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| LTC3855_15 Datasheet, PDF (34/44 Pages) Linear Technology – Dual, Multiphase Synchronous DC/DC Controller with Differential Remote Sense | |||
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LTC3855
Applications Information
MOSFET results in: RDS(ON) = 13m⦠(max), VMILLER =
2.6V, CMILLER â
150pF. At maximum input voltage with TJ
(estimated) = 75°C:
PMAIN
=
1.8V
20V
(15A
)2
[1+
(0.005)(75°C
â
25°C)]
â¢
(0.013â¦)
+
(20V)2

ï£ï£¬
15A
2


(2â¦)(150pF
)
â¢


5V
1
â 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:
ISC
=
(1/ 3)50mV
0.002â¦
â
1
2

ï£ï£¬
90ns(20V)
0.56 µH


=
6.7
A
A Renesas RJK0330DPB, RDS(ON) = 3.9mâ¦, is chosen for
the bottom FET. The resulting power loss is:
PSYNC
=
20V â 1.8V
20V
(15A
)2
â¢
1+ (0.005) ⢠(75°C â 25°C) ⢠0.0039â¦
PSYNC = 1W
CIN is chosen for an RMS current rating of at least 7.5A at
temperature assuming only channel 1 or 2 is on. COUT is
chosen with an equivalent ESR of 4.5m⦠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:
VORIPPLE = RESR (âIL) = 0.0045⦠⢠6.8A = 31mVPâP
Further reductions in output voltage ripple can be made
by placing a 100µF ceramic across COUT.
Typical Applications
20k
20k
RNTC2
100k
49.9k
RNTC1
100k
49.9k
0.1µF
63.4k
0.1µF
20k
1nF
20k
100pF
100pF
20k
15k 1nF
0.1µF
0.1µF
40.2k
TK/SS1
ITH1
VFB1
SGND
VFB2
ITH2
TK/SS2
SENSE2+
SENSE2â
DIFFP
86.6k
LTC3855
TG1
BOOST1
PGND1
BG1
VIN
INTVCC
EXTVCC
BG2
PGND2
BOOST2
0.1µF
CMDSH-3
2.2Ω
4.7µF
0.1µF
0.1µF CMDSH-3
100k
PGOOD1
PGOOD2
100k
10µF
s2
M1
RJK0305DPB
24.9k
3.01k
L1
0.68µH
M2
RJK0330DPB
+ 82µF
25V
s2
VIN
4.5V TO
20V
COUT1 +
100µF
6.3V
COUT2
330µF
4V
s2
VOUT1
2.5V
15A
M3
RJK0305DPB
L2
0.68µH
M4
RJK0330DPB
3.01k
24.9k
COUT3 +
100µF
6.3V
COUT4
330µF
4V
s2
VOUT2
1.8V
15A
L1, L2: VISHAY IHLP5050CE-01, 0.68µH
COUT1, COUT3: MURATA GRM32ER60J107ME20
COUT2, COUT4: KEMET T520V337M004ATE009
RNTC1, RNTC2: MURATA NCP18WF104J03RB
3855 F18
Figure 18. 2.5V, 15A and 1.8V, 15A Supply with NTC Temperature Compensated DCR Sensing, fSW = 350kHz
34
3855f
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