MAX8751ETJ+ Maxim Integrated Products, MAX8751ETJ+ Datasheet - Page 22

• Manufacturer Part Number: MAX8751ETJ+
• Description: IC CNTRLR CCFL INV 32-TQFN
• Manufacturer: Maxim Integrated Products
• Type: CCFL Controllerr

Specifications of MAX8751ETJ+

• Frequency: 30 ~ 80 kHz
• Current - Supply: 3.2mA
• Voltage - Supply: 6 V ~ 28 V
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 32-TQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Lead Free Status / Rohs Status: Details

Fixed-Frequency, Full-Bridge CCFL
Inverter Controller
The MAX8751 limits the secondary current even if the
IFB sense resistor is shorted or transformer secondary
current finds its way to ground without passing through
R1. ISEC monitors the voltage across the sense resistor
R2 connected between the low-voltage terminal of the
transformer secondary winding and ground. Determine
the value of R2 using the following equation:
where I
transformer secondary current during fault conditions,
and 1.26V is the typical value of the ISEC peak voltage-
when the secondary is shorted. To set the maximum
RMS secondary current in the circuit of Figure 1 to
22mA, set R2 = 40.2Ω.
The transformer is the most important component of the
resonant tank circuit. The first step in designing the
transformer is to determine the transformer turns ratio.
The ratio must be high enough to support the CCFL
operating voltage at the minimum supply voltage. The
transformer turns ratio N can be calculated as follows:
where V
normal operation, and V
voltage. If the maximum RMS lamp voltage in normal
operation is 800V and the minimum DC input voltage is
7V, the turns ratio should be greater than 120 turns.
The next step to design the resonant tank for CCFL is to
design the resonant frequency of the tank close to the
switching frequency set by the HF resistor. The resonant
frequency is determined by: the primary winding series
capacitor Cs, the secondary parallel capacitor Cp, the
transformer secondary leakage inductance L, and the
CCFL lamp operating resistance R
The simplified CCFL inverter circuit is shown in Figure
7(a). The full-bridge power stage is simplified and repre-
sented as a square-wave AC source. The resonant tank
circuit can be further simplified to Figure 7(b) by remov-
ing the transformer. C
capacitive divider reflected to the secondary and N is the
22
______________________________________________________________________________________
SEC(RMS)_MAX
LAMP(RMS)
Setting the Secondary Current Limit
Transformer Design and Resonant
R2
=
N
is the maximum RMS lamp voltage in
≥
S
2 x I
. 0 90
’ is the capacitance of the primary
V
IN(MIN)
is the desired maximum RMS
LAMP RMS
SEC(RMS)_MAX
x V
1 26
.
Component Selection
(
IN MIN
(
is the minimum DC input
V
L
)
.
)
transformer turns ratio.
Figure 8 shows the frequency response of the resonant
tank’s voltage gain under different load conditions. The
primary series capacitor is 1µF, the secondary parallel
capacitor is 15pF, the transformer turns ratio is 1:93,
and the secondary leakage inductance is 260mH.
Notice that there are two peaks, f
quency response. The first peak, f
nant peak determined by the secondary leakage
inductance (L) and the series capacitor reflected to the
secondary (Cs’):
The second peak, f
determined by the secondary leakage inductance (L),
the parallel capacitor (C
reflected to the secondary (C’
The actual resonant frequency is between these two
resonant peaks. When the lamp is off, the operating
point of the resonant tank is close to the parallel reso-
nant peak due to the lamp’s infinite impedance. The cir-
cuit displays the characteristics of a parallel-loaded
resonant converter. While in parallel-loaded resonant
operation, the inverter behaves like a voltage source to
generate the necessary striking voltage. Theoretically,
the output voltage of the resonant converter increases
until the lamp is ionized or until it reaches the IC’s sec-
ondary voltage limit. Once the lamp is ionized, the
equivalent-load resistance decreases rapidly and the
operating point moves toward the series-resonant
peak. While in series-resonant operation, the inverter
behaves like a current source.
The leakage inductance of the CCFL transformer is an
important parameter in the resonant tank design. The
leakage inductance values can have large tolerance and
significant variations among different batches. It is best
to work directly with transformer vendors on leakage
inductance requirements. The series capacitor Cs sets
the minimum operating frequency, which is approximate-
ly two times the series resonant peak frequency. The
series capacitor Cs can be chosen as below:
f
P
=
f
S
P
2π
, is the parallel resonant peak
=
P
2π
), and the series capacitance
L
Cs
1
Cs C
LCs
1
S
'
):
+
'
'
C
P
P
S
S
, is the series reso-
and f
P
, in the fre-