LM1949

SNLS349C – FEB 1995 – REVISED MARCH 2013

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Typical Circuit Waveforms

Figure 18.

APPLICATION HINTS

The injector driver integrated circuits were designed to be used in conjunction with an external controller. The

LM1949 derives its input signal from either a control oriented processor (COPS™), microprocessor, or some

other system. This input signal, in the form of a square wave with a variable duty cycle and/or variable frequency,

is applied to Pin 1. In a typical system, input frequency is proportional to engine RPM. Duty cycle is proportional

to the engine load. The circuits discussed are suitable for use in either open or closed loop systems. In closed

loop systems, the engine exhaust is monitored and the air-to-fuel mixture is varied (via the duty cycle) to

maintain a perfect, or stochiometric, ratio.

INJECTORS

Injectors and solenoids are available in a vast array of sizes and characteristics. Therefore, it is necessary to be

able to design a drive system to suit each type of solenoid. The purpose of this section is to enable any system

designer to use and modify the LM1949 and associated circuitry to meet the system specifications.

Fuel injectors can usually be modeled by a simple RL circuit. Figure 19 shows such a model for a typical fuel

injector. In actual operation, the value of L1 will depend upon the status of the solenoid. In other words, L1 will

change depending upon whether the solenoid is open or closed. This effect, if pronounced enough, can be a

valuable aid in determining the current necessary to open a particular type of injector. The change in inductance

manifests itself as a breakpoint in the initial rise of solenoid current. The waveforms at the sense input show this

occurring at approximately 130 mV. Thus, the current necessary to overcome the constrictive forces of that

particular injector is 1.3 amperes.

Figure 19. Model of a Typical Fuel Injector

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