L6567 STMicroelectronics, L6567 Datasheet - Page 8

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

L6567
Capacitive mode protection
During ignition and steady state the operating frequency is higher than the resonance frequency of the load
(L,C
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 V
on. At pin 9 there is an internal comparator with threshold V
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:
IC supply
At start up the IC is supplied with a current that flows through R
charges the external capacitor C
provide anymore the supply current. The easiest way to charge the C
a charge pump from the middle point of the half bridge.
To guarantee a minimum gate power MOS drive, the IC stops oscillating when V
restart once the V
starts operating at f = F
(that is: C
dard burning condition (feed forward and capacitive mode control). Excess charge on C
nal clamp that turns on at voltage V
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 C
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:
F
F
T
F
T
df/dt ---> C
Some useful formulas can well approximate the values:
If I
followi ng expression:
8/15
MIN
FEED FORWARD
PRE
PRE
DT
RHV
L
---> R
,R
---> R
& T
---> R
is greater than:
LAMP
IGN
P
REF
I
REF
capacitor is charged and discharged 15 times).Now the oscillator frequency is controlled as in stan-
SHUNT
---> C
and R
& C
---> C
f
STEADY STATE
, L, C
S
P
FILAMENT
F
will become higher than V
& R
F
MAX
I
L
R HV
, LAMP
& I
REF
RHV
,then the frequency shifts towards F
), so the transistors are turned on during the conduction time of the body diode in
------------- -
R
= MAX {F
15
S
R EF
. In steady state condition R
S CL
, the feed forward frequency is settled and the frequency value is fitted by the
.
MIN
F
F E ED FOR W AR D
, f
F
S HIGH1
MI N
FEED FORWARD
----------------- ----------------
8 R
. A minimum voltage hysteresis is guaranteed. The IC re-
R E F
CM TH
1
---------------------
121 C
, f
HV
I
C
CAPACITIVE MODE PROTECTION
MIN
R H V
(typ~20mV ): if V
is used as a mains voltage sensor, so it doesn’t
F
HV
. The timing of this frequency shifting is T
and an internal diode to the V
F
P
S
, C
capacitor (and to supply the IC) is to use
RS
I
, R
when the low side MOS FET is turned
REF
RS
S
and C
< V
is lower than V
CM TH
F
S
.
is drained by an inter-
capacitive mode is
}.
S HIGH2
S
pin which-
. It will
IGN