L6567 datasheet(8/15 Pages) STMICROELECTRONICS

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L6567

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Capacitive mode protection

During ignition and steady state the operating frequency is higher than the resonance frequency of the load

(L,CL,RLAMP and RFILAMENT), so the transistors are turned on during the conduction time of the body diode in

order to maintain Zero Voltage Switching.

If the operating frequency undershoots the resonance frequency ZVS doesn’t occur and causes hard switching

of the MOS transistors. The L6567 detects this situation by measuring VRS when the low side MOS FET is turned

on. At pin 9 there is an internal comparator with threshold VCM TH (typ~20mV ): if VRS <VCM TH capacitive mode is

assumed and the frequency is increased as long as this situation is present. The shift is determined by CI.

Steady state frequency

At any time during steady state the frequency is determined by the maximum on the following three frequencies:

fSTEADY STATE=MAX {FMIN,fFEED FORWARD,fCAPACITIVE MODEPROTECTION}.

IC supply

At start up the IC is supplied with a current that flows through RHV and an internal diode to the VS pin which-

charges the external capacitor CS. In steady state condition RHV is used as a mains voltage sensor, so it doesn’t

provide anymore the supply current. The easiest way to charge the CS capacitor (and to supply the IC) is to use

a charge pump from the middle point of the half bridge.

To guarantee a minimum gate power MOS drive, the IC stops oscillating when VS is lower than VSHIGH2. It will

restart once the VS will become higher than VSHIGH1. A minimum voltage hysteresis is guaranteed. The IC re-

starts operating at f = FMAX ,then the frequency shifts towards FMIN. The timing of this frequency shifting is TIGN

(that is: CP capacitor is charged and discharged 15 times).Now the oscillator frequency is controlled as in stan-

dard burning condition (feed forward and capacitive mode control). Excess charge on CS is drained by an inter-

nal clamp that turns on at voltage VSCL .

Ground pins

Pin 7(PGND) is the ground reference of the IC with respect to the application. Pin 11( SGND) provides a local

signal ground reference for the components connected to the pins CP,CI,RREF and CF.

Relationship between external components and sistem working condition

L6567 is designed to drive CFL and TL lamps with a minimum part count topology. This feature implies that each

external component is related to one or more circuit operating state.

This table is a short summary of these relationships:

FMIN ---> RREF &CF

FFEED FORWARD ---> CF &IRHV

TPRE &TIGN ---> CP &RREF

FPRE ---> RSHUNT,L,CL, LAMP

TDT ---> RREF

df/dt ---> CI

Some useful formulas can well approximate the values:

If IRHV is greater than:

, the feed forward frequency is settled and the frequency value is fitted by the

followi ng express ion:

MI N

RE F

⋅⋅

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

≅

RHV

REF

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

≥

F E ED FO R W AR D

IRHV

121 C

⋅

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

≅

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