VB027SP-E Application information
Doc ID 018828 Rev 1 9/14
3 Application information
3.1 Principle of operation
The VB027SP-E is mainly intended as high voltage power switch device driven by a logic
level input and interfaces directly to a high energy electronic ignition coil.
The input V
IN
of the VB027SP-E is fed from a low power signal generated by an external
controller that determines both dwell time and ignition point. During V
IN
high (≥ 4V) the
VB027SP-E increases current in the coil to the desired, internally set current level.
When the collector current exceeds 4.5 A, the diagnostic signal is turned high and it remains
so, until the load current reaches 5.8 A (second threshold). At that value, the diagnostic
signal is turned low, and the µC forces the V
IN
to the low state. During the coil current
switch-off, the primary voltage HV
C
is clamped by a series of Zener diodes at an internally
set value V
cl
, typically 360 V.
The collector current sensed through the R
sense
, is limited thanks to the “Current limiter”
block that, as soon as the I
cl
level is reached, forces the darlington (using the “Driver” block)
to limit the current provided.
The transition from saturation to desaturation, coil current limiting phase, must have the
ability to accommodate an overvoltage. A maximum overshoot of 20 V is allowed. There can
be some short period of time in which the output pin (HV
C
) is pulled below ground by a
negative current due to leakage inductances and stray capacitances of the ignition coil. This
can cause parasitic glitches on the diagnostic output. VB027SP-E has a built-in protection
circuit that allows to lock the p-buried layer potential of the linear stage to the collector
power, when the last one is pulled underground.
3.2 Thermal behaviour
You can see in the block diagram of the VB027SP-E (see Figure 1) a box called
overtemperature protection. The purpose of this circuit is to shift the current level at which
the first diagnostic is activated down of about 1 A.
This information can be managed by the micro that can take the corrective action in order to
reduce the power dissipation. This block is not an effective protection but just an
overtemperature detection. The shift down of the first flag level cannot be present for
temperatures lower than 125 °C.
As an example of its behavior 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 first flag threshold.
Example
I
C(DIAG1)
info after x ms (I
C(DIAG1
)=2.5A)
I
switch-off
info after kx ms.
As soon as the temperature rises over the overtemperature threshold, the first diagnostic is
shifted down to about 1.5 A and, in this example, the switch-off current is kx * 1.5 / 2.5.
