L6567 datasheet(6/15 Pages) STMICROELECTRONICS

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L6567

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Figure 1. Start up

Oscillator

The circuit starts oscillating when the voltage supply VS has reached the VS HIGH1 threshold. In steady state

condition the oscillator capacitor CF (at pin 12) is charged and discharged symmetrically with a current set main-

ly by the external resistor RREF connected to pin 10. The value of the frequency is determined by capacitor CF

and resistor RREF. This fixed value is called FMIN. A dead time TDT between the ON phases of the transistors

is provided for avoiding cross conduction, so the duty cycle for each is less than 50%. The dead time depends

on RREF value (fig. 7).

The IC oscillating frequency is between FMIN and FMAX = 2.5 · FMIN in all conditions.

Preheating mode

The oscillator starts switching at the maximum frequency FMAX. Then the frequency decreases at once to reach

the programmed preheating frequency (fig.2). The rate of decreasing (df/dt) is determined by the external ca-

pacitor CI (pin 14). The preheat time TPRE is adjustable with external components (RREF and CP). The preheat

current is adjusted by sense resistance RSHUNT. During the preheating time the load current is sensed with the

sense resistor RSHUNT (connected between pin 9-RS- and pin 7-PGND-). At pin 9 the voltage drop on RSHUNT

is sensed at the moment the low side MOS FET is turned off. There is an internal comparator with a fixed thresh-

old VPH:if VRS >VPH the frequency is decreased and if VRS <VPH the frequency is increased. If the VPH thresh-

old is reached, the frequency is held constant for the programmed preheating time TPRE.

TPRE is determined by the external capacitor CP (pin8) and by the resistor RREF:CP is charged 16 times with a

current that depends on RREF, and these 16 cycles determine the TPRE.

So the preheat mode is programmable with external components as far as TPRE is concerned (RREF &CP) and

as far as the preheating current is concerned (choosing properly RSHUNT and the resonant load components:

Land CL).

The circuit is held in the preheating mode when pin 8 (CP) is grounded.

In case FMIN is reached during preheat, the IC assumes an open load. Consequently the oscillation stops with

the low side MOS transistor gate on and the high side gate off. This condition is kept until V

S undershoots VSLOW1.

VSLOW1

VSHIGH1

low side mosfet

-VS

high side mosfet

TDT

TIME

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