Datasheet

LTC4156
46
4156f
Figure 13. Higher Frequency Ground Currents Follow Their
Incident Path. Slices in the Ground Plane Cause High Voltage
and Increased Emissions
Board Layout Considerations
The Exposed Pad on the backside of the LTC4156 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 LTC4156 as possible, and that there
be an unbroken ground plane under the LTC4156 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 LTC4156 exposed pad will
reduce output voltage ripple.
High frequency currents, such as the input current on the
LTC4156, 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 V
OUT
connected metal, which should generally be less than 1V
higher than BATGATE.
4156 F13
APPLICATIONS INFORMATION