Datasheet

LM1949
SNLS349C –FEB 1995–REVISED MARCH 2013 www.ti.com
FLYBACK ZENER
The purpose of zener Z
1
is twofold. Since the load is inductive, a voltage spike is produced at the collector of Q
1
anytime the injector is reduced. This occurs at the peak-to-hold transition, (when the current is reduced to one
fourth of its peak value), and also at the end of each input pulse, (when the current is reduced to zero). The
zener provides a current path for the inductive kickback, limiting the voltage spike to the zener value and
preventing Q
1
from damaging voltage levels. Thus, the rated zener voltage at the system peak current must be
less than the minimum breakdown of Q
1
. Also, even while Z
1
is conducting the majority of the injector current
during the peak-to-hold transition (see Figure 20), Q
1
is operating at the hold current level. This fact is easily
overlooked and, as described in the following text, can be corrected if necessary. Since the error amplifier in the
IC demands 94 mV across R
S
, Q
1
will be biased to provide exactly that. Thus, the safe operating area (SOA) of
Q
1
must include the hold current with a V
CE
of Z
1
volts. For systems where this is not desired, the zener anode
may be reconnected to the top of R
S
as shown in Figure 21. Since the voltage across the sense resistor now
accurately portrays the injector current at all times, the error amplifier keeps Q
1
off until the injector current has
decayed to the proper value. The disadvantage of this particular configuration is that the ungrounded zener is
more difficult to heat sink if that becomes necessary.
The second purpose of Z
1
is to provide system transient protection. Automotive systems are susceptible to a vast
array of voltage transients on the battery line. Though their duration is usually only milliseconds long, Q
1
could
suffer permanent damage unless buffered by the injector and Z
1
. There is one reason why a zener is preferred
over a clamp diode back to the battery line, the other reason being long decay times.
Figure 20. Circuit Waveforms
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