LTC3869-2_15 Datasheet, PDF (32/42 Pages) Linear Technology – Dual, 2-Phase Synchronous Step-Down DC/DC Controllers

LTC3869/LTC3869-2
Applications Information
For each channel, 0.1µF is selected for C1.
R1=
L
= ^0.56µH = 3.11k
(DCR_MAX at 25°C) • C1 1.8mΩ • 0.1µF
Choose R1 = 3.09k
The power loss in R1 at the maximum input voltage is:
P_LOSS R1 =
( V_{IN (MAX )}
− V_OUT
R1
)
•
V_OUT
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)
•
⎡
⎣⎢5V
1
– 2.6V
+
1
2.6V
⎤⎦⎥_(400kHz)
= 329mW + 288mW
= 617mW
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
⎞
⎟
⎠
=
6.7
A
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.
32
For more information www.linear.com/LTC3869
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