L497B 查看數據表（PDF） - STMicroelectronics
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L497B Datasheet PDF : 11 Pages
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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 capacitorCW beginsdischargingwith a constant
current l11D. When the set peak value of the coil cur-
rent is reached, this capacitor charges with a con-
stant current I11C = 13.3 x I11D, and the coil current
is kept constant by desaturationof the driven stage
and the external darlington.
The capacitor CT starts charging on the posi-
tive.edge of the Hall-effect input signal with a con-
stant current I10C. The dwell angle, and conse-
quentlythe starting point of the coil current conduc-
tion, is decided by the comparison between V10 and
V11.
A positive hysteresis is added to the dwell compa-
rator to avoid spurious effects and CT is rapidly dis-
charged on the negative edge of Hall-effects input
signal.
In this way the average voltage on CW increases if
the motor speed decreases and viceversa in order
to
maintain
constant
the
ratio
td
T
at any motor speed.
td
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 CT = CW as recommended
and if the values of the applicationcircuit of fig.4 are
used.
td
T
=
1
+
1
I11C
/
I11D
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.
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
Isense current flowing in the sensing resistor Rs on
the emitter of the external darlington. Isense is given
by :
Isense = Icoil + I14
When the voltage drop across Rs reaches the inter-
nal comparator thresholdvalue the feedbackloop is
activated and Isense kept constant (fig.1) forcing the
external darlington in the active region. In this con-
dition :
Isense = Icoil
Whena precisepeak coil currentis required Rs must
be trimmed or an auxiliary resistor divider (R10, R11)
added :
Ic
p
eak(A)
=
0R.3S2) 0⋅
R10
R11
+
1
SLOW RECOVERY CONTROL (fig. 2)
If Isense has not reached 94 % of the nominal value
just before the negativeedge of the Hall-effect input
signal, the capacitor Csrc and CW are quickly dis-
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 Tdesat; then the voltage on CSRC in-
creases linearly until the standby is reached.During
this recoverytime the CSRC voltageis convertedinto
a current which, substrated from the charging cur-
rent of the dwell capacitor, produces a Tdesat modu-
lation. This means that the Tdesat decreases slowly
until its value reaches,after a time TSRC, thenominal
7% value.
The time TSRC is given by:
Trsc = 12.9 R7 CSRC (ms)
where R7 isthe biasing resistor at pin 12 (in KΩ) and
Csrc the capacitor at pin 8 (in µF).
6/11
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