MAX8751ETJ+ Maxim Integrated Products, MAX8751ETJ+ Datasheet - Page 21

• 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

MOSFETs must be able to dissipate the conduction
losses plus the switching losses at both V
V
V
V
V
V
Conversely, if the losses at V
higher than the losses at V
MOSFETs with lower parasitic capacitance. If V
not vary over a wide range, the minimum power dissi-
pation occurs where the conduction losses equal the
switching losses.
Calculate the total conduction power dissipation of the
two MOSFETs using the following equation:
where I
following equation:
The low-side MOSFETs turn on with ZVS. If the switch-
ing frequency is close to resonant frequency, turn-on
power loss associated with high-side MOSFETs can be
ignored. However, the current is at peak when the
MOSFET is turned off. Calculate the turn-off switching
power dissipation of the MOSFET using the following
equation:
where C
MOSFETs and I
when the MOSFET is being turned off.
IN(MAX)
IN(MAX)
IN_MAX
IN(MIN)
IN(MIN)
PD
PRI
. Calculate both terms. Ideally, the losses at
RSS
, with lower losses in between. If the losses at
, consider increasing the size of the MOSFETs.
SWITCH
are significantly higher than the losses at
should be roughly equal to the losses at
is the primary current calculated using the
PD
is the reverse transfer capacitance of the
I
I
LIM_MIN
LIM_MAX
CONDUCT
GATE
=
______________________________________________________________________________________
I
PRI
2
is the peak gate-drive sink current
=
x C
=
=
P
R
=
OUT MAX
R
RSS
DS(ON)_MAX
I
η
DS(ON)_MIN
PRI
IN(MIN)
x V
380mV
420mV
_
x V
IN(MAX)
I
2
IN
GATE
x R
Fixed-Frequency, Full-Bridge CCFL
IN
2
, consider choosing
DS ON
x f
(
SW
are significantly
)
x I
IN_MIN
PRI
IN
does
and
The MAX8751 senses the lamp current flowing through
resistor R1 (Figure 1) connected between the low-volt-
age terminal of the lamp and ground. The voltage
across R1 is fed to IFB and is internally full-wave recti-
fied. The MAX8751 controls the desired lamp current
by regulating the average of the rectified IFB voltage.
To set the RMS lamp current, determine R1 as follows:
where I
790mV is the typical value of the IFB regulation point
specified in the Electrical Characteristics table. To set
the RMS lamp current to 6mA, the value of R1 should
be 148Ω. The closest standard 1% resistors are 147Ω
and 150Ω. The precise shape of the lamp-current
waveform depends on lamp parasitics. The resulting
waveform is an imperfect sinusoid waveform, which has
an RMS value that is not easy to predict. A high-fre-
quency true RMS current meter (such as Yokogawa
2016) should be used to measure the RMS current and
make final adjustments to R1. Insert this meter between
the sense resistor and the lamp’s low-voltage terminal
to measure the actual RMS current.
The MAX8751 limits the transformer secondary voltage
during startup and lamp-out faults. The secondary volt-
age is sensed through the capacitive voltage-divider
formed by C3 and C4 (Figure 1). The voltage of VFB is
proportional to the CCFL voltage. The selection of the
parallel resonant capacitor C1 is described in the
Selecting the Resonant Components section. Smaller
values for C3 result in higher efficiency due to lower cir-
culating current. If C3 is too small, the resonant opera-
tion is affected by the panel parasitic capacitance.
Therefore, C3 is usually chosen to be between 10pF
and 18pF. After the value of C3 is set, select C4 based
on the desired maximum RMS secondary voltage
V
where the 2.34V is the typical value of the VFB peak
voltage when the lamp is open. To set the maximum
RMS secondary voltage to 1800V with C3 selected to
be 12pF, C4 must be less than or equal to 13nF.
LAMP(RMS)_MAX
LAMP(RMS)
Setting the Secondary Voltage Limit
Inverter Controller
C4
=
R
1
:
2 x V
=
is the desired RMS lamp current and
2 2
LAMP(RMS)_MAX
Setting the Lamp Current
2.32V
π
x I
x
LAMP RMS
790
mV
(
)
x C3
21