Datasheet

LTC3854
22
3854fb
Choosing an RJK0330DPB for the bottom FET will pro-
vide:
BV
DSS
= 30V
R
DS(ON)
= 3.9mΩ maximum at 25°C, V
GS
= 4.5V
Q
G
= 27nC, typical, at V
GS
= 4.5V
P
SYNC
=
20V − 1.2V
20V
• 15A
( )
2
• 1+ 0.005 • 100°C – 25°C
( )
(
)
• 3.9mΩ
P
SYNC
= 1.1W
Assuming a thermal resistance of 40°C/W for the main and
synchronous FETs, the resulting junction temperatures at
an ambient of 60°C will be 82°C and 104°C, respectively.
APPLICATIONS INFORMATION
Figure 6. 1.2V/15A Converter from Design Example
Some airflow may be required for higher ambient tem-
peratures. A maximum MOSFET junction temperature of
110°C at worst case ambient generally provides adequate
margin.
Given a typical Q
G
of 8nC for the RJK0305DPB and 27nC
for the RJK0330DPB and the 400kHz switching frequency,
the current supplied by INTV
CC
will be:
I
GATECHG
= (8nC + 27nC) • 400kHz = 14mA
The resulting controller temperature at 60°C and a 20V
input will be:
T
J
= 60°C + 20V • 14mA • 76°C/W = 81°C
which is well under the maximum junction temperature
of 125°C.
3854 F06
LTC3854
0.1µF
0.1µF
180µF
25V
4.7µF
D1
100pF
2200pF
L
0.56µH
1.8mΩ DCR
3.09k
0.1µF
C
OUT1
330µF
×2
V
OUT
1.2V
15A
V
IN
4.5V TO 28V
4.99k
10k
3.9k
V
IN
TG
BOOST
SW
INTV
CC
BG
GND
RUN/SS
ITH
FB
SENSE
–
SENSE
+
M1
M2
C
OUT2
47µF
10µF
25V
×2
D1 = CMDSH-3
M1 = RENESAS RJK0305DPB
M2 = RENESAS RJK0330DPB
L = VISHAY IHLP4040DZ-01 0.56µH
C
OUT1
= SANYO 2R5TPE330M9
C
OUT2
= MURATA GRM31CR60J476K
+