Rainbow Electronics MAX8759 - Manual (Page 26)

MAX8759
Applications Information
MOSFETs
The MAX8759 requires four external n-channel power
MOSFETs: NL1, NL2, NH1, and NH2 to form a full-
bridge inverter circuit. The controller senses the on-state
drain-to-source voltage of the two low-side MOSFETs
NL1 and NL2 to detect the transformer primary current,
so the R
DS(ON)
of NL1 and NL2 should be matched. For
instance, if dual MOSFETs are used to form the full
bridge, NL1 and NL2 should be in one package. Since
the MAX8759 uses the low-side MOSFET R
DS(ON) for
primary overcurrent protection, the lower the MOSFET
R
DS(ON)
, the higher the current limit. Therefore,
the user should select a dual logic-level n-channel
MOSFET with low R
DS(ON) to minimize conduction loss,
and keep the primary current limit at a reasonable level.
The regulator uses ZVS to softly turn on each of four
switches in the full bridge. ZVS occurs when the exter-
nal power MOSFETs are turned on when their respec-
tive drain-to-source voltages are near 0V (see the
Resonant Operation section). ZVS effectively eliminates
the instantaneous turn-on loss of MOSFETs caused by
C
OSS
(drain-to-source capacitance) and parasitic
capacitance discharge, and improves efficiency and
reduces switching-related EMI.
Setting the Lamp Current
The MAX8759 senses the lamp current flowing through
sense resistors connected between the low-voltage ter-
minals of the lamps and ground. The voltages across
the sense resistors are fed to IFB1 and IFB2 and are
internally full-wave rectified. The MAX8759 controls the
desired lamp current by regulating the average of the
rectified IFB_ voltages. To set the RMS lamp current in
a single-lamp application, determine the value of the
sense resistor as follows:
where I
LAMP(RMS)
is the desired RMS lamp current and
785mV is the typical value of the IFB1 regulation point
specified in the Electrical Characteristics table. To set
the RMS lamp current to 6mA, the value of the sense
resistor should be 148
Ω. The closest standard 1%
resistors are 147
Ω and 150Ω. The precise shape of the
lamp-current waveform, which is dependent on lamp
parasitics, influences the actual RMS lamp current. Use
a true RMS current meter to make final adjustments.
Setting the Secondary Voltage Limit
The MAX8759 limits the transformer secondary voltage
during startup and lamp-out faults. The secondary volt-
age is sensed through the capacitive voltage-divider
formed by C4 and C5 (Figure 1). The VFB voltage is
proportional to the CCFL voltage. The selection of the
parallel resonant capacitor C1 is described in the
Transformer Design and Resonant Component
Selection section. C4 is usually between 10pF and
22pF. After the value of C4 is determined, select C5
R
mV
I
LAMP RMS
1
785
2 2
=
Ч
Ч
π
( )
Low-Cost, SMBus, CCFL Backlight Controller
26 ______________________________________________________________________________________
BIT 7 (R/ W) BIT 6 (R/ W) BIT 5 (R/ W) BIT 4 (R/ W) BIT 3 (R/ W) BIT 2 (R/ W) BIT 1 (R/ W) BIT 0 (R/ W)
ALSHL7 ALSHL6 ALSHL5 ALSHL4 ALSHL3 ALSHL2 ALSHL1 ALSHL0
ALSHL[7..0]: The highest brightness setting that can be set based on ALS inputs.
ALS High-Limit Register [0x06] (POR = 0xFF)
The value in this read-write register reflects the highest
possible brightness value the inverter can set based on
inputs from the ALS. Users can change this value so
that they can control the effect of ALS. A write-byte
cycle to register 0x06 sets the highest possible bright-
ness value that can be set based on ALS inputs. If the
brightness setting due to ALS is higher than the value
written to this register, the inverter immediately decreas-
es the brightness setting to the newly written value. A
read-byte cycle to register 0x06 returns the current
maximum brightness value that can be set based on
ALS inputs. The default value of register 0x06 is 0xFF,
which corresponds to the maximum brightness.