LM1949

SNLS349C – FEB 1995 – REVISED MARCH 2013

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Q1 Power Dissipation:

(1)

SWITCHING INJECTOR DRIVER CIRCUIT

The power dissipation of the system, and especially of Q1, can be reduced by employing a switching injector

driver circuit. Since the injector load is mainly inductive, transistor Q1 can be rapidly switched on and off in a

manner similar to switching regulators. The solenoid inductance will naturally integrate the voltage to produce the

required injector current, while the power consumed by Q1 will be reduced. A note of caution: The large

amplitude switching voltages that are present on the injector can and do generate a tremendous amount of radio

frequency interference (RFI). Because of this, switching circuits are not recommended. The extra cost of

shielding can easily exceed the savings of reduced power. In systems where switching circuits are mandatory,

extensive field testing is required to guarantee that RFI cannot create problems with engine control or

entertainment equipment within the vicinity.

The LM1949 can be easily modified to function as a switcher. Accomplished with the circuit of Figure 23, the only

additional components required are two external resistors, RA and RB. Additionally, the zener needs to be

reconnected, as shown, to RS. The amount of ripple on the hold current is easily controlled by the resistor ratio of

RA to RB. RB is kept small so that sense input bias current (typically 0.3 mA) has negligible effect on VH. Duty

cycle and frequency of oscillation during the hold state are dependent on the injector characteristics, RA, RB, and

the zener voltage as shown in the following equations.

(2)

10

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