MAX8729 Maxim Integrated Products, MAX8729 Datasheet - Page 21

• Manufacturer Part Number: MAX8729
• Description: Constant-Frequency / Half-Bridge CCFL Inverter Controller
• Manufacturer: Maxim Integrated Products

The MAX8729 requires two external n-channel power
MOSFETs to form a half-bridge inverter circuit to drive
the transformer primary. Since the positive half-cycle
and negative half-cycle are symmetrical, the same type
of MOSFET should be used for the high-side and low-
side switches. When selecting the MOSFET, focus on
the voltage rating, current rating, on-resistance
(R
Select a MOSFET with a voltage rating at least 25%
higher than the maximum input voltage of the inverter.
For example, if the maximum input voltage is 24V, the
voltage rating of the MOSFET should be 30V or higher.
The current rating of the MOSFET should be higher
than the peak primary current at the minimum input
voltage and full brightness. Use the following equation
to estimate the primary peak current I
where P
V
mated efficiency at the minimum input voltage.
Assuming the half bridge drives four CCFLs and the
maximum output power of each lamp is 4.5W, the total
maximum output power is 18W. If the minimum input
voltage is 8V and the estimated efficiency is 75% at
that input, the peak primary current is approximately
4.3A. Therefore, power MOSFETs with a DC current rat-
ing of 5A or greater are sufficient.
Since the regulator senses the on-state, drain-to-source
voltage of both MOSFETs to detect the transformer prima-
ry current, the lower the MOSFET R
current limit is. Therefore, the user should select n-chan-
nel MOSFETs with low R
loss, and keep the primary current limit at a reasonable
level. Use the following equation to estimate the maxi-
mum and minimum values of the primary current limit:
Both MOSFETs must be able to dissipate the conduc-
IN
DS(ON)
_
MIN
is the minimum input voltage, and is the esti-
), total gate charge, and power dissipation.
OUT
_
I
PEAK PI
I
I
MAX
LIM MIN
LIM MAX
Applications Information
______________________________________________________________________________________
_
_
_
is the maximum output power,
Constant-Frequency, Half-Bridge CCFL
=
=
=
DS(ON)
R
R
2
V
DS ON MAX
DS ON MIN
IN MIN
xP
320
480
(
_
(
OUT MAX
to minimize conduction
mV
)_
mV
)_
DS(ON)
_
x
η
PEAK
, the higher the
_
MOSFETs
PRI
:
tion losses plus the switching losses at both V
and V
at 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:
Both MOSFETs turn on with ZVS condition, so there is
no switching power dissipation associated with the
MOSFET. However, the current is at peak when the
MOSFET is turned off. Calculate the total turn-off
switching power dissipation of the two MOSFETs using
the following equation:
where C
MOSFETs and I
when the MOSFET is being turned off.
IN(MAX)
IN(MAX)
IN(MIN)
IN(MIN)
IN_MAX
PD
PRI
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
Inverter Controller
is the primary current calculated using the
should be roughly equal to the losses at
PD
. Calculate both terms. Ideally, the losses
is the reverse transfer capacitance of the
CONDUCT
GATE
=
I
PRI
2
is the peak gate-drive sink current
=
x C
P
=
OUT MAX
RSS
η
I
PRI
IN(MIN)
x V
_
x V
IN(MAX)
I
2
IN
GATE
x R
IN
2
, consider choosing
DS ON
x f
(
SW
are significantly
)
x I
PRI
IN
IN_MIN
does
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