LTC3854_15 Datasheet, PDF (22/28 Pages) Linear Technology – Small Footprint, Wide VIN Range Synchronous Step-Down DC/DC Controller

LTC3854
APPLICATIONS INFORMATION
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.
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.
0.1µF
2200pF
3.9k
V_IN
LTC3854 ^TG
RUN/SS
ITH
BOOST
100pF
SW
10k
4.99k
INTV_CC
FB
SENSE^–
BG
SENSE⁺ GND
0.1µF
D1
M1
180µF
25V
L
0.56µH
1.8mΩ DCR
3.09k ^0.1µF
4.7µF
M2
10µF
25V
×2
V_IN
4.5V TO 28V
V_OUT
1.2V
15A
+ _COUT1
330µF
×2
C_OUT2
47µF
D1 = CMDSH-3
M1 = RENESAS RJK0305DPB
M2 = RENESAS RJK0330DPB
L = VISHAY IHLP4040DZ-01 0.56µH
C_OUT1 = SANYO 2R5TPE330M9
C_OUT2 = MURATA GRM31CR60J476K
3854 F06
Figure 6. 1.2V/15A Converter from Design Example
3854fb
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