L9918 datasheet(30/109 Pages) STMICROELECTRONICS

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Figure 14. Excitation high side power MOS current measurement: working principle

VBatt

Field

Power

DMOS

IEXC

Sense

DMOS

Scaled IEXC

ADC

If the current measured on EXC pin is greater than IFOCP for more than TFOC_filter, overcurrent event is detected

and EXC duty cycle is set to 0 %.

Overcurrent flag can be enabled or disabled depending by OVC_FLAG_EN NVM bit programming.

During overcurrent event the device retry EXC power stage activation accordingly to EXC PWM period.

4.2.7

Current limitation

In some cases it is needed to limit the torque required by the alternator to the engine. A direct way to do that is to

limit the EXC current flowing into the rotor.

Once a current limitation threshold has been set by LIN or by default (I_MAX, IFELD_MAXB NVM bits), L9918

adjust its output (i.e. its EXC DC) so to do not overcome the limit.

Adopted Current limitation threshold is the lower between default and LIN thresholds.

During its usual functioning, EXC DC is driven by voltage regulation loop control, whose target is to maintain a

certain voltage (set point) on car loads. As soon as the EXC current overcome the current limitation threshold,

EXC duty cycle is not calculated by voltage regulation loop anymore. Current limitation regulation become

dominant and the EXC DC is calculated by current limitation block, whose target is to keep EXC current fixed to

the selected threshold.

L9918 can implement current limitation by two different strategies, selectable by ILIM_MODE NVM bit:

•

Current limitation regulation loop: is the strategy implemented when ILIM_MODE = 0.

When this setting is chosen, current regulation is performed by an internal PI loop, whose parameters can

be fully programmable by NVM bits (IE_I_ANT, IE_P_ANT and IE_P_ANT_PRE for pre-excitation).

•

Fixed DC slope regulation: DC variation is performed applying a fixed slope, selectable by IE_GR_RAMPE

NVM bits.

A fixed offset on current limitation threshold can be selected by IFELD_OFFSET NVM bits.

Current limitation is available even into pre-excitation state.

L9918

EXC characteristics

DS13000 - Rev 2

page 30/109

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