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L484
MAGNETIC PICKUP IGNITION CONTROLLER
L484November 1991
MAGNETIC PICKUP IGNITION CONTROLLER DIRECT DRIVING OF THE EXTERNAL DAR-
LINGTON. OPERATES WITH A WIDE RANGE OF MA-
GNETIC PICKUP TYPES. CHARGING ANGLE (DWELL) CONTROL. COIL CURRENT PEAK LIMITATION . CONTINUOUS COIL CURRENT PROTECTION. TACHOMETER SIGNAL OUTPUT. EXTERNAL DARLINGTON OVERVOLTAGE
PROTECTION. LOAD DUMP AND REVERSE BATTERY PRO-
TECTION. POSSIBILITY OF SPARK POINT DELAYING
(ANTI KNOCK SYSTEM)
DESCRIPTIONThe L484 is an integrated circuit designed for use
with an NPN darlington in breakerless ignition sy-
stems with magnetic pickup sensors and high ener-
gy ignition coils.
A key feature of the L484 is flexibility. It can be used
with a wide variety of magnetic sensors thanks to the
special design which has two input pins from the pic-
kup ; the first is the zero crossing detector for the
ignition command and the second pin is used to cal-
culate the dwell time. Moreover another pin is used
to adapt the L484 to various pickup types.
Other features of the device include darlington over-
voltage protection, dump protection, a supply volt-
age range of 6-28 V.
BLOCK DIAGRAM 1/11
ABSOLUTE MAXIMUM RATINGS
THERMAL DATA
PIN CONNECTION
L4842/11
PIN FUNCTIONS (refer to fig. 2)* this function is particularly useful in antiknock systeme because provides a spark time delay. anyway the current limitation, the pemanent con
duction protection and the dump protection are operating even when pin 9 is at a low level.
L4843/11
PIN FUNCTIONS (continued)
ELECTRICAL CHARACTERISTICS (VS = 14.4V ; TJ = - 40 to 125°C unless otherwise specified ; referredto the test circuit)
L4844/11
DUMP PROTECTION (*) Note 1 : TD/T is given by the formula :
TD 1 = 1 + I7D/I7C
(**) Note 2 : K = 1 + I7D/I7C
K value depends on the pick–up used in the application ; typically K = 0.1
(***) Note 3 : the permanent conduction protection is guaranteed over the full temperature range
CIRCUIT OPERATIONThe L484 controls the charging angle (dwell) and
the peak value of the primary current in the coil over
the full range of operating conditions.
The coil current is limited to a predetermined level
by means of a negative feedback circuit including a
current sensing resistor, a comparator, the driver
stage and the power switch.
The dwell control circuit keeps the output stage in its
active region during current limitation. The time the
output stage is operating in the active region (desa-
turation time), is sufficient to compensate for possi-
ble variation in the energy stored due to the
acceleration of the motor ; moreover this time is li-
mited to avoid excessive power dissipation.
MAGNETIC PICK–UP CHARACTERISTICS
The typical magnetic pickup waveforms are shown
in fig. 1, the amplitude of the signal being a function
of the frequency. However on the market there are
many types of magnetic pickup, of which the wave-
forms may differ very much. Adjusting the value of
the resistor R11 on pin 2 and/or adding a resistor Ra
between the pin 6 (dwell adjust) and pin 11, as
shown in the application circuit, it is possible to adapt
the L484 to a wide range of magnetic pickup wave-
forms.
Particularly by means of the resistor R11 on pin 2 it
is possible to define the maximum advance of the
conduction start into the coil . This is very useful at
high pick–up frequency.
CONTROL OF THE DWELL ANGLE
The dwell angle control circuit defines the conduc-
tion time of the output darlington, versus the speed
of rotation, the supply voltage and the charac-
teristics of the coil.
In each cycle the time the transistor operates in the
active region is compared with a reference time and
the error signal amplified to advance or delay the
conduction in the next cycle. To limit the power dis-
sipation the desaturation time is typically fixed
to 10% of the period T.
At very low frequencies the ON thershold is fixed at
200mV of the input signal and the desaturation time
is mainly determined by the peak waveform. This
positive threshold also prevents permanent conduc-
tion when the motor is stopped. When the input fre-
quency increases the dwell control gradually sets
the desaturation time to 10% of the period. At higher
frequencies the ON threshold becomes negative to
permit a conduction angle of more than 50% always
keeping desaturation time to 10% of the period.
CURRENT LIMITING
The current in the coil is measured by means of a
voltage drop across a suitable resistor in the emitter
lead of the power transistor. When the threshold
voltage (260mV typ) is reached, the coil current is
kept constant via a feedback loop.
L4845/11
DARLINGTON OVERVOLTAGE LIMITATION
The darlington is protected against overvoltage by
means of an external divider R5 /R6 (pin 15) and an
internal zener. This zener drives the external darlin-
gton in order to limit the collector voltage.
CHARGING ANGLE SIGNAL OUTPUT
This signal is intended for tachometer applications
(pin 5). It consists of an open collector stage with
current internally limited at 10mA
"POWER ON" SIGNAL INPUT
In the low status this input forces the external dar-
lington into conduction (pin 9). This control input can
be used together with the conduction time informa-
tion coming from pin 5 to bypass the normal dwell
time calculation. When an external logic control is
used to recognize particular engine condition (as in
anti Knock system).
PROTECTION CIRCUITSPERMANENT CONDUCTION PROTECTION
This function is intended to prevent continuous cur-
rent conduction in the final stage when the magnetic
pickup is open or intermittent. The duration of the in-
tervention is set by means of a capacitor 1 at pin 3.
Grounding pins 3 or 4, this protection is eliminated.
The inhibit function at pin 4 is particularly useful
when an external logic control is used to disable the
permanent conduction protection.
REVERSE BATTERY AND DUMP PROTECTION
Due to the external resistors R6, R7, R8, R10 the de-
vice is protected against reverse battery. The load
dump protection withstands up to 100V with a decay
time ≤ 300ms. The intervention threshold for load
dump is fixed by means of an external divider con-
nected to pin 11.
OTHER APPLICATION INFORMATION
If the supply is voltage disconnected - or the battery
wire is broken - while the current is flowing through
the coil, the external diode D1 keeps the coil current
from recirculating into the device : in this way both
device and darlington are protected.
The zener diode Dz, connected between pin 14 and
GND, allows to withstand positive spikes up to 200V.
The device - used in the recommended application
circuit - satisfies the ISO/DP 7637/1 overvoltage
standard.
Figure 1 : Typical Magnetic Pick–up Waveform and L484 Response at low and high fequency.
L4846/11
