PRINCIPLE OF OPERATION

The VB027ASP is mainly intended as a high

voltage power switch device driven by a logic

level input and interfaces directly to a high energy

electronic ignition coil.

The input Vin of the VB027ASP is fed from a low

power signal generated by an external controller

that determines both dwell time and ignition point.

During Vin high (

≥ 4V) the VB027ASP increases

current in the coil to the desired, internally set

current level.

After reaching this level, the coil current remains

constant until the ignition point, that corresponds

to the transition of Vin from high to low (typ. 1.9V

threshold).

During the coil current switch-off, the primary

voltage HVc is clamped at an internally set value

Vcl, typically 345V.

The transition from saturation to desaturation, coil

current limiting phase, must have the ability to

accomodate

an

overvoltage.

A

maximum

overshoot of 20V is allowed.

THERMAL BEHAVIOUR

You can see in the block diagram of the

VB027ASP

a

box

called

overtemperature

protection. The purpose of this circuit is to shift

the current level at witch the 1st diagnostic is

activated down of about 2.5A.

This information can be managed by the micro

that can take the corrective actions in order to

reduce the power dissipation.

This block is not an effective protection but just

an overtemperature detection. The shift down of

the

1st

flag

level

cannot

be

present

for

temperatures lower than 125

As an example of its behaviour you can suppose

a very simple motor management system in

which the micro does just a simple arithmetic

calculation to decide when to switch off the

device after the 1st flag threshold.

EXAMPLE:

Iflag info after X msec. (Iflag = 4.5A)

Iswitch off after K*X msec.

As soon as the temperature rises over the

overtemp. threshold, the 1st diagnostic is shifted

down to about 2.5A and in this example, the

switch off current will be K*X*2.5/4.5 helping the

system to reduce the power dissipation.

As you can see this behaviour is not a protection

but just a feedback for the micro.

FEEDBACK

When the collector current exceeds 4.5A, the

feedback signal is turned high and it remains so,

until the load current reaches 5.8A (second

threshold), at that value, the feedback signal is

turned low.

OVERVOLTAGE

The VB027BSP can withstand the following

transients of the battery line:

Ω)

Ω)

VB027ASP

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