MAX8785AETI+T Maxim Integrated Products, MAX8785AETI+T Datasheet - Page 15

• Manufacturer Part Number: MAX8785AETI+T
• Description: IC CCFL CTRL 28-TQFN
• Manufacturer: Maxim Integrated Products
• Type: CCFL Controllerr

Specifications of MAX8785AETI+T

• Current - Supply: 1.5mA
• Voltage - Supply: 4.5 V ~ 28 V
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 28-TQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Frequency: -
• Lead Free Status / Rohs Status: Details

The MAX8785A has protection against faulty connections
of PZT to the PC board. The OLF pin is used to detect
high-voltage conditions on the secondary side of the
transformer. When the OLF voltage exceeds 1.2V (typ), a
2.8mA current source starts charging the TFLT capacitor.
When V
set and the MAX8785A stops switching. For details, see
the Setting the Arc Protection Threshold section.
The MAX8785A senses the voltage across both low-
side MOSFETs at LX1 and LX2. If the voltage exceeds
the internal 400mV (typ) current-limit threshold, the
MAX8785A turns off the respective MOSFET to prevent
the transformer primary current from increasing further.
The MAX8785A requires four external n-channel power
MOSFETs to form a full-bridge inverter circuit to drive the
transformer primary. When selecting the MOSFET, focus
on the voltage rating, current rating, on-resistance
(R
Select a MOSFET with a voltage rating at least 25% high-
er 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 cur-
rent 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, assum-
ing the full bridge drives one CCFL and maximum
output power of 4.5W. If the minimum input voltage is
8V and the estimated efficiency is 75% at that input, the
peak primary current is approximately 1.1A. Therefore,
power MOSFETs with a DC current rating of 1.4A or
greater are sufficient.
Since the regulator senses the on-state, drain-to-source
voltage of both MOSFETs to detect the transformer
primary current, the lower the MOSFET R
higher the current limit would be. Therefore, the user
should select n-channel MOSFETs with low R
IN_MIN
DS(ON)
TFLT
is the minimum input voltage, and η the esti-
), total gate charge, and power dissipation.
OUT_MAX
exceeds the threshold of 3V, the fault latch is
I
PEAK PRI
Applications Information
_
______________________________________________________________________________________
is the maximum output power,
Primary Side Current Limit
=
PEAK_PRI
2
V
IN MIN
×
Open PZT Protection
P
_
OUT MAX
:
_
×
η
MOSFETs
DS(ON)
DS(ON)
Piezoelectric Transformers
, the
to
Full-Bridge Controller for
minimize conduction loss, and keep the primary current
limit at a reasonable level. Use the following equation to
estimate the maximum and minimum values of the pri-
mary current limit:
Both MOSFETs must be able to dissipate the conduction
losses, as well as the switching losses at both V
and 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 dissipa-
tion 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
lowing equation and R
where P
Both MOSFETs turn on with the ZVS condition, as the
switching frequency is the same as the resonance fre-
quency of the tank, so there is no switching power dis-
sipation associated with high-side 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
rent when the MOSFET is being turned off.
IN(MAX)
IN(MAX)
IN(MIN)
IN(MIN)
IN_MAX
PD
PRI
OUT_MAX
, consider increasing the size of the MOSFETs.
RSS
, with lower losses in between. If the losses at
are significantly higher than the losses at
SWTICH
should be roughly equal to the losses at
is the primary current calculated using the fol-
PD
. Calculate both terms. Ideally, the losses at
is the reverse transfer capacitance of the
I
I
LIM MIN
LIM MAX
CONDUCT
GATE
is the output power of the lamp.
=
I
_
_
PRI
2
DS(ON)
=
×
is the peak gate-drive sink cur-
=
P
=
C
OUT MAX
=
R
R
η
RSS
DS ON MAX
I
PRI
DS ON MIN
IN(MIN)
×
370
is MOSFET on-resistance:
(
430
_
V
(
2
×
IN(MAX)
IN
I
GATE
×
V
mV
)_
mV
IN
)_
R
, consider choosing
2
DS ON
×
(
f
are significantly
SW
)
×
I
PRI
IN
IN_MIN
does
15