Datasheet
LTC3813
29
3813fb
Verify that the Si7848DP is also a good choice for the
bottom MOSFET by checking its power dissipation at
current limit and minimum input voltage, assuming a
junction temperature of 30°C above a 70°C ambient
(ρ
100°C
= 1.4):
P
BOT
= 0.5
6.5A
1 0.5
2
(1.4) (0.009)
+
1
2
(24V)
2
6.5A
1
0.5
(2)(400pF)
•
1
12V 3.5
V
+
1
3.5V
(250kHz)
= 1.06W + 0.30W = 1.36W
T
J
= 70°C + 1.36W • 20°C/W = 97°C
The junction temperature will be signifi cantly less at
nominal current, but this analysis shows that careful at-
tention to heat sinking on the board will be necessary in
this circuit.
Since V
IN
is always between 6.2V and 14V, it can be con-
nected directly to the INTV
CC
and DRV
CC
pins.
C
OUT
is chosen for an RMS current rating of about 5A at
85°C. The output capacitors are chosen for a low ESR
of 0.018Ω to minimize output voltage changes due to
inductor ripple current and load steps. The ripple voltage
will be only:
V
OUT(RIPPLE)
= (5A)
1
250kHz • 330μF
+
0.018
1 0.5
= 0.25V (about 1%)
A 0A to 5A load step will cause an output change of up to:
ΔV
OUT(STEP)
= ΔI
LOAD
• ESR = 5A • 0.018Ω
= 90mV
An optional 10μF ceramic output capacitor is included
to minimize the effect of ESL in the output ripple. The
complete circuit is shown in Figure 14.
APPLICATIONS INFORMATION
Figure 14. 12V Input Voltage to 24V/5A
PGOOD
SYNC
PGOOD
V
RNG
I
TH
SGND
U
VIN
SHDN
V
FB
SGND PGND
PGND
SS
PLL/LPF
C
OFF
100pF
R
OFF
403k
C
SS
1000pF
V
IN
12V
V
OUT
24V
5A
C
C2
470pF
R
C
250k
R
FB2
1k
R
FB1
29.4k
LTC3813
EXTV
CC
TG
SENSE
–
BG
BGRTN
DRV
CC
INTV
CC
NDRV
BOOST
3813 F14
C
B
0.1μF
C
DRVCC
0.1μF
C
VCC
1μF
R
UV2
10k
R
UV1
115k
133k
20k
V
OUT
DB
BAS19
M2
Si7848DP
C
OUT
330μF
35V
2x
SHDN
1
4
5
6
7
8
9
10
11
12
13
14
28
27
21
20
19
18
17
16
15
C
C1
47pF
C
IN1
68μF
20V
C
IN2
1μF
20V
SW
SENSE
+
26
25
L1
5.9μH
M1
Si7848DP
D1
B1100
V
OFF
I
OFF