LTC1430
PPLICATI
power inductor will generally be the most massive single
componentontheboard;itcanrequireamechanicalhold-
down in addition to the solder on its leads, especially if it
is a surface mount type.
O U
W
U
A
S I FOR ATIO
majority of the power lost in the converter; even assuming
that they consume 100% of the power used by the
converter, that’s only 3.7W 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.
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
3.3V/10A output will dissipate only (33W/90%) • 10% =
3.7W. The power MOSFETs generally account for the
V
OUT
+
SENSE
NC
LTC1430
R1
FB
–
R2
SENSE
NC
LTC1430 • F12
Figure 12. Using External Resistors to Set Output Voltages
5V
100Ω
1µF
TOTAL
880µF
(220µF
10V ×4)
+
+
4.7µF
35V
1N4148
V
PV
CC2
CC
0.1µF
GND
PGND
PV
CC1
G1
M1A*
M1B*
LTC1430
0.1µF
2.7µH/15A
I
NC
NC
MAX
I
FB
3.3V
FREQSET
+
SENSE
TOTAL
SHDN
COMP
G2
M2*
+
1980µF
(330µF
6.3V ×6)
FB
–
NC
SS
SENSE
C1
220pF
R
C
7.5k
GND
PGND
PGND
C
C
C
SS
4700pF
0.01µF
* MOTOROLA MTD20N03HL
LTC1430 • F11
GND
Figure 11. Typical Schematic Showing Layout Considerations
14
Linear [ Linear ]
