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DWELL ANGLE CONTROL
The dwell angle control circuit calculates the con-
duction time D for the output transistor in relation to
the speed of rotation, to the supply voltage and to
the characteristics of the coil.
On the negative edge of the Hall-effect input signal
the capacitor C
W
begins discharging with a constant
current l
11D
. When the set peak value of the coil cur-
rent is reached, this capacitor charges with a con-
stant current I
11C
= 13.3 x I
11D
, and the coil current
is kept constant by desaturation of the driven stage
and the external darlington.
The capacitor C
T
starts charging on the posi-
tive.edge of the Hall-effect input signal with a con-
stant current I
10C
. The dwell angle, and conse-
quently the starting point of the coil current conduc-
tion, is decided by the comparison between V
10
and
V
11
.
A positive hysteresis is added to the dwell compa-
rator to avoid spurious effects and C
T
is rapidly dis-
charged on the negative edge of Hall-effects input
signal.
In this way the average voltage on C
W
increases if
the motor speed decreases and viceversa in order
to maintain constant the ratio
t
d
T
at any motor speed.
t
d
T
is kept constant (and not
D
T
= cost) to control
the power dissipation and to have sufficient time to
avoid low energy sparks during acceleration.
DESATURATION TIMES IN STATIC
CONDITIONS
In static conditions and if C
T
= C
W
as recommended
and if the values of the application circuit of fig.4 are
used.
t
d
T
=
1
1 + I
11C
/
I
11D
DESATURATION TIMES IN LOW AND HIGH
FREQUENCY OPERATION
Due to the upper limit of the voltage range of pin 11,
if the components of fig.4 are used, below 10 Hz
(300 RPM for a 4 cylinder engine) the OFF time
reaches its maximum value (about 50 ms) and then
the circuit gradually loses control of the dwell angle
because D = T – 50 ms.
Over 200 Hz (6000 RPM for a 4 cylinder engine) the
available time for the conduction is less than 3.5 ms.
If the used coil is 6 mH, 6A, the OFF time is reduced
to zero and the circuit loses the dwell angle control.
TRANSIENT RESPONSE
The ignition system must deliver constant energy
even during the condition of acceleration and decel-
eration of the motor below 80Hz/s. These conditions
can be simulated by means of a signal gene-rator
with a linearly modulated frequency between 1 Hz
and 200 Hz (this corresponds to a change between
30 and 6000 RPM for a 4 cylinders engine).
CURRENT LIMIT
The current in the coil is monitored by measuring the
I
sense
current flowing in the sensing resistor R
s
on
the emitter of the external darlington. I
sense
is given
by :
I
sense
= I
coil
+ I
14
When the voltage drop across Rs reaches the inter-
nal comparator threshold value the feedback loop is
activated and I
sense
kept constant (fig.1) forcing the
external darlington in the active region. In this con-
dition :
I
sense
= I
coil
When a precise peak coil current is required R
s
must
be trimmed or an auxiliary resistor divider (R
10
, R
11
)
added :
Icpeak (A) =
0.320
RS)
⋅
R10
R11
+ 1
SLOW RECOVERY CONTROL (fig. 2)
If I
sense
has not reached 94 % of the nominal value
just before the negative edge of the Hall-effect input
signal, the capacitor C
src
and C
W
are quickly dis-
charged as long as the pick-up signal is "low". At the
next positive transition of the input signal the load
current starts immediately, producing the maximum
achievable T
desat
; then the voltage on C
SRC
in-
creases linearly until the standby is reached. During
this recovery time the C
SRC
voltage is converted into
a current which, substrated from the charging cur-
rent of the dwell capacitor, produces a T
desat
modu-
lation. This means that the T
desat
decreases slowly
until its value reaches, after a time T
SRC
, the nominal
7% value.
The time T
SRC
is given by:
T
rsc
= 12.9 R7 C
SRC
(ms)
where R
7
is the biasing resistor at pin 12 (in KΩ) and
C
src
the capacitor at pin 8 (in µF).
L497
6/11
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