LTC3731
28
3731fc
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 MOSFETs’ AC gate loss:
(3)Q
G
V
IN
V
DSSPEC
(f)= (6)(15nC)
V
IN
V
DSSPEC
(4E5)
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.
APPLICATIONS INFORMATION
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’s AC loss (V
IN
= 12V) 2.25W 3.75%
Main switch’s DC loss 0.87W 1.5%
Synchronous switch AC loss 0.19W 0.3%
Synchronous switch DC loss 7.2W 12%
Power path loss 3.7W 6.1%
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.
