LTC4240IGN datasheet(26/28 Pages) LINER

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LTC4240

4240f

CURRENT FLOW

TO SOURCE

*ADDITIONAL DETAILS OMITTED FOR CLARITY. DRAWING IS NOT TO SCALE!

4240 F18

TRACK WIDTH W:

0.03" PER AMPERE

ON 1OZ Cu FOIL

CURRENT FLOW

TO LOAD

CURRENT FLOW

TO LOAD

SENSE

RESISTOR

VIA TO

GND PLANE

GND

5VOUT

5VIN

VIA/PATH

TO GND

GATE

CTIMER

POWER

MOSFET

LTC4240CGN*

SIMILAR LAYOUT

FOR 3.3V RAIL

NOT SHOWN

Note (see front page schematic) that the 12V and –12V

show 0.01

µF snubber capacitors. This is consistent with

the CPCI specification since we also recommend a 10

Ω

snubber resistor. The 12VIN pin is the most sensitive to

high energy large voltage transients. A transient voltage

suppressor with a breakdown voltage between 13.2V and

15V is advisable. The TVS should also be able to dissipate

at least 150W. The SMAJ12CA can be used for both 12VIN

and VEEIN. Place the TVS close to the LTC4240. See front

page schematic.

Figure 18. Recommended Layout for Power MOSFET,

Sense Resistor and GATE Components for the 5V Rail.

Similar Layout for 3.3V Rail Not Shown

APPLICATIO S I FOR ATIO

PCB Layout Considerations

For proper operation of the LTC4240’s circuit breaker

function, a 4-wire Kelvin connection to the sense resistors

is highly recommended. A recommended PCB layout for

the sense resistor, the power MOSFET, and the GATE drive

components around the LTC4240 is illustrated in

Figure 18. The drawing is not to scale and is only intended

to show the low resistance, external high current path. In

hot swap applications where load currents can reach 10A,

narrow PCB tracks exhibit more resistance than wider

tracks and operate at more elevated temperatures. Since

the sheet resistance of 1 ounce copper is approximately

0.5m

Ω/square, track resistances add up quickly in high-

current applications. Thus, to keep PCB track resistance

and temperature rise to a minimum, the suggested trace

width in these applications for 1 ounce copper is 0.03" for

each ampere of DC current.

In order to help dissipate the heat generated by the power

MOSFET, the copper trace connected to the drain should

be made as large as possible.

In the majority of applications, it will be necessary to use

plated-through vias to make circuit connections from

component layers to power and ground layers internal to

the PC board. For 1 ounce copper plating, a general rule is

1A of DC current per via, making sure the via is properly

dimensioned so that solder completely fills any void. For

other plating thicknesses, check with your PCB fabrication

facility.

Power MOSFET and Sense Resistor Selection

Table 7 lists some current MOSFET transistors that are

available. Table 8 lists some current sense resistors that

can be used with the LTC4240’s circuit breakers. Table 9

lists supplier web site addresses for discrete components

mentioned throughout the LTC4240 data sheet. High

current applications should select a MOSFET with very

low on-resistance and good transient thermal character-

istics.

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