L9524C datasheet(21/27 Pages) STMICROELECTRONICS

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L9524C

Functional description

21/27

4.14

Power regulation

The power through each glow plug (here expressed by VRMS which is the root-mean-square

voltage across one glow plug) depends on the battery voltage VBAT and the duty cycle tG

on/TG of the PWM signal applied to the external power MOSFET’s:

Equation 2

In order to regulate the power through the glow plugs the L9524C measures VBAT and

adjusts tG on/TG of the gate drivers (G1...4) such that VRMS = VRMS ref, where VRMS ref

represents the desired power through each glow plug.

The desired power VRMS ref is given by the input duty cycle tCI on/TCI which represents the

desired output duty cycle at a nominal battery voltage of 12V:

Equation 3

As a result, the actual output duty cycle of the gate drivers is given by:

Equation 4

Note:

The L9524C varies both the on time tG on and the period TG of the PWM output signal to

vary the duty cycle tG on/TG.

The accuracy of the power regulation is given by

ΔV

RMS = VRMS - VRMS ref.

The output jitter (electrical characteristics Item 8.8) is not taken in considuration while the

average is zero over some periodes.

RMS

BAT

G on

------------

⋅

RMS ref

12V

CI on

--------------

⋅

G on

------------

12V

BAT

--------------

⎝⎠

⎛⎞ 2 tCI on

--------------

⋅

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