ltc3869gn-2 Linear Technology Corporation, ltc3869gn-2 Datasheet - Page 31

• Manufacturer Part Number: ltc3869gn-2
• Description: Ltc3869/ltc3869-2 - Dual, 2-phase Synchronous Step-down Dc/dc Controllers
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
For each channel, 0.1µF is selected for C1.
Choose R1 = 3.09k
The power loss in R1 at the maximum input voltage is:
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
the DCR sense application is 1.8mΩ (max). As a result of
the lower conduction losses from the switch node to V
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
2.6V, C
(estimated) = 75°C:
R1=
P
P
⎡
⎣ ⎢
(
= 329mW + 288mW
= 617mW
MAIN
0.013Ω
5V – 2.6V
LOSS
MILLER
(DCR
1
R1=
SENSE
=
1.8V
20V
)
MAX
+ 20V
≅ 150pF . At maximum input voltage with T
(V
application whereas the inductor DCR for
(
+
IN(MAX)
(
2.6V
at 25°C) • C1
15A
L
1
)
2
)
DS(ON)
⎤
⎦ ⎥
⎛
⎜
⎝
2
(
− V
15A
400kHz
[
R1
1+ (0.005)(75°C – 25°C)
2
OUT
⎞
⎟ 2Ω
⎠
= 13mΩ (max), V
( )
=
) • V
)
1.8mΩ • 0.1µF
OUT
(
150pF
0.56µH
)
•
MILLER
= 3.11k
]
•
OUT
=
J
,
For a 2mΩ sense resistor, a short-circuit to ground will
result in a folded back current of:
A Renesas RJK0330DPB, R
the bottom FET. The resulting power loss is:
C
temperature assuming only channel 1 or 2 is on. C
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:
Further reductions in output voltage ripple can be made
by placing a 100µF ceramic across C
IN
P
V
I
P
⎡ ⎣
SC
is chosen for an RMS current rating of at least 7.5A at
1+ 0.005
SYNC
SYNC
ORIPPLE
=
(
(
= 1W
=
1/ 3
0.002Ω
20V – 1.8V
= R
)
)
50mV
20V
• 75°C – 25°C
ESR
LTC3869/LTC3869-2
(
(∆I
–
L
) = 0.0045Ω • 6.8A = 31mV
2
(
1
15A
⎛
⎜
⎝
90ns(20V)
DS(ON)
0.56µH
)
2
)
⎤ ⎦ • 0.0039Ω
•
= 3.9mΩ, is chosen for
OUT
⎞
⎟ = 6.7A
⎠
.
31
OUT
P–P
3869f
is