LTC4099 datasheet(46/52 Pages) LINER

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LTC4155

4155fc

stable behavior, but if engaged, it will result in much less

power delivery to the system load and battery, depending

on the magnitude of input resistance.

If the undervoltage current limit feature is disabled and the

input power source is resistive, the voltage will continue

to fall through the falling undervoltage lockout threshold,

eventually shutting down that input channel and resetting

its input current limit back to the default setting. When

the input voltage recovers, the channel will restart in the

default current limit setting.

Board Layout Considerations

The Exposed Pad on the backside of the LTC4155 package

must be securely soldered to the PC board ground. This

is the primary ground pin in the package, and it serves as

the return path for both the control circuitry and the syn-

chronous rectifier. Furthermore, due to its high frequency

switching circuitry, it is imperative that the input capacitor

be as close to the LTC4155 as possible, and that there

be an unbroken ground plane under the LTC4155 and its

external input bypass capacitors. Additionally, minimizing

the area between the SW pin trace and inductor will limit

high frequency radiated energy.

The output capacitor carries the inductor ripple current.

While not as critical as the input capacitor, an unbroken

ground plane from this capacitor’s ground return to the

inductor, input capacitor, and LTC4155 Exposed Pad will

reduce output voltage ripple.

High frequency currents, such as the input current on the

LTC4155, tend to find their way on the ground plane along

a mirror path directly beneath the incident path on the top

of the board. If there are slits or cuts in the ground plane

due to other traces on that layer, the current will be forced

to go around the slits. If high frequency currents are not

allowed to flow back through their natural least-area path,

excessive voltage will build up and radiated emissions will

occur (see Figure 13). There should be a group of vias

directly under the grounded backside leading directly

down to an internal ground plane. To minimize parasitic

inductance, the ground plane should be as close as pos-

sible to the top plane of the PC board (layer 2).

The BATGATE pin has limited drive current. Care must be

taken to minimize leakage to adjacent PC board traces,

which may significantly compromise the 15mV ideal di-

ode forward voltage. To minimize leakage, the trace can

be guarded on the PC board by surrounding it with VOUT

connected metal, which should generally be less than 1V

higher than BATGATE.

Figure 13. Higher Frequency Ground Currents Follow Their

Incident Path. Slices in the Ground Plane Cause High Voltage

and Increased Emissions

APPLICATIONS INFORMATION

4155 F13

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