kia3802f Korea Electronics (KEC GROUP), kia3802f Datasheet - Page 5

• Manufacturer Part Number: kia3802f
• Description: Lcd Backlight Inverter Controller
• Manufacturer: Korea Electronics (KEC GROUP)

2005.12.6
Functional Description
1) Main Frequency (Lamp Operation Frequency)
A resistor RT and a capacitor CT determine the main frequency of
IC. The main frequency can be calculated as below equation:
2) Open Lamp Protection
Open lamp protection in the ignition time is provided through both
OVP and TIMER pin to ensure a rated voltage is achieved and a
required timing is satisfied.
If time that TIMER capacitor charges to 3V is long than the CCFL
ignition time, it can prevent over-operating by protection circuit.
3) Error Amplifier
The CCFL current is regulated through the error amplifier. Error
amplifier output (CMP) voltage and ramp wave (CT) make the
PWM pulses. The non-inverting reference is at 2.5V nominal.
4) OVP
When the output voltage reaches the threshold, it commands the
PWM controller to maintain the current driving level. This ensures
that output gets sufficient striking voltage while operating the power
transformer safely. The OVP threshold is set at 2V nominal.
5) On/Off Function
The Enable pin provides the function to turn on and off the output
without shut down the supply voltage. The threshold of the Enable is
set at 2.0V.
6) Soft-start
The soft-start function is provided with a capacitor connected to SST
pin. It provides a rate of rise for the pulse width where switches are
turned on. A 2.2
second period for striking the lamp.
7) Burst-mode Dimming
The burst-mode frequency is determined by a capacitor connected to
LCT pin.
The frequency can be calculated by:
To compare the input of V
the PWM pulses for Burst-mode Dimming. The LPWM pin is pulled
to V
floating state to make the bright portion. A less than 0.6V input on
V
Transformer Design
1. Transformer Turns Ratio
Calculation of the required turns ratio involves the estimation of two
DIM
REF
pin will obtain 100% brightness.
Main [kHz] =
to make the dark portion of the CCFL output burst and the
f [Hz] =
f
capacitor connected on the SST pin can set a 0.4
LCT[nF]
DIM
CT[nF]
pin with the ramp wave(LCT) makes
Revision No : 0
6.8 10
1 10
RT[k ]
RT[k ]
6
2
KIA3802F
critical factors.
1). Estimating Minimum Primary Voltage.
The minimum primary voltage is obtained from the value of the low
supply voltage.
2). Estimating Maximum Secondary Voltage.
The secondary is composed of a ballast capacitor, a voltage divide
capacitor and the lamp parasitic capacitor.
The voltage across the ballast capacitor can be calculated as below
equation:
The maximum secondary voltage can be calculated as below
equation:
Transformer turns ratio is:
2. Required Number of Secondary Turns
From the core constant provided by the core manufacturer, you will
need the effective core cross sectional area(Ae).
From the Faraday's law, the required number of secondary turns
would be:
3. Required Number of Primary Turns
From the turns ratio and number of secondary turns
4. Determination of Wire Gauge
Wire gauge can be done by an acceptable current density.
From the assumed current density (500cmil) and the current that
flows through wires, the required Circular Mils is:
From the AWG table, pick a wire-gauge that would fit limit.
Using the selected wire diameter and bobbin information, determine
if this wire gauge will fit in the space allotted. (when the space
factor[SF] is accounted for)
N
secondary
V
Vsecondary =
Ballast C
Nprimary =
Cmils = C
N =
=
= X
V
secondary
Ballast
V
V
secondary
primary
V
DLim
N
Lamp
secondary
B
N
2
I
+
Lamp
A
V
I
e
t
RMS
max
Ballast C
10
8
2
5/5