Linear LTC3875, Datasheet

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LTC3875
36
3875fb
For more information www.linear.com/LTC3875
APPLICATIONS INFORMATION
Choose R1 = 4.64k and R2 = 931Ω.
The maximum DCR of the inductor is 0.34mΩ. The
V
SENSE(MAX)
is calculated as:
V
SENSE(MAX)
= I
PEAK
• DCR (Max) = 12mV
The current limit is chosen to be 15mV. If temperature
variation is considered, please refer to Inductor DCR
Sensing Temperature Compensation with NTC Thermistor.
The power dissipation on the topside MOSFET can be
easily estimated. Choosing an Infineon BSC050NE2LS
MOSFET results in: R
DS(ON)
= 7.1mΩ (max), V
MILLER
=
2.8V, C
MILLER
≅
35pF. At maximum input voltage with T
J
(estimated) = 75°C:
P
MAIN
=
1.5V
20V
30A
( ) 2
1 + 0.005
( )
75 ° C– 25 ° C
( )
  •
0.0071 Ω
( )
+ 20V
( ) 2
30A
2




2 Ω
( )
35pF
( )
•
1
5.5V – 2.8V
+
1
2.8V

 

400kHz
( )
= 599mW + 122mW
= 721mW
For a 0.32mΩ DCR, a short-circuit to near ground will
result in a folded back current of:
I
SC
=
1/ 3
( )
15mV
0.0032 Ω
–
1
2
90ns 20V
( )
0.33µH

 

=
12.9A
An Infineon BSC010NE2LS, R
DS(ON)
= 1.1mΩ, is chosen
for the bottom FET. The resulting power loss is:
P
SYNC
=
20V –1.5V
20V
30A
( ) 2
•
1 + 0.005
( )
• 75 ° C– 25 ° C
( )
  •0.001 Ω
P
SYNC
= 1.14W
C
IN
is chosen for an equivalent RMS current rating of at
least 13.7A. C
OUT
is chosen with an equivalent ESR of
4.5mΩ for low output ripple. The output ripple in continu-
ous mode will be highest at the maximum input voltage.
The output voltage ripple due to ESR is approximately:
V
ORIPPLE
= R
ESR
( ∆ I
L
) = 0.0045Ω • 10A = 45mV
P-P
Further reductions in output voltage ripple can be made
by placing a 100µF ceramic capacitor across C
OUT
.
Thermal Balancing Converter Example
If thermal balancing function is desired, connecting
ENTMPB pin to ground enables the temperature balancing
function, but disables the inductor DCR sensing temperature
compensation function. For a 4-phase design select
TRSET1,2,3,4 = 10k, then R
TAVG
= 2.5k. The resistance
vs temperature slope of NTC connected to the TCOMP
pin need to be modified according to the inductor current
correction range. Please refer to temperature balancing
with NTC thermistor example shown in Figure 17.