Datasheet

LM1949
www.ti.com 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:
R
S
= Sense Resistor (0, 1Ω)
V
H
= Sense Input Hold Voltage (.094V)
V
P
= Sense Input Peak Voltage (.385V)
V
Z
= Z
1
Zener Breakdown Voltage (33V)
V
BATT
= Battery Voltage (14V)
L
1
= Injector Inductance (.002H)
R
1
= Injector Resistance (1Ω)
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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