DocID10990 Rev 13 5/20
L9911 Circuit operation
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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-
6 V. Consequently it doesn't need, in the standard application, any external component.
Anyway an external capacitor (2.2 μF) 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.
1.5 Self start function
The device wakes up by the phase signal only. The phase signal must have a voltage
amplitude higher than VPHL1 and a frequency higher than EN-x.
