LTC4155 datasheet(22/26 Pages) AD

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LTC4041

Rev A

For more information www.analog.com

APPLICATIONS INFORMATION

2. To minimize parasitic inductance, the ground plane

should be as close as possible to the top plane of

the PC board (Layer 2). High frequency currents in

the hot loop tend to flow along a mirror path on the

ground plane which is directly beneath the incident

path on the top plane of the board as illustrated in

Figure 4. If there are slits or cuts or drill-holes in this

mirror path on the ground plane due to other traces,

the current will be forced to go around the slits. When

high frequency currents are not allowed to flow back

through their natural least-area path, excessive volt-

age will build up and radiated emissions will occur.

So every effort should be made to keep the hot-loop

current path as unbroken as possible.

3. The other important components that need to be

placed close to the pins are the supercapacitor (con-

nected to the SCAP pin) and the inductor L1. Even

though the current through these components is

continuous, they can change very abruptly due to

a sudden change in load demand. Also, their traces

should be wide enough to handle currents as high as

the NMOS current limit (typical 6.5A) in backup boost

mode.

4. Locate the VSYS dividers for BSTFB and RSTFB near

the IC but away from the switching components.

Kelvin the top of the resistor dividers to the positive

terminal of CSYS. The bottom of the resistor dividers

should return to the ground plane away from the hot-

loop current path. The same is true for the PFI divider

and the CAPFB divider.

5. The exposed pad on the backside of the LTC4041

package must be securely soldered to the PC board

ground and also must have a group of vias con-

necting it to the ground plane for optimum thermal

performance. Also this is the only ground pin in the

package, and it serves as the return path for both the

control circuitry and the switching converter.

6. The IGATE pin for controlling the gates of the external

pass transistors has extremely limited drive current.

Care must be taken to minimize leakage to adjacent

PC board traces. To minimize leakage, the trace can

be guarded on the PC board by surrounding it with

VSYS connected metal.

Figure 4. High Frequency Ground Currents Follow

Their Incident Path. Slices in the Ground Plane

Cause High Voltage and Increased EMI

4041 F04

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