LTC3738 Datasheet, PDF (29/32 Pages) Linear Technology – 3-Phase Buck Controller for Intel VRM9/VRM10 with Active Voltage Positioning

LTC3738
APPLICATIO S I FOR ATIO
result that is within 10% to 20% of the final application.
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 1.3W + 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 3.2W.
Total of all three main MOSFET’s AC loss:
P_MAIN
≈
3(V_IN)²
45A
(2)(3)
(2Ω)(140pF)
⎛⎝⎜
5V
1
– 1.8V
+
1.81V ^{⎞⎠⎟ (400kHz)}
=
6.3W
This totals 0.14W at V_IN = 8V, 0.315W at V_IN = 12V and
0.875W at V_IN = 20V.
Total of all three synchronous MOSFET’s AC loss:
(3)Q_G
V_IN
V_DSSPEC
(f)
=
(3)(16nC)
V_IN
V_DSSPEC
(400kHz)
This totals 0.085W at V_IN = 8V, 0.128W at V_IN = 12V and
0.213W 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)(400kHz)
This totals 1.26W.
The total output power is (1.3V)(45A) = 58.5W and the
total input power is approximately 67W so the % loss of
each component is as follows:
Main switch AC loss (V_IN = 12V) 0.315W 0.47%
Main switch DC loss
1.3W 1.9%
Synchronous switch AC loss 0.128W 0.2%
Synchronous switch DC loss 3.2W 4.8%
Power path loss
3.375W 5.05%
The numbers above represent the values at V_IN = 12V.
3738f
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