FAN7544 Ver la hoja de datos (PDF) - Fairchild Semiconductor
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FAN7544 Datasheet PDF : 14 Pages
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FAN7544
Operating Mode
Vcc
Vth(st)
VCPH
5V
2.9V
ICPHL=1.6µA
ICPHH=10µA
f
fpre
fpre
fign
frun
frun
preheating ignition
run
tph
tsw
0V < VCPH < 2.9V ;Preheating Frequency
iCT
=
I RT
+
IPH − IRT
(5V − 2.9V
) (5V
− 2.9V )
=
I PH
t
2.9V ≤ VCPH < 5V ; Frequency Sweep
( ) iCT
=
I RT
+
IPH − IRT
(5V − 2.9V
)
5V − VCPH
t
VCPH ≥ 5V
iCT = I RT
; Run Frequency
t
Figure 12. Timing Diagram
The FAN7544 has three operating mode as was shown in the figure 12.
1) Preheating Mode
The preheating mode is defined as the IC’s internal status is in when the lamp filaments are being heated to correct emission
temperature. This is necessary for maximizing lamp life and reducing the required ignition voltage. As soon as the Vcc
exceeds the UVLO high threshold , the preheating time set-up capacitor, CPH starts to be charged by the internal 1.6µA cur-
rent source until the VCPH reaches 2.9V. From 0V to 2.9V of the VCPH after the VCPH start to be charged, the switching fre-
quency throughout the preheating mode is determined by CT and RPH and the preheating time is decided by the CPH and the
1.6µA current source. (∆TPRE=CPH×2.9V/1.6µA)
2) Ignition mode
The ignition mode is defined as the IC’s internal status is in when a high voltage is established across the lamp necessary for
igniting the lamp. When the VCPH exceeds 2.9V, the FAN7544 enters the ignition mode, and moves to the run mode when the
VCPH exceeds 5V. In this period, the internal 10µA current source charges the external preheating timing capacitor(CPH) in
order to increase noise immunity with sharp slop of the VCPH. The ignition time is decided CPH and internal 10µA current
source(∆TIGN=CPH×∆VCPH2/ICPHH). In this mode, the switching frequency is determined by CT, RPH and RT because the
ICT is decided by the following equation.
( ) ICT
=
I RT
+
IRPH − IRT
2.1
5V −VCPH
3) Run Mode
After the lamp has successfully ignited, the FAN7544 enters run mode. The run mode is defined as the IC’s internal status is in
when 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 timing capacitor CT. As soon as the VCPH exceeds 5V, the protection masking mode
is disable and the IC can enter the protection mode.
Protection Mode
If the voltage at the RPH pin decreases below 2V during the run mode, the FAN7544 enters the protection mode and all gate
drive outputs(OUT1 & OUT2) are latched off in the low state and the VCPH is decreased to 0V. To exit the protection mode,
the Vcc must go down below the UVLO low threshold.
8
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