Datasheet

17
LTC3718
3718fa
and double check the assumed T
J
in the MOSFET:
P
VV
V
A
W
BOT
=






Ω
=
25 125
25
99
2
1 15 0 013
018
2
.–.
.
.
(. )(. )
.
T
J
= 50°C + (0.18W)(50°C/W) = 59°C
Now check the power dissipation of the top MOSFET at
current limit with ρ
90°C
= 1.35:
P
V
V
A
V A pF kHz
W
TOP
=
()()
Ω
()
+
()()()( )( )
=
125
25
99 135 0013
17 25 99 60 300
087
2
2
.
.
...
.. .
.
T
J
= 50°C + (0.87W)(50°C/W) = 93.5°C
C
IN
is chosen for an RMS current rating of about 6A at
temperature. The output capacitors are chosen for a low
ESR of 0.005Ω to minimize output voltage changes due to
inductor ripple current and load steps. The ripple voltage
will be only:
∆V
OUT(RIPPLE)
= ∆I
L(MAX)
(ESR)
= (2.6A) (0.005Ω) = 13mV
However, a 0A to 6A load step will cause an output change
of up to:
∆V
OUT(STEP)
= ∆I
LOAD
(ESR) = (6A) (0.005Ω) = 30mV
The inductor for the boost converter is selected by first
choosing an allowable ripple current. The boost converter
will be operating in discontinous mode. If we select a ripple
current of 170mA for the boost converter, then:
L
V
V
V
mA MHz
H=
−






=µ
33 1
33
5
170 1 4
47
.
.
()(.)
.
The complete circuit is shown in Figure 7.
APPLICATIO S I FOR ATIO
WUUU
Figure 7. Design Example: 1.25V/±6A at 300kHz from 2.5V
C
SS
0.1µF
C2
100pF
R
C
4.75k
R
PG
100k
RUN/SS
V
ON
PGOOD
V
RNG
I
TH
SGND1
I
ON
V
FB1
V
REF
SHDN
SGND2
V
FB2
BOOST
TG
SW1
SENSE
+
SENSE
–
PGND1
BG
INTV
CC
V
IN1
V
IN2
PGND2
SW2
1
2
3
4
5
6
7
8
9
10
11
12
24
23
22
21
20
19
18
17
16
15
14
13
LTC3718
C1 820pF
R
R1
10k
R
ON
237k
PGOOD
R
F1
12.1k
R
F2
37.4k
R
F3
10k
R
R2
39.2k
C
F4
1000pF
C
IN2
4.7µF
L2
4.7µH
M2
IRF7811A
D2
B340A
3718 F07
L1
1µH
D3
MBR0520
C
VCC1
10µF
C
B
0.33µF
D
B
CMDSH-3
C
OUT
270µF
×2
M1
IRF7811A
C
IN2
330µF
V
IN
2.5V
V
OUT
1.25V
±6A
C
IN1
22µF
×2
D1
B340A