LTC3731 Linear Technology, LTC3731 Datasheet - Page 27

• Manufacturer Part Number: LTC3731
• Description: Synchronous Buck Switching Regulator Controller
• Manufacturer: Linear Technology

www.DataSheet4U.com
PACKAGE DESCRIPTIO
This totals 1W at V
at V
Total of all three synchronous MOSFET’s AC gate loss:
This totals 0.08W at V
0.19W at V
experience the Miller capacitance dissipation issue that
the main switch does because the bottom switch turns on
P
( )
3
IN
MAIN
Q
= 20V.
G
V
≈
DSSPEC
⎛
⎜
⎝
3
V
IN
(
5
IN
V
V
IN
= 20V. The bottom MOSFET does not
– .
)
1
2
1 8
IN
( ) ( )(
f
( )( )
45
2 3
= 8V, 2.25W at V
V
=
A
IN
+
6 15
(
= 8V, 0.12W at V
1 8
2
Ω
.
1
V
)(
nC
1000
⎞
⎟
⎠
(
)
400
V
DSSPEC
pF
V
kHz
U
IN
IN
)
= 12V and 6.25W
)
=
(
4 5
IN
6 3
E
.
= 12V and
)
W
when its drain is close to ground.
The Schottky rectifier loss assuming 50ns nonoverlap
time:
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:
The numbers above represent the values at V
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
loss with the DC loss. A smaller, less expensive MOSFET
can actually perform better in the task of the main switch.
2 • 3(0.7V)(15A)(50ns)(4E5)
Main switch’s AC loss (V
Main switch’s DC loss
Synchronous switch AC loss
Synchronous switch DC loss
Power path loss
MILLER
IN
= 12V) 2.25W 3.75%
to better balance the AC
0.87W 1.5%
0.19W 0.3%
7.2W
3.7W
LTC3731
IN
= 12V. It
12%
6.1%
27
3731fa