NIS5101

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11

An overload condition is one in which the FET is fully

circuit condition occurs when either the load has shorted or

upon turn on, as the load capacitor to the hot swap device

initially looks like a short circuit.

A single resistor will determine both the short circuit and

overload current. For example, a 110

W resistor would

result in a 1 A current limit when charging the capacitance

at turn on, but once the FET is fully enhanced, it would

allow the load to operate at a current up to 2.5 A. Once the

2.5 A limit is reached, any further reduction in load

impedance will result in a short circuit condition and the

current will be reduced to 1 amp.

As with all SMART HotPlug devices, the current limit

will never shut down the limiter. Only the thermal limit will

stop the flow of current to the load. Once the current is

stopped due to the thermal limit, it will remain off until

input power is recycled for the latching version, or it will

continuously retry to start again if it is the auto−retry

version.

of the curves and temperatures are shown below:

where “y” is the desired ILimit value.

Main/Mirror MOSFET Current Ratio. The ratio varies

with current and sense resistance. The key parameter that

it is important to know is that the current sense reference

voltage of the device is 50 mV. Knowing this information,

it is possible to use Figure 2 on the datasheet for the current

limit to calculate the ratio for any condition.

For ”normal” operating condition, the overload curve

would apply. If a 100

sense current would be 50 mV/ 100

W at the current limit

level, which results in 500

mA. The drain current is 2.7 A

under this condition, so the ratio is 5400:1.

Same analysis can be made for “short circuit” conditions,

the only difference is that the short circuit curve of

Figure 2 is used to do the ratio calculations instead.

There is a 5

W resistor in series with the sense cells. This

has a tolerance of about 10% and should be taken into

account when making the above calculations.

Turn−onSurge: During the turn−on event, there is a large

amount of energy dissipated due to the linear operation of

the power device. The energy rating is the amount of energy

that the device can absorb before the thermal limit circuit

will shut the unit down. This is very important specially for

the latch off device as it determines the maximum load

capacitance that the device can charge before the thermal

limit shuts the device down. The calculation of this is not

very simple as it depends on several factors such as the

therefore, it is recommended to do lab evaluations for these

purposes. Figure 19 shows the device’s thermal transient

response for minimum pad.

0.01

0.1

1

10

100

0.00001

0.0001

0.001

0.01

0.1

1

10

100

1000

Figure 19. Thermal Transient Response

TIME (seconds)