LM1949

www.ti.com

SNLS349C – FEB 1995 – REVISED MARCH 2013

PEAK AND HOLD CURRENTS

The drop across the sense resistor is created by the solenoid current, and when this drop reaches the peak

threshold level, typically 385 mV, the IC is tripped from the peak state into the hold state. The IC now behaves

duration of the input signal at Pin 1. This mode of operation is preferable when working with solenoids, since the

current required to overcome kinetic and constriction forces is often a factor of four or more times the current

necessary to hold the injector open. By holding the injector current at one fourth of the peak current, power

In the circuit of Figure 1, it was known that the type of injector shown opens when the current exceeds 1.3 amps

and closes when the current then falls below 0.3 amps. In order to guarantee injector operation over the life and

of 0.1

Ω. Dividing the peak and hold thresholds by this factor gives peak and hold currents through the solenoid of

3.85 amps and 0.94 amps respectively.

amp, IR drops within the component leads or printed circuit board may create substantial errors unless

appropriate care is taken. The sense input and sense ground leads (Pins 4 and 5 respectively), should be Kelvin

easy solution to this problem on double-sided PC boards (without plated-through holes) is to have the high

TIMER FUNCTION

The purpose of the timer function is to limit the power dissipated by the injector or solenoid under certain

conditions. Specifically, when the battery voltage is low due to engine cranking, or just undercharged, there may

not be sufficient voltage available for the injector to achieve the peak current. In the Figure 18 waveforms under

the low battery condition, the injector current can be seen to be leveling out at 3 amps, or 1 amp below the

normal threshold. Since continuous operation at 3 amps may overheat the injectors, the timer function on the IC

each input pulse. For systems where the timer function is not needed, it can be disabled by grounding the TIMER

Pin (Pin 8). For systems where the initial peak state is not required, (i.e., where the solenoid current rises

immediately to the hold level), the timer can be used to disable the peak function. This is done by setting the time

out and disable the peak condition before the solenoid current has had a chance to rise above the hold level.

The actual range of the timer in injection systems will probably never vary much from the 3.9 milliseconds shown

in Figure 1. However, the actual useful range of the timer extends from microseconds to seconds, depending on

only by stray capacitances for low values and by leakages for large values.

reset time at the end of each controller pulse is determined by the supply voltage and the timing capacitor value.

Thus, a 0.1 µF cap is reset in approximately 25 µs.

COMPENSATION

Compensation of the error amplifier provides stability for the circuit during the hold state. External compensation

(from Pin 2 to Pin 3) allows each design to be tailored for the characteristics of the system and/or type of

Darlington power device used. In the vast majority of designs, the value or type of the compensation capacitor is

not critical. Values of 100 pF to 0.1 µF work well with the circuit of Figure 1. The value shown of 0.1 µF (disc)

provides a close optimum in choice between economy, speed, and noise immunity. In some systems, increased

0.1 µF capacitor. This is, however, rarely necessary.

Copyright © 1995–2013, Texas Instruments Incorporated

Submit Documentation Feedback

7

Product Folder Links: LM1949