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LTC3854_15 Datasheet, PDF (22/28 Pages) Linear Technology – Small Footprint, Wide VIN Range Synchronous Step-Down DC/DC Controller | |||
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LTC3854
APPLICATIONS INFORMATION
Choosing an RJK0330DPB for the bottom FET will pro-
vide:
BVDSS = 30V
RDS(ON) = 3.9m⦠maximum at 25°C, VGS = 4.5V
QG = 27nC, typical, at VGS = 4.5V
PSYNC
=
20V â 1.2V
20V
â¢
(15A)2
⢠(1+ 0.005 ⢠(100°C â 25°C)) ⢠3.9mâ¦
PSYNC = 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.
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 QG of 8nC for the RJK0305DPB and 27nC
for the RJK0330DPB and the 400kHz switching frequency,
the current supplied by INTVCC will be:
IGATECHG = (8nC + 27nC) ⢠400kHz = 14mA
The resulting controller temperature at 60°C and a 20V
input will be:
TJ = 60°C + 20V ⢠14mA ⢠76°C/W = 81°C
which is well under the maximum junction temperature
of 125°C.
0.1µF
2200pF
3.9k
VIN
LTC3854 TG
RUN/SS
ITH
BOOST
100pF
SW
10k
4.99k
INTVCC
FB
SENSEâ
BG
SENSE+ GND
0.1µF
D1
M1
180µF
25V
L
0.56µH
1.8mΩ DCR
3.09k 0.1µF
4.7µF
M2
10µF
25V
Ã2
VIN
4.5V TO 28V
VOUT
1.2V
15A
+ COUT1
330µF
Ã2
COUT2
47µF
D1 = CMDSH-3
M1 = RENESAS RJK0305DPB
M2 = RENESAS RJK0330DPB
L = VISHAY IHLP4040DZ-01 0.56µH
COUT1 = SANYO 2R5TPE330M9
COUT2 = MURATA GRM31CR60J476K
3854 F06
Figure 6. 1.2V/15A Converter from Design Example
3854fb
22
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