FAN7532
9
1) Preheating Mode
The preheating mode is defined as the IC’s internal status
when the V
CPH
is between 0V and 2.9V. During preheating,
the current that flows through the ballast circuit heats the
lamp filaments. This is necessary for maximizing lamp life
and reducing the required ignition voltage. When the V
cc
exceeds the UVLO high threshold, the preheating time set-up
capacitor, CPH, starts being charged by the internal 1.5
μ
A
current source until the V
CPH
reaches 2.9V. Until the VCPH
reaches
2.9V
the switching frequency throughout the pre-
heating mode is determined by CT and RPH.
The preheating time is determined by the CPH and the 1.5
μ
A
current source. Therefore, the preheating time is determined
by equation (3):
The preheating frequency is determined by the amount of
charging and discharging current to the CT capacitor. The
charging and discharging current during preheating mode is
decided by equation (4):
Additionally, the charging and discharging time of the CT
capacitor during preheating mode is decided by equation (5):
Finally, the FAN7532’s preheating frequency in the preheat-
ing period is determined by equation (6):
2) Ignition
Mode
The ignition mode is defined as the IC’s internal status when
V
CPH
is approximately between 2.9V and 5V. During igni-
tion, the operating frequency is decreased to a pre-deter-
mined value.
At the same time, a very high voltage for igniting the lamp is
established across the lamp. When the V
CPH
exceeds 2.9V,
the FAN7532 enters the ignition mode.
Once
V
CPH
exceeds
5V, the device enters the run mode described in the following
section.
In the ignition period, the internal 10mA current source
charges the external preheating timing capacitor (CPH) in
order to increase noise immunity with the sharp slope of the
V
CPH
. The ignition time is determined by the CPH and inter-
nal 10mA current source (DTIGN=CPH*DVCPH/ICPHH).
-In this mode, the switching frequency is determined by
CT, RPH and RT. Therefore, the charging and
discharging currents change according to V
CPH
are
determined by equation (7):
3) Run Mode
After the lamp has successfully ignited, the FAN7532 enters
run mode. The run mode is defined as the IC’s internal status
when V
CPH
is higher than 5V. In this mode, the lamp is
being driven with a normal power level after the lamp is dis-
charged. The run mode switching frequency is determined by
the timing resistor RT and the timing capacitor CT. When the
V
CPH
exceeds 5V, the protection-masking mode is disabled
and the IC can enter the protection mode. The running fre-
quency is determined by the amount of charging and dis-
charging current to CT capacitor.
The charging and discharging currents during preheating
mode are decided by the equation (8):
The charging and discharging time of the CT capacitor dur-
ing preheating mode is determined by the equation (9):
0V
VCPH
2.9V
<
<
Preheating Frequency
;
iCT
2.9V
IRT
VCPH
5V
<
2.9V
5V ; Frequency Sweep
–
(
)
-------------–
5V
2.9V
–
(
)
IPH
=
+
=
<
iCT
IRT
5V
2.9V
Run Frequency
;
–
(
)
-------------–
5V
VCPH
–
(
)
+
=
VCPH
5V
≥
iCT
IRT
=
tpre
CPH
=
Ich
arg
ing
--------------------------
×
(3)
Ipre_ch
4
Rph
6.25
-----------
×
=
(4)
Ipre_disch
4
Rph
×
----------------------------
Vref
4V: constant
=
×
=
Ipre_ch
CT
Ipre_ch
Ipre_disch
-------------------
×
=
(5)
tpre_disch
CT
---------------------------
dVct
3.45V: constant
=
×
=
fpre
+
2
tpre_ch
tpre_disch
(
)
×
--------------------------------------------------------------------
=
(6)
ICT
IRT
------------------------------
5V
–
2 1
VCPH
–
(
)
+
=
7
( )
Irun_ch
2
Rt
6.25
-----------
×
=
(8)
Irun_disch
2
Rt
×
-----------------------
Vref
4V: constant
=
×
=