Obsolete Product(s) - Obsolete Product(s)
DUMP PROTECTION
Symbol
Paramater Test Conditions Min. Typ. Max. Unit
V
DZ
Zener Dump (pin 12) I
pin12
= 2mA 7.5 9.5 V
(*) Note 1 : TD/T is given by the formula :
TD 1
=
T 1 + I7D/I7C
(**) Note 2 :
TD K
=
T 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 OPERATION
The 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 (de-
saturation time), is sufficient to compensate for
possible variation in the energy stored due to the ac-
celeration of the motor ; moreover this time is limited
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 R
11
on pin 2 and/or adding a resistor R
a
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 R
11
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 dissi-
pation 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.
L484
5/11
