LTC3731H_15 Datasheet, PDF (27/34 Pages) Linear Technology – 3-Phase, 600kHz, Synchronous Buck Switching Regulator Controller

LTC3731H
Applications Information
Total of all three main MOSFETs’ DC loss:
( ) P_MAIN
=
N^{}
V_OUT
V_IN




I_MAX
N


2
1+ d
R_DS(ON) + C_INESR Loss
This totals 0.87W + C_INESR loss (at 90°C).
Total of all three synchronous MOSFETs’ DC loss:
P_SYNC
=
N ^{} 1–
V_OUT
V_IN




I_MAX
N


2
(1+
)d R_DS(ON)
This totals 7.2W at 90°C.
Total of all three main MOSFETs’ AC loss:
P_MAIN
≈
3(V_IN)²
45A
(2)(3)
(2Ω)(1000pF)

5V
1
– 1.8V
+
1.81V ^{ (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 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.
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.
3731Hfb
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