28
LTC3733/LTC3733-1
3733f
APPLICATIO S I FOR ATIO
WUUU
The temperature of the MOSFET’s die temperature may
require interative calculations if one is not familiar typical
performance. A maximum operating junction tempera-
ture of 90° to 100°C for the MOSFETs is recommended
for high reliability applications.
Common output path DC loss:
PN
I
N
R R C Loss
COMPATH
MAX
L SENSE OUTESR
≈
+
()
+
2
This totals 3.375W + C
OUTESR
loss.
Total of all three main MOSFET’s DC loss:
PN
V
V
I
N
R C Loss
MAIN
OUT
IN
MAX
DS ON INESR
=
+
()
+
2
1δ
()
This totals 0.83W + C
INESR
loss.
Total of all three synchronous MOSFET’s DC loss:
PN
V
V
I
N
R
SYNC
OUT
IN
MAX
DS ON
=
+
()
11
2
–
()
δ
This totals 5.4W.
Total of all three main MOSFET’s AC loss:
PV
A
pF
VV V
kHz W
MAIN IN
≈Ω
+
=
3
45
23
2 1000
1
518
1
18
400 6 3
2
()
()()
()( )
–. .
().
This totals 1W at V
IN
= 8V, 2.25W at V
IN
= 12V and 6.25W
at V
IN
= 20V.
Total of all three synchronous MOSFET’s AC loss:
() () ()( ) ( )361545Q
V
V
fnC
V
V
E
G
IN
DSSPEC
IN
DSSPEC
=
This totals 0.08W at V
IN
= 8V, 0.12W at V
IN
= 12V and
0.19W at V
IN
= 20V. The bottom MOSFET does not
experience the Miller capacitance dissipation issue that
the main switch does because the bottom switch turns on
when its drain is close to ground.
The Schottky rectifier loss assuming 50ns nonoverlap
time:
2 • 3(0.7V)(15A)(50ns)(4E5)
This totals 1.26W.
The total output power is (1.3V)(45A) = 58.5W and the
total input power is approximately 60W so the % loss of
each component is as follows:
Main switch AC loss (V
IN
= 12V) 2.25W 3.75%
Main switch DC loss 0.83W 1.4%
Synchronous switch AC loss 0.19W 0.3%
Synchronous switch DC loss 5.4W 9%
Power path loss 3.375W 5.6%
The numbers above represent the values at V
IN
= 12V. It
can be seen from this simple example that two things can
be done to improve efficiency: 1) Use two MOSFETs on the
synchronous side and 2) use a smaller MOSFET for the
main switch with smaller C
MILLER
to better balance the AC
loss with the DC loss. A smaller, less expensive MOSFET
can actually perform better in the task of the main switch.
