LT4239 datasheet(18/24 Pages) AD

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LT4239

Rev. 0

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In addition, a resistor RDRAIN of 100kΩ with1% tolerance

is placed on the DRAIN pin for sensing the MOSFET’s

drain voltage accurately. A RC filter consisting of a 1kΩ

resistor and a 1µF ceramic bypass capacitor is also placed

on the VCC pin so that TVS is not required from the input

supply to ground.

Layout Considerations

To achieve accurate current sensing, Kelvin connections

for the sense resistors are required. The PCB layout of

Kelvin sensing traces should be balanced and symmetri-

cal to balance the parasitics on the PCB. Even with a good

PCB layout, it is necessary to use a resistor network to

average the voltages sensed across the individual parallel

sense resistors. In the 12V/100A application shown in

Figure 11, the SENSE2 and OUTK pins of the LT4239 are

joined to the six 0.5mΩ sense resistors with an array of

1Ω resistors. The resulting voltage between the SENSE2

and OUTK pins is the average of all the voltages across

the 0.5mΩ sense resistors, effectively Kelvin sensing the

six 0.5mΩ resistors. The same arrangement can also be

expanded for Kelvin sensing twelve 1mΩ resistors. The

Kelvin sensing connection from RS1 10mΩ is not con-

nected to OUTK so as not to affect the effective sense

resistance between the SENSE2 and OUTK pins. A rec-

ommended PCB layout is shown in Figure 6.

The PCB traces associated with the power path through

the MOSFETs should have low resistance. The suggested

trace width for 2oz copper foil is 0.01-inch per ampere

or wider to keep PCB trace resistance, voltage drop, and

temperature rise to a minimum. Note that 2oz copper foil

exhibits a sheet resistance of about 0.25mΩ/square. Small

resistances add up quickly in high current applications.

To improve noise immunity, place the resistive divider to

the UV pin close to the device and keep traces to supply

input and GND short. It is also important to place the

bypass capacitor C1 as close as possible between VCC and

ground. A 0.1µF capacitor C2 from the UV pin to ground

also helps reject supply noise. The 100kΩ resistor RDRAIN

for monitoring the MOSFET’s drain voltage should also be

placed near the DRAIN pin. If a surge suppressor, Z1, is

used, it should be placed between the IN and OUT using

wide traces.

APPLICATIONS INFORMATION

Try to avoid placing the ground plane immediately under

the power MOSFETs. If the MOSFETs fail and overheat, that

could result in a catastrophic failure as the input supply gets

shorted to ground when the insulation between them fails.

RS5

CURRENT FLOW

TO LOAD

VIA TO GND PLANE

R18

R19

•••

RS4

R16

R17

•••

RS3

R14

R15

•••

RS2

R12

R13

•••

RS7

R22

R23

•••

RS6

R20

R21

•••

OUT

•••

RVCC

RDRAIN

SENSE2

OUTK

GATE2

4239 F06

LT4239

Figure 6. Recommended Layout

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