L9918 datasheet(2/109 Pages) STMICROELECTRONICS

2 / 109 page

Functional description

The device is an alternator voltage regulator suited for 12 V system, able to communicate with ECU through LIN

communication protocol.

The device is housed into an alternator brush holder and it provides current to the excitation coil of the alternator

(rotor) through a high side power MOSFET driver. The device is suitable for alternators with different poles pair

number. An active freewheeling circuit is also integrated into the device.

Target function of the device is to modulate the rotor current in order to keep the voltage on car loads steady to a

target value (set point), whatever the vehicle demand.

The set point target is typically commanded by the ECU and it is achieved using an internal voltage reference. Set

point is communicated along with many others functional parameters during the regular device functioning.

Slew rate control in combination with external filtering of the interface lines helps to realize electromagnetic

compatibility.

Figure 1. Simplified application diagram

Alternator

Regulator

Diodes

Bridge

Rotor

Field

Stator

ECU

Battery

Load

B+

LIN

EXC

GND

1.1

External component required

The capacitors C1 (2.2 μF ceramic suggested) and C2 (220 pF suggested) to suppress radio frequency injection

have to be connected as close as possible between the related pin and GND.

L9918

Functional description

DS13000 - Rev 2

page 2/109

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