LTC3730_15 Datasheet, PDF (25/28 Pages) Linear Technology – 3-Phase, 5-Bit Intel Mobile VID, 600kHz, Synchronous Buck Controller

LTC3730
APPLICATIO S I FOR ATIO
Efficiency Calculation
To estimate efficiency, the DC loss terms include the input
and output capacitor ESR, each MOSFET R_DS(ON), induc-
tor resistance R_L, the sense resistance R_SENSE and the
forward drop of the Schottky rectifier at the operating
output current and temperature. Typical values for the
design example given previously in this data sheet are:
Main MOSFET R_DS(ON) = 7mΩ (9mΩ at 90°C)
Sync MOSFET R_DS(ON) = 7mΩ (9mΩ at 90°C)
C_INESR = 20mΩ
C_OUTESR = 3mΩ
R_L = 2.5mΩ
R_SENSE = 3mΩ
V_SCHOTTKY = 0.8V at 15A (0.7V at 90°C)
V_OUT = 1.3V
V_IN = 12V
I_MAX = 45A
δ = 0.5%/°C
N=3
f = 400kHz
The main MOSFET is on for the duty factor V_OUT/V_IN and
the synchronous MOSFET is on for the rest of the period
or simply (1 – V_OUT/V_IN). Assuming the ripple current is
small, the AC loss in the inductor can be made small if a
good quality inductor is chosen. The average current,
I_OUT, is used to simplify the calaculations. The equation
below is not exact but should provide a good technique
for the comparison of selected components and give a
result that is within 10% to 20% of the final application.
Determining the MOSFETs’ die temperature may require
iterative calculations if one is not familiar with typical
performance. A maximum operating junction temperature
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 MOSFETs’ DC loss:
( ) ^PMAIN
=
⎛
N_⎝⎜
V_OUT
V_IN
⎞ ⎛ _IMAX
⎠⎟ ⎝⎜ N
⎞²
⎠⎟
1+
δ
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+ δ
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 _{(2Ω)(1000pF}
(2)(3)
)
⎛⎝⎜
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.
3730fa
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