LTC3731 Datasheet, PDF (27/32 Pages) Linear Technology – 3-Phase, 600kHz, Synchronous Buck Switching Regulator Controller

LTC3731
PACKAGE DESCRIPTIO
P_MAIN ≈ 3(V_IN)² _(42)5(A3)(2Ω)(1000pF)
⎛⎝⎜
5V
1
– 1.8V
+
1
1.8V
⎞⎠⎟
(400kHz)
=
6.3W
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 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.
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.
3731fa
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