FAN7532 Ver la hoja de datos (PDF) - Fairchild Semiconductor
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Fabricante
FAN7532 Datasheet PDF : 16 Pages
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0V ≤ VCPH ≤ 2.9V, ;Preheating Frequency
ICT
= IRT
+
IPH − IRT
(5V − 2.9V
)
(5V
− 2.9V ) = IPH
2.9V ≤ VCPH ≤ 5V, ;Ignition Frequency
ICT
= IRT
+
(
IPH − IRT
5V − 2.9V
)
(
5V
) − VCPH
VCPH ≥ 5V
ICT = IRT ,
;Run Frequency
4. Preheating Mode
The preheating mode is defined as the IC’s internal
status when the VCPH 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 VCC exceeds the UVLO high
threshold, the preheating time set-up capacitor, CPH,
starts being charged by the internal 1.5µA current source
until the VCPH reaches 2.9V. Until the VCPH reaches
2.9V, the switching frequency throughout the preheating
mode is determined by CT and RPH.
The preheating time is determined by the CPH and the
1.5µA current source. Therefore, the preheating time is
determined by equation (3):
t pre
= CPH × VCPH
Icharging
(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):
Ipre _ ch
=
4 × Vref
RPH
(4)
( ) Ipre _ disch = 4 ×
6.25 ×Vref
RPH
,
Vref
= 4V
(Constant )
The charging and discharging time of the CT capacitor
during preheating mode is decided by equation (5):
tpre _ ch
= CT
× dVCT
Ipre _ ch
(5)
tpre _ disch
= CT
× dVCT
Ipre _ disch
,
dVCT=3.45V (Constant)
Finally, the FAN7532’s preheating frequency in the
preheating period is determined by equation (6):
( ) fpre
=
2×
1
tpre _ ch + tpre _ disch
(6)
5. Ignition Mode
The ignition mode is defined as the IC’s internal status
when VCPH is approximately between 2.9V and 5V.
During ignition, the operating frequency is decreased to
a pre-determined value. At the same time, a very high-
voltage for igniting the lamp is established across the
lamp. When the VCPH exceeds 2.9V, the FAN7532
enters the ignition mode. Once VCPH 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) to increase noise immunity with the sharp slope of
the VCPH. The ignition time is determined by the CPH
and internal 10mA current source ( ΔTign = CPH × ΔVCPH ). In this
ICPH
mode, the switching frequency is determined by CT,
RPH, and RT. Therefore, the charging and discharging
currents change according to VCPH and are determined
by equation (7).
ICT
= IRT
+
IPH − IRT
(5V − 2.9V
)
(5V
) − VCPH
(7)
6. 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 VCPH is higher than 5V. In this
mode, the lamp is being driven with a normal power level
after the lamp is discharged. The run mode switching
frequency is determined by the timing resistor RT and
the timing capacitor CT. When the VCPH exceeds 5V, the
protection-masking mode is disabled and the IC can
enter the protection mode. The running frequency is
determined by the amount of charging and discharging
current to CT capacitor.
The charging and discharging currents during preheating
mode are decided by the equation (8):
Irun _ ch
=
2 × Vref
RT
(8)
Irun _ disch
= 2 × (6.25 ×Vref ) ,
RT
Vref=4V (Constant)
trun _ ch
= CT
×
dVCT
Irun _ ch
(9)
trun _ disch
= CT
× dVCT
Irun _ disch
, dVCT=3.45V
(Constant)
Finally, the preheating frequency in the preheating period
using the FAN7532 is determined by the equation (10):
( ) frun
=
2×
1
trun _ ch + trun _ disch
(10)
© 2006 Fairchild Semiconductor Corporation
FAN7532 Rev. 1.0.2
10
www.fairchildsemi.com
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