LTC2977 datasheet(82/138 Pages) AD

82 / 138 page

LTM4664A

Rev. 0

For more information www.analog.com

EMI PERFORMANCE

The SW_Cn pin provides access to the midpoint of the

power MOSFETs in LTM4664A’s power stages.

Connecting an optional series RC network from SW_Cn to

GND can dampen high frequency (~30MHz+) switch node

ringing caused by parasitic inductances and capacitances

in the switched-current paths. The RC network is called a

snubber circuit because it dampens (or “snubs”) the res-

onance of the parasitics, at the expense of higher power

loss. To use a snubber, choose first how much power to

allocate to the task and how much PCB real estate is avail-

able to implement the snubber. For example, if PCB space

allows a low inductance 0.5W resistor to be used then the

capacitor in the snubber network (CSW) is computed by:

CSW =

PSNUB

VINS3n(MAX)

2 •fSW

where VIN3n(MAX) is the maximum input voltage that the

input to the power stage (VINn) will see in the application,

and fSW is the DC/DC converter’s switching frequency

of operation. CSW should be NPO, C0G or X7R-type (or

better) material.

The snubber resistor (RSW) value is then given by:

RSW =

5nH

CSW

The snubber resistor should be low ESL and capable of

withstanding the pulsed currents present in snubber cir-

cuits. A value between 0.7Ω and 4.2Ω is normal.

A 2.2nF snubber capacitor is a good value to start with in

series with the snubber resistor to ground. The no load

input quiescent current can be monitored while selecting

different RC series snubber components to get a increased

power loss versus switch node ringing attenuation.

SAFETY CONSIDERATIONS

The LTM4664A modules do not provide galvanic isolation

from VIN to VOUT. There is no internal fuse. If required,

a slow blow fuse with a rating twice the maximum input

current needs to be provided to protect each unit from

catastrophic failure.

The fuse or circuit breaker should be selected to limit the

current to the regulator during overvoltage in case of an

internal top MOSFET fault. If the internal top MOSFET

fails, then turning it off will not resolve the overvoltage,

thus the internal bottom MOSFET will turn on indefinitely

trying to protect the load. Under this fault condition, the

input voltage will source very large currents to ground

through the failed internal top MOSFET and enabled

internal bottom MOSFET. This can cause excessive heat

and board damage depending on how much power the

input voltage can deliver to this system. A fuse or circuit

breaker can be used as a secondary fault protector in

this situation. The device does support over current and

overtemperature protection.

DUAL 25A/30A PSM APPLICATIONS INFORMATION

LTC2977 Datasheet(PDF) 82 Page - Analog Devices

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