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L6567
6/15
Figure 1. Start up
Oscillator
The circuit starts oscillating when the voltage supply V
S
has reached the V
S HIGH1
threshold. In steady state
condition the oscillator capacitor C
F
(at pin12) is charged anddischarged symmetrically with a current setmain-
ly by the external resistor R
REF
connected to pin 10. The value of the frequency is determined by capacitor C
F
and resistor R
REF
. This fixed value is called F
MIN
. A dead time T
DT
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 R
REF
value (fig. 7).
The IC oscillating frequency is between F
MIN
and F
MAX
= 2.5 · F
MIN
in all conditions.
Preheatingmode
The oscillator starts switching at the maximum frequency F
MAX
. 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 C
I
(pin 14). The preheat time T
PRE
is adjustable with external components (R
REF
and C
P
). The preheat
current is adjusted by sense resistance R
SHUNT
. During the preheating time the load current is sensed with the
sense resistor R
SHUNT
(connected between pin 9-R
S
- and pin 7-PGND-). At pin 9 the voltage drop on R
SHUNT
is sensed at themoment the low side MOS FET is turnedoff. There is an internal comparator with a fixed thresh-
old V
PH
: ifV
RS
> V
PH
the frequency isdecreased and ifV
RS
<V
PH
thefrequency is increased. If the V
PH
thresh-
old is reached, the frequency is held constant for the programmed preheating time T
PRE
.
T
PRE
is determined by the external capacitor C
P
(pin8) and by the resistor R
REF
: C
P
is charged 16 times with a
current that depends on R
REF
, and these 16 cycles determine the T
PRE
.
So the preheat mode is programmable with external components as far as T
PRE
is concerned (R
REF
&C
P
) and
as far as the preheating current is concerned (choosing properly R
SHUNT
and the resonant load components:
L and C
L
).
The circuit is held in the preheating mode when pin 8 (C
P
) is grounded.
In case F
MIN
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 V
SLOW1
.
V
SLOW1
V
SHIGH1
V
G
low side mosfet
V
G
-V
S
high side mosfet
C
F
V
S
T
DT
TIME
0
0
0
0