Datasheet

LTC3852
29
3852f
The power dissipation on the topside MOSFET can be easily
estimated. Choosing Vishay SIR438DP MOSFETs results
in: R
DS(ON)
= 0.0023W, C
MILLER
= 445pF. At maximum
input voltage with T (estimated) = 50°C:
P
MAIN
=
1.5V
5.5V
15
()
2
1+ 0.005
()
50°C − 25°C
()
⎡
⎣
⎤
⎦
• 0.0023Ω
()
+ 5.5V
()
2
15A
2
⎛
⎝
⎜
⎞
⎠
⎟
2Ω
()
445pF
()
•
1
5 − 1
+
1
1
⎡
⎣
⎢
⎤
⎦
⎥
400kHz
()
= 108mW
A short-circuit to ground will result in a folded back current of:
I
SC
=
1
4
()
65mV
0.003Ω
–
1
2
90ns 5.5V
()
400nH
⎛
⎝
⎜
⎞
⎠
⎟
= 4.8A
APPLICATIONS INFORMATION
with a typical value of R
DS(ON)
and d = (0.005/°C)(25°C)
= 0.125. The resulting power dissipated in the bottom
MOSFET is:
P
SYNC
=
5.5V–1.5V
5.5V
15A
()
2
1.125
()
0.0023Ω
()
=423mW
which is less than under full-load conditions.
C
IN
is chosen for an RMS current rating of at least 9A at
temperature. C
OUT
is chosen with an ESR of 0.02W 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
(DI
L
) = 0.02W (5.1A) = 102mV
P-P
TYPICAL APPLICATIONS
Figure 16. High Effi ciency 1.5V/15A Step-Down Converter From Design Example
C8
2200pF
1
2
3
JP1
RUN
R9
5.9k
C5
0.1µF
2.2µF
10V
0603
C1
JP2
MODE
1
2
3
4
R5
68.1k
1%
R16
100k
R17
100k
C9
150pF
C11
0.01µF
V
IN
INTV
CC
100k
PGOOD
ON
OFF
CCM
BURST
PS
+
GND
V
IN
2.7V TO 5.5V
C
IN1
220µF
6.3V
L1
0.4µH
C
IN1
: SANYO 6TPE220MI
C
OUT1
: AVX 12106D107MAT2A
C
OUT2
: SANYO 4TPE330MI
D1: CENTRAL SEMI CMDSH-3
L1: VITEC 59PR9875N
Q1, Q3: VISHAY SILICONIX SiR438DP
C
IN5
10µF
16V
1206
×2
C
OUT1
100µF
6.3V
C
OUT2
330µF
4V
×2
V
OUT
1.5V/15A
Q1
D1
Q3
C15
4.7µF
10V
C6
0.1µF
C12
1000pF
RS1
0.002
1%
+
GND
R11
100
R20
17.4k
1%
R19
20k
1%
R12
100
24
GND1 GND2
FB
BG
TG
BOOST
SW
INTV
CC
V
IN2
V
IN1
V
PUMP
SENSE
–
SENSE
+
C
–
C
+
MODE/
PLLIN
FREQ/PLLFLTR
RUN
TRACK/SS
PGOOD
SHDN
I
TH
GND
LTC3852
3852 F16