LM1949

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SNLS349C – FEB 1995 – REVISED MARCH 2013

Figure 21. Alternate Configuration for Zener Z1

POWER DISSIPATION

The power dissipation of the system shown in Figure 1 is dependent upon several external factors, including the

frequency and duty cycle of the input waveform to Pin 1. Calculations are made more difficult since there are

many discontinuities and breakpoints in the power waveforms of the various components, most notably at the

peak-to-hold transition. Some generalizations can be made for normal operation. For example, in a typical cycle

of operation, the majority of dissipation occurs during the hold state. The hold state is usually much longer than

the peak state, and in the peak state nearly all power is stored as energy in the magnetic field of the injector,

later to be dumped mostly through the zener. While this assumption is less accurate in the case of low battery

voltage, it nevertheless gives an unexpectedly accurate set of approximations for general operation.

The following nomenclature refers to Figure 1. Typical values are given in parentheses:

RS

= Sense Resistor (0, 1

Ω)

VH

= Sense Input Hold Voltage (.094V)

VP

= Sense Input Peak Voltage (.385V)

VZ

= Z1 Zener Breakdown Voltage (33V)

VBATT = Battery Voltage (14V)

L1

= Injector Inductance (.002H)

R1

= Injector Resistance (1

Ω)

n

= Duty Cycle of Input Voltage of Pin 1 (0 to 1)

f

= Frequency of Input (10 Hz to 200 Hz)

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