MAX6495ATT/VT(2012) 查看數據表（PDF） - Maxim Integrated

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MAX6495ATT/VT
(Rev.:2012)

72V, Overvoltage-Protection Switches/Limiter Controllers with an External MOSFET

Maxim Integrated 

72V, Overvoltage-Protection Switches/

Limiter Controllers with an External MOSFET

where IGATE is GATE’s 100µA sourcing current, ILOAD

is the load current at startup, and COUT is the output

capacitor.

MOSFET Selection

Select external MOSFETs according to the application

current level. The MOSFET’s on-resistance (RDS(ON))

should be chosen low enough to have a minimum volt-

age drop at full load to limit the MOSFET power dissipa-

tion. 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 = ILOAD2 x RDS(ON)

where P is the power dissipated in the MOSFET, ILOAD

is the output load current, and RDS(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 = VDS x ILOAD

where VDS is the voltage across the MOSFET’s drain

and source.

Thermal Shutdown

The MAX6495–MAX6499 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–

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 TJ = +160°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’s junction temperature cools by 20°C. Thermal-over-

load protection is designed to protect the MAX6495–

MAX6499 and the external MOSFET in the event of cur-

rent-limit fault conditions. For continuous operation, do

not exceed the absolute maximum junction-temperature

rating of TJ = +150°C.

Peak Power-Dissipation Limit

The MAX6495–MAX6499 activate an internal 100mA

pulldown on GATE when SHDN goes low, OVSET

exceeds its threshold or UVSET falls below its threshold.

Once the voltage on GATE falls below the OUTFB volt-

age, current begins to flow from OUTFB to the 100mA

pulldown through the internal clamp diode, discharging

the output capacitors.

Depending on the output capacitance and the initial volt-

age, a significant amount of energy may be dissipated

by the internal 100mA pulldown. To prevent damage to

the device ensure that for a given overvoltage threshold,

the output capacitance does not exceed the limit provid-

ed in Figure 4. This output capacitance represents the

sum of all capacitors connected to OUTFB, including

reservoir capacitors and DC-DC input filter capacitors.

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:

• The device’s ambient temperature

• The output capacitor (COUT)

• The output load current (IOUT)

• The overvoltage threshold limit (VOV)

100,000

MAXIMUM OUTPUT CAPACITANCE

vs. OVERVOLTAGE THRESHOLD

10,000

1000 SAFE OPERATING AREA

100

10

0

10 20 30 40 50 60 70

OVERVOLTAGE THRESHOLD (V)

Figure 4. Safe Operating Area for 100mA Pulldown.

10

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