Datasheet
LTC3855
34
3855f
applicaTions inForMaTion
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
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
.
L1, L2: VISHAY IHLP5050CE-01, 0.68µH
C
OUT1
, C
OUT3
: MURATA GRM32ER60J107ME20
C
OUT2
, C
OUT4
: KEMET T520V337M004ATE009
RNTC1, RNTC2: MURATA NCP18WF104J03RB
3855 F18
TG1
BOOST1
PGND1
BG1
V
IN
INTV
CC
EXTV
CC
BG2
PGND2
BOOST2
TK/SS1
I
TH1
V
FB1
SGND
V
FB2
I
TH2
TK/SS2
SENSE2
+
SENSE2
–
DIFFP
20k
0.1µF
40.2k
SENSE1
–
SENSE1
+
RUN1
ITEMP1
ITEMP2
FREQ
MODE/PLLIN
PHSASMD
CLKOUT
SW1
DIFFN
DIFFOUT
RUN2
I
LIM1
I
LIM2
PGOOD1
PGOOD2
PGOOD1
PGOOD2
NC
SW2
TG2
63.4k
49.9k
R
NTC1
100k
20k
86.6k
100k
100k
20k
2.2Ω
4.7µF
0.1µF
0.1µF
LTC3855
1nF
20k
0.1µF
100pF
10µF
s2
82µF
25V
s2
1nF
15k
0.1µF
100pF
0.1µF
49.9k
R
NTC2
100k
20k
CMDSH-3
M4
RJK0330DPB
M3
RJK0305DPB
M2
RJK0330DPB
M1
RJK0305DPB
CMDSH-3
0.1µF
L1
0.68µH
L2
0.68µH
24.9k
24.9k
3.01k
3.01k
V
OUT2
1.8V
15A
V
IN
4.5V TO
20V
V
OUT1
2.5V
15A
+
C
OUT1
100µF
6.3V
C
OUT2
330µF
4V
s2
+
C
OUT3
100µF
6.3V
C
OUT4
330µF
4V
s2
+
Figure 18. 2.5V, 15A and 1.8V, 15A Supply with NTC Temperature Compensated DCR Sensing, f
SW
= 350kHz
Typical applicaTions