LTC3350
31
3350fc
For more information
www.linear.com/LTC3350
applicaTions inForMaTion
Connect the drain of the top MOSFET and cathode of
the top diode directly to the positive terminal of C
OUT
.
Connect the source of the bottom MOSFET and anode
of the bottom diode directly to the negative terminal
of C
OUT
. This capacitor provides the AC current to the
MOSFETs.
2. Ground is referenced to the negative terminal of the
V
CAP
decoupling capacitor in step-down mode and to
the negative terminal of the V
OUT
decoupling capacitor
in step-up mode. The negative terminal of C
OUT
should
be as close as possible to the negative terminal of
C
CAP
by placing the capacitors next to each other and
away from the switching loop described above. The
combined IC SGND pin/PGND paddle and the ground
returns of C
INTVCC
and C
DRVCC
must return to the com-
bined negative terminal of C
OUT
and C
CAP
.
3. Effective grounding techniques are critical for success-
ful DC/DC converter layouts. Orient power components
such that switching current paths in the ground plane
do not cross through the SGND pin and exposed pad
on the backside of the LTC3350 IC. Switching path
currents can be controlled by orienting the MOSFET
switches, Schottky diodes, the inductor, and V
OUT
and
V
CAP
decoupling capacitors in close proximity to each
other.
4. Locate V
CAP
and V
OUT
dividers near the part and away
from switching components. Kelvin the top of resistor
dividers to the positive terminals of C
CAP
and C
OUT
,
respectively. The bottom of the resistive dividers should
go back to the SGND pin. The feedback resistor con-
nections should not be run along the high current feeds
from the C
OUT
capacitor.
5. Route ICAP and VCAP sense lines together, keep them
short. Same with VOUTSP and VOUTSN. Filter com-
ponents should be placed near the part and not near
the sense resistors. Ensure accurate current sensing
with Kelvin connections at the sense resistors. See
Figure 10.
6. The trace from the positive terminal of the input current
sense resistor, R
SNSI
, to the VOUTSP pin carries the
part’s quiescent and gate drive currents. To maintain
accurate measurement of the input current keep this
trace short and wide by placing R
SNSI
near the part.
7. Locate the DRV
CC
and BST decoupling capacitors in
close proximity to the IC. These capacitors carry the
MOSFET drivers’ high peak currents. An additional 0.1μF
ceramic capacitor placed immediately next to the DRV
CC
pin can help improve noise performance substantially.
8. Locate the small-signal components away from high
frequency switching nodes (BST, SW, TG, and BG). All
of these nodes have very large and fast moving signals
and should be kept on the output side of the LTC3350.
9. The input ideal diode senses the voltage between V
IN
and VOUTSP. V
IN
should be connected near the source
of the input ideal diode MOSFET. VOUTSP is used for
Kelvin sensing the input current. Place the input cur-
rent sense resistor, R
SNSI
, near the input ideal diode
MOSFET with a short, wide trace to minimize resistance
between the drain of the ideal diode MOSFET and R
SNSI
.
10. The output ideal diode senses the voltage between
VOUTSN and VCAP. VCAP is used for Kelvin sensing
the charge current. Place the output ideal diode near
the charge current sense resistor, R
SNSC
, with a short,
wide trace to minimize resistance between the source
of the ideal diode MOSFET and R
SNSC
.
11. The INFET and OUTFET pins for the external ideal diode
controllers have extremely limited drive current. Care
must be taken to minimize leakage to adjacent PC board
traces. 100nA of leakage from these pins will introduce
an additional offset to the ideal diodes of approximately
10mV. To minimize leakage, the INFET trace can be
guarded on the PC board by surrounding it with VOUT
connected metal. Similarly, the OUTFET trace should be
guarded by surrounding it with VCAP connected metal.
12. The VCC2P5 bypass capacitor should return to ground
away from switching and gate drive current paths.
Figure 10. Kelvin Current Sensing
3350 F10
DIRECTION OF SENSED CURRENT
R
SNSC
OR
R
SNSI
TO VCAP
OR
VOUTSN
TO ICAP
OR
VOUTSP
