LTC1649 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1649' PDF : 16 Pages View PDF
LTC1649
APPLICATIONS INFORMATION
and output capacitors, the source of Q2, the LTC1649 GND
pin, the output return and the input supply return all
clustered at one point. Figure 9 is a modified schematic
showing the common connections in a proper layout. Note
that at 10A current levels or above, current density in the
PC board itself can become a concern; traces carrying high
currents should be as wide as possible.
Power Component Hook-Up/Heat Sinking
As current levels rise much above 1A, the power compo-
nents supporting the LTC1649 start to become physically
large (relative to the LTC1649, at least) and can require
special mounting considerations. Input and output ca-
pacitors need to carry high peak currents and must have
low ESR; this mandates that the leads be clipped as short
as possible and PC traces be kept wide and short. The
power inductor will generally be the most massive single
component on the board; it can require a mechanical hold-
down in addition to the solder on its leads, especially if it
is a surface mount type.
The power MOSFETs used require some care to ensure
proper operation and reliability. Depending on the current
levels and required efficiency, the MOSFETs chosen may
be as large as TO-220s or as small as SO-8s. High
efficiency circuits may be able to avoid heat sinking the
power devices, especially with TO-220 type MOSFETs. As
an example, a 90% efficient converter working at a steady
2.5V/10A output will dissipate only (25W/90%)10% =
2.8W. The power MOSFETs generally account for the
majority of the power lost in the converter; even assuming
that they consume 100% of the power used by the
converter, that’s only 2.8W spread over two or three
devices. A typical SO-8 MOSFET with a RON suitable to
provide 90% efficiency in this design can commonly
dissipate 2W when soldered to an appropriately sized
piece of copper trace on a PC board. Slightly less efficient
or higher output current designs can often get by with
standing a TO-220 MOSFET straight up in an area with
some airflow; such an arrangement can dissipate as much
as 3W without a heat sink. Designs which must work in
high ambient temperatures or which will be routinely
overloaded will generally fare best with a heat sink.
22Ω
+
10µF
DCP
VIN
PVCC2
PVCC1
CPOUT
G1
+
1µF
10µF 1k
VCC
IFB
LTC1649
0.1µF
C+
1µF
C–
IMAX
G2
RIMAX
C1 RC
CC
COMP
FB
SS
SHDN
GND
CSS
VIN
+
CIN
Q1
L1
VOUT
R1
Q2
+
COUT
R2
14
SHDN
1649 F09
Figure 9. Typical Schematic Showing Layout Considerations
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