L497 STMicroelectronics, L497 Datasheet - Page 6

• Manufacturer Part Number: L497
• Description: HALL EFFECT PICKUP IGNITION CONTROLLER
• Manufacturer: STMicroelectronics

L497
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 capacitorC
current l
rent is reached, this capacitor charges with a con-
stant current I
is kept constant by desaturationof the driven stage
and the external darlington.
The capacitor C
tive.edge of the Hall-effect input signal with a con-
stant current I
quentlythe starting point of the coil current conduc-
tion, is decided by the comparison between V
V
A positive hysteresis is added to the dwell compa-
rator to avoid spurious effects and C
charged on the negative edge of Hall-effects input
signal.
In this way the average voltage on C
the motor speed decreases and viceversa in order
to maintainconstantthe ratio
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
and if the values of the application circuit of fig.4 are
used.
DESATURATION TIMES IN LOW AND HIGH
FREQUENCY OPERATION
Due to the upperlimit of the voltagerange of pin 11,
if the components of fig.4 are used, below 10 Hz
(300 RPM for a 4 cylinder engine) the OFF time
reachesits 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 cylinderengine) the
availabletime for the conductionis 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.
6/11
t
T
11
d
.
is kept constant (and not
11D
. When the set peak value of the coil cur-
11C
W
10C
t
T
d
beginsdischargingwith a constant
= 13.3 x I
T
. The dwell angle, and conse-
starts charging on the posi-
1
I
11C
T
1
11D
= C
t
T
/ I
d
, and the coil current
D
T
11D
at any motor speed.
W
as recommended
= cost) to control
T
W
is rapidly dis-
increases if
10
and
TRANSIENT RESPONSE
The ignition system must deliver constant energy
even duringthe conditionof accelerationand decel-
erationof the motor below80Hz/s.Theseconditions
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 currentin thecoil is monitoredby measuringthe
I
the emitter of the external darlington. I
by :
When the voltage drop across Rs reaches the inter-
nal comparator thresholdvalue the feedbackloop is
activated and I
external darlington in the active region. In this con-
dition :
Whena precisepeak coil currentis requiredR
be trimmed or an auxiliary resistor divider (R
added :
SLOW RECOVERY CONTROL (fig. 2)
If I
just before the negativeedge of the Hall-effect input
signal, the capacitor C
chargedas long as the pick-up signalis ”low”. At the
next positive transition of the input signal the load
current starts immediately, producingthe maximum
achievable T
creases linearly until the standby is reached.During
this recoverytime the C
a current which, substrated from the charging cur-
rent of the dwell capacitor, produces a T
lation. This means that the T
until its valuereaches,after a time T
7% value.
The time T
T
where R
C
sense
rsc
src
sense
= 12.9 R7 C
the capacitor at pin 8 (in F).
current flowing in the sensing resistor R
has not reached 94 % of the nominal value
7
Ic p eak A
isthe biasing resistor at pin 12 (in K ) and
SRC
desat
sense
is given by:
I
SRC
sense
; then the voltage on C
I
sense
kept constant (fig.1) forcing the
(ms)
= I
SRC
0.320
src
RS
coi l
= I
and C
voltageis convertedinto
coil
desat
+ I
R10
R11
14
W
decreases slowly
SRC
are quickly dis-
sense
, thenominal
1
desat
is given
10
SRC
modu-
s
, R
must
s
11
on
in-
)