LTC3733_15 Datasheet, PDF (28/32 Pages) Linear Technology – 3-Phase, Buck Controllers for AMD CPUs

LTC3733/LTC3733-1
APPLICATIO S I FOR ATIO
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:
( ) ^P_COMPATH
≈
N^{}
I_MAX
N
 ²
R_L
+ R_SENSE
+ C_OUTESR Loss
This totals 3.375W + C_OUTESR loss.
Total of all three main MOSFET’s DC loss:
( ) ^P_MAIN
=
N^{}
V_OUT
V_IN



I_MAX
N

2
1+ δ
R_DS(ON)
+ C_INESR Loss
This totals 0.83W + C_INESR loss.
Total of all three synchronous MOSFET’s DC loss:
( ) ^P_SYNC
= N^{}1–
V_OUT
V_IN



I_MAX
N

2
1+ δ
R_DS(ON)
This totals 5.4W.
Total of all three main MOSFET’s AC loss:
P_MAIN
≈
3(V_IN)²
45A _{(2Ω)(1000pF)}
(2)(3)

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 MOSFET’s AC 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 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.
3733f
28