LM1949

SNLS349C – FEB 1995 – REVISED MARCH 2013

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FLYBACK ZENER

The purpose of zener Z1 is twofold. Since the load is inductive, a voltage spike is produced at the collector of Q1

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 Q1 from damaging voltage levels. Thus, the rated zener voltage at the system peak current must be

less than the minimum breakdown of Q1. Also, even while Z1 is conducting the majority of the injector current

during the peak-to-hold transition (see Figure 20), Q1 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 RS, Q1 will be biased to provide exactly that. Thus, the safe operating area (SOA) of

Q1 must include the hold current with a VCE of Z1 volts. For systems where this is not desired, the zener anode

may be reconnected to the top of RS 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 Q1 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 Z1 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, Q1 could

suffer permanent damage unless buffered by the injector and Z1. 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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