L9911

Circuit operation

Doc ID10990 Rev 12

5/19

1

Circuit operation

The alternator's voltage regulator performs the following main functions:

1.

voltage regulation

2.

protection

3.

control fault diagnosis

4.

load response control

1.1

Voltage regulation

The alternator voltage is compared with a reference voltage in an amplifier, whose output

determines the switching frequency of output power MOS whose current excites the coil of

the alternator; as the regulator is in fixed-frequency topology, the field switching frequency

does not depend on the alternator characteristics. The regulators have an integrated filter in

the voltage sensing path guaranteeing the correct behaviour of the devices also when the

rectifier diodes feature very high switching spikes. The internal filtering allows the usage of

the device also with very long cables connecting the alternator to the battery with an

impedance so high to cause a superimposed ripple on the alternator voltage higher than 5-

6V. Consequently it doesn't need, in the standard application, any external component.

Anyway an external capacitor (2.2µF) must be inserted between A+ and ground when using

the device with very long cables.

1.2

Protection

It is present a protection against short circuits of the lamp and the relay power drivers (L)

and of the field power driver (F+), a thermal drivers shutdown protection and an overvoltage

protection of L power drivers.

1.3

Diagnosis

The circuit detects fault conditions related to the phase and F+ status and receives

informations from one of the three alternator phases. In order to prevent spurious

indications, fault warnings are not displayed immediately but are delayed by a fixed time.

1.4

Load response control

The internal circuit regulates the soft start characteristics (activated always at engine start)

and the soft attack characteristics.