LTC2978
APPLICATIONS INFORMATION
PCB ASSEMBLY AND LAYOUT SUGGESTIONS
The proposed stencil design enables out-gassing of the
solderpasteduringreflowaswellasregulatingthefinished
solder thickness.
Bypass Capacitor Placement
The LTC2978 requires 0.1µF bypass capacitors between
PC Board Layout
the V
pins and GND, the V
pin and GND, and the
DD33
DD25
REFP pin and REFM pin. If the chip is being powered from
the V 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.
Mechanical stress on a PC board and soldering-induced
stress can cause the LTC2978’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.
PWR
Exposed Pad Stencil Design
Unused ADC Sense Inputs
The LTC2978’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.
Connect all unused ADC sense inputs (V
SENSEMn
or
SENSEPn
V
) 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.
V
SENSEP
LTC2978
SENSEP
100k
V
100k
2978 F33
Figure 33. Connecting Unused Inputs to GND
2978 F32
Figure 32. Suggested Screen Pattern for Die Attach Pad
2978fc
77
Linear Systems [ Linear Systems ]
