Datasheet

LTC2978A
79
2978afa
For more information www.linear.com/LTC2978A
Figure 32. Suggested Screen Pattern for Die Attach Pad
Figure 33. Connecting Unused Inputs to GND
APPLICATIONS INFORMATION
2978a F32
100k
100k
V
SENSEP
V
SENSEM
LTC2978A
2978a F33
PCB ASSEMBLY AND LAYOUT SUGGESTIONS
Bypass Capacitor Placement
The LTC2978A requires 0.1µF bypass capacitors between
the V
DD33
pins and GND, the V
DD25
pin and GND, and the
REFP pin and REFM pin. If the chip is being powered from
the V
PWR
input, then that pin should also be bypassed
to GND by a 0.1µF capacitor. In order to be effective,
these capacitors should be made of high quality ceramic
dielectric such as X5R or X7R and be placed as close to
the chip as possible.
Exposed Pad Stencil Design
The LTC2978A’s package is thermally and electrically
efficient. This
is enabled by the exposed die attach pad
on the under side of the package which must be soldered
down to the PCB or mother board substrate. It is a good
practice to minimize the presence of voids within the
exposed pad inter-connection. Total elimination of voids
is difficult, but the design of the exposed pad stencil is
key. Figure 32 shows a suggested screen print pattern.
The proposed stencil design enables out-gassing of the
solder paste during reflow as well as regulating the finished
solder thickness. See IPC7525A.
PC Board Layout
Mechanical stress
on a PC board and soldering-induced
stress
can cause the LTC2978A’s reference voltage and
voltage drift to shift. A simple way to reduce these stress-
related shifts is to mount the IC near the short edge of the
PC board, or in a corner. The board edge acts as a stress
boundary, or a region where the flexure of the board is
minimal.
Unused ADC Sense Inputs
Connect all unused ADC sense inputs (V
SENSEPn
or
V
SENSEMn
) to GND. In a system where the inputs are
connected to removable cards and may be left floating
in certain situations, connect the inputs to GND using
100k resistors. Place the 100k resistors before any filter
components, as shown in Figure 33, to prevent loading
of the filter.