L6567 STMicroelectronics, L6567 Datasheet - Page 6

• Manufacturer Part Number: L6567
• Description: HIGH VOLTAGE DRIVER FOR CFL
• Manufacturer: STMicroelectronics

L6567
Figure 1. Start up
Oscillator
The circuit starts oscillating when the voltage supply V
condition the oscillator capacitor C
ly by the external resistor R
and resistor R
is provided for avoiding cross conduction, so the duty cycle for each is less than 50%. The dead time depends
on R
The IC oscillating frequency is between F
Preheating mode
The oscillator starts switching at the maximum frequency F
the programmed preheating frequency (fig.2). The rate of decreasing (df/dt) is determined by the external ca-
pacitor C
current is adjusted by sense resistance R
sense resistor R
is sensed at the moment the low side MOS FET is turned off. There is an internal comparator with a fixed thresh-
old V
old is reached, the frequency is held constant for the programmed preheating time T
T
current that depends on R
So the preheat mode is programmable with external components as far as T
as far as the preheating current is concerned (choosing properly R
L and C
The circuit is held in the preheating mode when pin 8 (C
In case F
the low side MOS transistor gate on and the high side gate off. This condition is kept until V
6/15
PRE
REF
PH
is determined by the external capacitor C
: if V
L
value (fig. 7).
I
MIN
).
(pin 14). The preheat time T
RS
V
high side mosfet
is reached during preheat, the IC assumes an open load. Consequently the oscillation stops with
G
V
low side mosfet
REF
> V
-V
G
SHUNT
S
PH
. This fixed value is called F
the frequency is decreased and if V
(connected between pin 9-R
V
C
REF
S
0
0
0
F
0
REF
, and these 16 cycles determine the T
connected to pin 10. The value of the frequency is determined by capacitor C
F
(at pin 12) is charged and discharged symmetrically with a current set main-
V
SLOW1
PRE
SHUNT
MIN
is adjustable with external components (R
and F
MIN
P
. During the preheating time the load current is sensed with the
(pin8) and by the resistor R
. A dead time T
MAX
S
V
- and pin 7-PGND-). At pin 9 the voltage drop on R
SHIGH1
S
RS
P
= 2.5 · F
) is grounded.
has reached the V
MAX
< V
. Then the frequency decreases at once to reach
PH
MIN
DT
the frequency is increased. If the V
SHUNT
PRE
between the ON phases of the transistors
in all conditions.
.
T
DT
and the resonant load components:
REF
PRE
S HIGH1
: C
is concerned (R
P
PRE
is charged 16 times with a
threshold. In steady state
REF
S
.
undershoots V
and C
TIME
P
). The preheat
REF
PH
&C
S LOW1
thresh-
P
SHUNT
) and
.
F