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OB3328N High Performance CCFL Controller Full Bridge On-Bright
On-Bright Electronics  On-Bright
OB3328N Datasheet PDF : 14 Pages
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High Performance CCFL Controller Full Bridge
Function Description
High Efficiency Operation
No energy is further delivered to the CCFL load.
The OB3328 CCFL controller is desi gned to drive
the inverter system in full bridge top ology, the
resonant mo de full b ridge switche s conve rts
e unregulated DC voltage to pure
waveforms for CCFL operating with high efficiency
fid and low EMI emission. The resonant frequency of
the tank i s set by th e tran sformer lea kage
inductance, prima ry seri es capacitor and
a secondary parallel capacitor.
The OB3328 provides a high performance solution
n with a low system cost. One reference designs for
o single lamp appli cation is shown in figure 3 o n
page 9.
B Enable the Controller
OB3328 is activated by a pplying logic high to the
to ENA input. Control i s TTL logic compatible. The
controller is e nabled when the voltage at ENA pin
l is highe r than 1.1V. Toggling the ENA sign al
resets the state machi ne hen ce restart s the
tia inverter system.
Lamp Ignition and Striking Voltage Regulation
n A much higher voltag e than that in normal
operation is required to ignite CCFL, especially for
e aged la mp o r in low ambiance tempe rature. The
programmable stri king freque ncy, vol tage an d
fid ignition time set by RT, RI, VS, and STIME pins
ensure suffici ent voltage and time fo r a ny CCFL
ignition. RI in paralleled with RT pin is u sed to set
n the oscillator frequency which is close to resonant
frequency of the tank at ignition stage thus provide
o sufficient stri king voltage. The output voltage is
divided by the cap acitive voltage divider (forme d
C by C12 and C13 shown in figure 3 on page 9). The
divided sig nal is fed into VS pin and its pea k is
t compared wi th internal 3.0V threshol d voltage.
Consequently, the output voltage is regu lated and
h limited. The stri king voltage can be ap proximated
rig as:
3.0V C13
2 • C12
-BOnce the out put voltage reache s the target level,
and if CS pin voltage is le ss tha n 350 mV, the IC
nwill initiate the stri king timer. An internal constant
3.0uA current source starts to charge the capacitor
O connected to STIME pin. Voltage higher than 3.0V
Voltage at CS pin greater than 300mV indicates
the lamp being ignited and capacitor at STIME pin
will be discha rged. Capacitor in app ropriate value
is req uired to provide sufficient time (typically, 1
second) to ignite the lamp. Ignition time is
approximated as:
T(sec) = C[uF]
Over Voltage Protection
During n ormal ope rating, if VS peak voltag e
reaches the internal threshold of 3.0V, the
controller im mediately sh uts do wn th e inverte r.
OVP is a latch shutd own and could only be re set
by toggling ENA pin.
Soft Start and Soft On/off
External ca pacitor connected to CM P/SST pin
provides soft start and soft on/off control. At start
up, an intern al current source starts to charge the
capacitor. Consequently, voltage at CM P/SST pin
increases gradually and so as to the overlap tim e
of the PWM Gate si gnals. This soft st art control
helps to redu ce the MOS FET inru sh current an d
voltage stre sses, thus expand the lam p life. The
slope of the soft st
art ΔV / ΔT can be
approximated as:
ΔV = 3 ×10-6 × 36
ΔT C × RT[kohm]
Once lamp s are ignite d, the ca pacitor connected
to CMP/SST pin performs the loop com pensation
function. In intern al bu rst m ode dimming o r
external b urst (PWM) mo de dimmi ng conditions,
the voltage rampi ng up and do wn a t this pin
performs a soft on/off control function in each burst
Normal Operation and Striking Frequency
The ope ration freque ncy is determi ned by the
external resi stor con nected to RT pin. The
operation frequency is calculated by th e following
Fop (KHz) = RT(Kohm)
At ignition st age, RI pi n is internally connected to
RT pin, there fore external resistor at RI pin is in
at STIME pin indicates an ignition timeout. Under parallel with the external resi stor at RT pin. The
such cir cumstance, P WM Gat e out puts are
striking fre quency can be cal culated by the
disabled and thus power s witches are turned off.
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