Sense Input Hold Voltage

vs Junction Temperature

Normalized Timer Function

vs Junction Temperature

LM1949N Junction

Temperature Rise Above

Ambient vs Supply Voltage

TLH5062 – 5

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 proces-

input signal in the form of a square wave with a variable

duty cycle andor 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 cir-

cuits 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 3 shows such a model for a typical fuel injector In

TLH5062 – 6

FIGURE 3 Model of a Typical Fuel Injector

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 on Page 2

at the sense input show this occurring at approximately 130

mV Thus the current necessary to overcome the constric-

tive forces of that particular injector is 13 amperes

PEAK AND HOLD CURRENTS

The peak and hold currents are determined by the value of

into saturation The injector current will rise exponentially

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

closed loop system to maintain the hold reference voltage

from the peak level to the hold level it remains there for the

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 dissipation in the sole-

In the circuit of

Figure 1 it was known that the type of injec-

tor shown opens when the current exceeds 13 amps and

closes when the current then falls below 03 amps In order

to guarantee injector operation over the life and tempera-

ture range of the system a peak current of approximately 4

ing the peak and hold thresholds by this factor gives peak

and hold currents through the solenoid of 385 amps and

094 amps respectively

Different types of solenoids may require different values of

Note that for large currents above one 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 respec-

should not be allowed to flow through any part of these

traces or connections An easy solution to this problem on

double-sided PC boards (without plated-through holes) is to

have the high current trace and sense trace attach to the

TIMER FUNCTION

The purpose of the timer function is to limit the power dissi-

pated 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 cur-

rent In the

Figure 2 waveforms under the low battery condi-

tion the injector current can be seen to be leveling out at 3

5