LTC4221IGN datasheet(23/28 Pages) LINER

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LTC4221

4221f

the input supply line, exhibiting a peak overshoot to 2.5

times the steady-state value. This peak is followed by a

damped sinusoidal response whose duration and period

are dependent on the resonant circuit parameters. This

can cause detrimental damage to board components

unless measures are taken.

The energy stored in the lead/trace inductance is easily

controlled with snubbers and/or transient voltage sup-

pressors. Even when ferrite beads are used for electro-

magnetic interference (EMI) control, the low saturating

current of ferrite will not pose a major problem if the

transient voltage suppressors with adequate ratings are

used. The transient associated with a GATE turn off can be

controlled with a snubber and/or transient voltage sup-

pressor. Snubbers such as RC networks are effective

especially at low voltage supplies. The choice of RC is

usually determined experimentally. The value of the snub-

ber capacitor is usually chosen between 10 to 100 times

the MOSFET COSS. The value of the snubber resistor is

typically between 3Ω to 100Ω. When the supply exceeds

7V or EMI beads exist in the wire harness, a transient

voltage suppressor and snubber are recommended to clip

off large spikes and reduce the ringing. For supply volt-

ages of 6V or below, a snubber network should be suffi-

cient to protect against transient voltages. These protection

networks should be mounted very close to each of

LTC4221’s two supply voltages using short lead lengths to

minimize lead inductance. This is shown schematically in

the Typical Application on the front page of this data sheet.

In many cases, a simple short-circuit test can be per-

formed to determine the need of the transient voltage

suppressor. Additional overvoltage protection is provided

by the FB

n pins.

APPLICATIO S I FOR ATIO

PCB Layout Considerations

A recommended layout for the SENSE resistors, the

power MOSFETs, VCC transient protection devices and

GATE drive components around the LTC4221 is shown in

Figure 16. For proper operation of the LTC4221’s elec-

tronic circuit breaker, a 4-wire Kelvin connection to each

SENSE resistor is used. Also, PCB layout for the external

N-channel MOSFETs emphasizes optimal thermal man-

agement of MOSFET power dissipation to keep θJA as low

as possible. The VCC transient protection devices are

positioned close to the supply pins to reduce lead induc-

tance and thus overshoot voltage.

In Hot Swap applications where load currents can reach

10A or more, PCB track width must be appropriately sized

to keep track resistance and temperature rise to a mini-

mum. Consult Appendix A of LTC Application Note 69 for

details on sizing and calculating trace resistances as a

function of copper thickness.

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 1oz copper foil plating, a good starting

point 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.

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