LTC4240IGN datasheet(20/28 Pages) LINER

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LTC4240

4240f

On Resistance

The CompactPCI specification limits the total IR drop of

the FET plus the IR drop of the sense resistor to 100mV.

For a nominal sense resistor of 0.005

Ω, if the user limits

the 3.3V supply load current to 8.7A, then the maximum

FET resistance should be less than 0.0063

Ω. Similarly, for

a 6.2A load current on the 5V supply and a 0.007

Ω sense

resistor, the maximum 5V FET resistance should be

0.0088

Ω. Note that above values of FET resistance are

worst case over temperature (on the FET’s datasheet, find

the resistance vs temperature curve and de-rate the room

temperature maximum value).

Breakdown Voltage

The maximum DC voltage that can appear across the

drain/source of the external power FET is 5V +10%. During

transient events and hot swap conditions, parasitic induc-

tances could cause ringing up to 3 times the supply

voltage. The use of voltage transient suppressors at the 5V

and 3.3V inputs can limit these voltage swings to less than

10V (see front page schematic). Similarly, the largest DC

voltage that is likely to appear across the gate is 12V

+10%. Voltage suppressors on the 12VIN node will also

limit the transient spikes on that node. Additionally, the

total capacitance on the GATE node will serve to filter fast

voltage noise spikes. FETs with a minimum rating of

±20V

on both the drain/source and the gate/source are recom-

mended.

Steady State Power Dissipation

For a user selected maximum load current of 8.7A on the

3.3V power supply and a 0.0063

Ω maximum FET resis-

tance, the DC power dissipation is:

(IMAX)2(RDSON,MAX) = (8.7)(8.7)(0.0063) = 0.477W

This is within the SOA limits of most power FETs.

sense resistor drops to 0.0049

Ω and the largest value of

threshold voltage increases to 60mV. This results in a trip

current of 12.2A.

–

5VIN

RSENSE

5VSENSE

VCB

VCB(MAX) = 60mV

VCB(NOM) = 55mV

VCB(MIN) = 50mV

ILOAD(MAX)

LTC4240*

*ADDITIONAL DETAILS

OMITTED FOR CLARITY

4240 F09

Figure 9. Circuit Breaker Equivalent

Circuit for Calculating RSENSE

APPLICATIO S I FOR ATIO

Plug-in board designers are thus limited to using less than

9.8A when a nominal 0.005

Ω resistor is used. Using more

than 9.8A runs the risk of turning off the external FET.

Since the CompactPCI specification allows a maximum

1A/pin, at least 10 pins must be used to supply 9.8A. This

implies that only the 3.3V supply can use a 0.005

Ω

resistor, since the 5V supply has a maximum of 8 pins

available. To adhere to the 1A/pin specification, the 5V

sense resistor should be larger than the 3.3V sense

resistor. Typical applications show a nominal 0.007

Ω

resistor, which results in a 7.04A maximum deliverable

current to the plug-in board loads. The 7.04A current

implies at least 7 pins on the 5V connector. Note that the

thermal considerations of the external FET will also place

limitations on the maximum allowable current.

5V and 3.3V External FET Selection

The LTC4240 uses external power FETs to limit and

modulate the current delivered by the 3.3V and 5V sup-

plies. There are several parameters to consider when

selecting the FET:

1. On resistance.

2. Gate and drain breakdown voltage.

3. Steady state and transient power dissipation.

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