AN1320 datasheet(2/20 Pages) STMICROELECTRONICS

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AN1320 APPLICATION NOTE

2/20

Figure 1. L6574 Block Diagram

3.0 L6574: HOW TO SET FREQUENCIES, TIMING, FAULT SIGNALS IN AN “ANALOG APPLICATION”

In this paragraph we will have a snap shot of the L6574 working, just as far as the characteristics important for

the micro interface are concerned. For further details please refer to [1] and [2].

The L6574 typical behavior is shown in fig. 2: there is a starting frequency (fMAX or fPRE) that is constant for a

set preheat time TPRE, than there is a frequency shift towards fmin that last 0.1 of TPRE and is called TIGN or TSH.

Figure 2. Frequency shift

Leads 1, 2, 3, 4 are used to set frequencies and timing.

The capacitor connected to pin 1[CPRE] sets the preheat time:

TPRE = K1 · CPRE

The ignition time, or better, the time to let frequency shift from preheat value to the min. value is one tenth of TPRE.

The current that charges and discharges the capacitor connected to pin 3 (CF) sets the half bridge oscillating

frequency.

The current that charges CF is set by the current that flows out from pin 2 and 4 during preheat and from pin 4

GND

VREF

Imin

RIGN

VCO

EN1

VTHE

EN2

VBOOT

OUT

CBOOT

LOAD

H.V.

LVG

DETECTION

HVG

BOOTSTRAP

DRIVER

HVG

DRIVER

LVG DRIVER

Vthpre

Ifs

CPRE

VREF

Imax

RPRE

OP AMP

OPOUT

OPIN-

OPIN+

DEAD

TIME

DRIVING

LOGIC

CONTROL

LOGIC

Ipre

LEVEL

SHIFTER

D97IN493B

fmax

fmin

freq

time

Tpre

Tsh

normal operation

preheating phase

ignition phase

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