
, depends on the load charac-
teristics, on V
i
and on the supply voltage value (V
s
,
pin 1). During the t
on
time, I
load
is converter into a
voltage signal by means of the external sensing
resistance R
s
connected to pin 10. This signal,
amplified by the op amp and converted by the
transconductance amplifier OTA, charges the exter-
nal RC network at pin 8 (R1, C1). The voltage at this
pin is sensed by the inverting input of a comparator.
The voltage on the non-inverting input of this one
is fixed by the external voltage V
i
(pin 7).
After t
r
, the comparator switches and the output
stage "source" is switched off. The comaprator out-
put is confirmed by the voltage on the non-inverting
input, which decreases of a costant fraction of V
i
(1/10), allowing hysteresis operation. The 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
comparator is just the value proportional to I
load
. In
the second case, when the current decreases too
quickly, the comparator senses the voltage signal
stored in the R1 C1 network.
In the first case t
1
depends on the load charac-
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
of t
1
)t
1
≥
1/10 R1.C1. Note that C1 should be
chosen in the range 2.7 to 10 nF for stability reasons
of the OTA).
After t
1
, the comparator switches again: the output
is confirmed by the voltage on the non-inverting
input, which reaches V
i
again (hysteresis).
Now the cycle starts again: t
2
, t
4
and t
6
have the
same characteristics as t
r
, while t
3
and t
5
are similar
to t
1
. The peak current I
p
depends on V
i
as shown
in the typical transfer function of fig.3.
It can be seen that for V
i
lower than 450 mV the
device is not operating.
For V
i
greater than 600 mV, the L294 has a tran-
sconductance of 1A/V with R
s
= 0.2
Ω
. For V
i
in-
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 protection).
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), regard-
less 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
(i.e. for a sink current greater than 1V/R
s
) the output
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
through V
ss
(pin 4). The DIAG signal is reset and
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
