Datasheet

LTC3869/LTC3869-2
32
38692fa
For more information www.linear.com/LTC3869
For a 2mΩ sense resistor, a short-circuit to ground will
result in a folded back current of:
I
SC
=
1/ 3
( )
50mV
0.002Ω
–
1
2
90ns(20V)
0.56µH
⎛
⎝
⎜
⎞
⎠
⎟
= 6.7A
A Renesas RJK0330DPB, R
DS(ON)
= 3.9mΩ, is chosen for
the bottom FET. The resulting power loss is:
P
SYNC
=
20V – 1.8V
20V
15A
( )
2
•
1+ 0.005
( )
• 75°C – 25°C
( )
⎡
⎣
⎤
⎦
• 0.0039Ω
P
SYNC
= 1W
C
IN
is chosen for an RMS current rating of at least 7.5A at
temperature assuming only channel 1 or 2 is on. C
OUT
is
chosen with an equivalent ESR of 4.5mΩ for low output
ripple. The output ripple in continuous 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 • 6.8A = 31mV
P–P
Further reductions in output voltage ripple can be made
by placing a 100µF ceramic across C
OUT
.
applicaTions inForMaTion
For each channel, 0.1µF is selected for C1.
R1=
L
(DCR
MAX
at 25°C) • C1
=
0.56µH
1.8mΩ • 0.1µF
= 3.11k
Choose R1 = 3.09k
The power loss in R1 at the maximum input voltage is:
P
LOSS
R1=
(V
IN(MAX)
− V
OUT
) • V
OUT
R1
The resulting power loss for R1 is 11mW for channel 1
and 7mW for channel 2.
The sum of the sense resistor and DCR is 2.5mΩ (max)
for the R
SENSE
application whereas the inductor DCR for
the DCR sense application is 1.8mΩ (max). As a result
of the lower conduction losses from the switch node to
V
OUT
, the DCR sensing application has higher efficiency.
The power dissipation on the topside MOSFET can be
easily estimated. Choosing a Renesas RJK0305DPB
MOSFET results in: R
DS(ON)
= 13mΩ (max), V
MILLER
=
2.6V, C
MILLER
≅ 150pF. At maximum input voltage with
T
J
(estimated) = 75°C:
P
MAIN
=
1.8V
20V
15A
( )
2
1+(0.005)(75°C – 25°C)
[ ]
•
0.013Ω
( )
+ 20V
( )
2
15A
2
⎛
⎝
⎜
⎞
⎠
⎟
2Ω
( )
150pF
( )
•
1
5V – 2.6V
+
1
2.6V
⎡
⎣
⎢
⎤
⎦
⎥
400kHz
( )
= 329mW + 288mW
= 617mW