DUMP PROTECTION

Symbol

Paramater

Test Conditions

Min.

Typ.

Max.

Unit

Zener Dump (pin 12)

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

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.

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