L497 datasheet(6/11 Pages) STMICROELECTRONICS

6 / 11 page

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-

quently the 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 maintainconstantthe ratio

at any motor speed.

is kept constant (and not

= 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 application circuit of fig.4 are

used.

+ I11C /I11D

DESATURATION TIMES IN LOW AND HIGH

FREQUENCY OPERATION

Due to the upperlimit of the voltage range 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 cylinder engine) 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 during the conditionof acceleration and decel-

erationof the motor below 80Hz/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 monitored by measuring the

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

When a precisepeak coil current is required Rs must

be trimmed or an auxiliary resistor divider (R10,R11)

added :

Ic p eak

(A)=

0.320

)

⋅ 



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-

charged as long as the pick-up signal is ”low”. At the

next positive transition of the input signal the load

current starts immediately, producing the maximum

achievable Tdesat; then the voltage on CSRC in-

creases linearly until the standby is reached.During

this recovery time 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 is the biasing resistor at pin 12 (in K

Ω)and

Csrc the capacitor at pin 8 (in

µF).

L497

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