72V, Overvoltage-Protection Switches/Limiter
Controllers with an External MOSFET
MAX6495鈥揗AX6499
MOSFET Selection
Select external MOSFETs according to the application
current level. The MOSFET鈥檚 on-resistance (R
DS(ON)
)
should be chosen low enough to have a minimum volt-
age drop at full load to limit the MOSFET power dissi-
pation. Determine the device power rating to
accommodate an overvoltage fault when operating the
MAX6495/MAX6496/MAX6499 in overvoltage-limit mode.
During normal operation, the external MOSFET dissi-
pates little power. The power dissipated in the MOSFET
during normal operation is:
P = I
LOAD
2
x R
DS(ON)
where P is the power dissipated in the MOSFET, I
LOAD
is the output load current, and R
DS(ON)
is the drain-to-
source resistance of the MOSFET.
Most power dissipation in the MOSFET occurs during a
prolonged overvoltage event when operating the
MAX6495/MAX6496/MAX6499 in voltage-limiter mode.
The power dissipated across the MOSFET is as follows
(see the
Thermal Shutdown in Overvoltage-Limiter
Mode
section):
P = V
DS
x I
LOAD
where V
DS
is the voltage across the MOSFET鈥檚 drain
and source.
Thermal Shutdown in Overvoltage-Limiter Mode
When operating the MAX6495/MAX6496/MAX6499 in
overvoltage-limit mode for a prolonged period of time, a
thermal shutdown is possible. The thermal shutdown is
dependent on a number of different factors:
鈥?/div>
The device鈥檚 ambient temperature
鈥?/div>
The output capacitor (C
OUT
)
鈥?/div>
The output load current (I
OUT
)
鈥?/div>
The overvoltage threshold limit (V
OV
)
鈥?/div>
The overvoltage waveform period (t
OV
)
鈥?/div>
The power dissipated across the package (P
DISS
)
During an initial overvoltage occurrence, the discharge
time (鈭唗
1
) of C
OUT
, caused by I
OUT
and I
GATEPD
. The
discharge time is approximately:
鈭唗
1
=
C
OUT
V
OV
脳
0.95
(I
OUT
+
I
GATEPD
)
Thermal Shutdown
The MAX6495鈥揗AX6499 thermal-shutdown feature
turns off GATE if it exceeds the maximum allowable
thermal dissipation. Thermal shutdown also monitors
the PC board temperature of the external n-channel
MOSFET when the devices sit on the same thermal
island. Good thermal contact between the MAX6495鈥?/div>
MAX6499 and the external n-channel MOSFET is essen-
tial for the thermal-shutdown feature to operate effec-
tively. Place the n-channel MOSFET as close to
possible to OUTFB.
When the junction temperature exceeds T
J
= +160掳C,
the thermal sensor signals the shutdown logic, turning
off the GATE output and allowing the device to cool.
The thermal sensor turns the GATE on again after the
IC鈥檚 junction temperature cools by 20掳C. Thermal-over-
load protection is designed to protect the MAX6495鈥?/div>
MAX6499 and the external MOSFET in the event of cur-
rent-limit fault conditions. For continuous operation, do
not exceed the absolute maximum junction-tempera-
ture rating of T
J
= +150掳C.
where V
OV
is the overvoltage threshold, I
OUT
is the
load current, and I
GATEPD
is the GATE鈥檚 100mA pull-
down current.
Upon OUT falling below the threshold point, the
MAX6495/MAX6496/MAX6499s鈥?charge-pump current
must recover and begins recharging the external GATE
voltage. The time needed to recharge GATE from -V
D
to the MOSFET鈥檚 gate threshold voltage is:
鈭唗
2
=
C
ISS
V
GS(TH)
+
V
D
I
GATE
where C
ISS
is the MOSFET鈥檚 input capacitance,
V
GS(TH)
is the MOSFET鈥檚 gate threshold voltage, V
D
is
the internal clamp (from OUTFB to GATE) diode鈥檚 for-
ward voltage and I
GATE
is the charge-pump current
(100碌A typ).
GATE
鈭唗
2
OUTFB
鈭唗
1
鈭唗
OV
鈭唗
3
Figure 4. MAX6495/MAX6496/MAX6499 Timing
10
______________________________________________________________________________________
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