LTC3813 Datasheet, PDF (29/32 Pages) Linear Technology – 100V Current Mode Synchronous Step-Up Controller

LTC3813
APPLICATIONS INFORMATION
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)
•


12V
1

3.5V
+
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 significantly 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 •
330
μF
+
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.
PGOOD
SHDN
R_UV1
115k
133k
20k
C_SS
1000pF
C_OFF
100pF
V_OUT
R_OFF
403k
LTC3813
1 _IOFF
BOOST ²⁸
4
5
V_OFF
V_RNG
6 _PGOOD
7 _SYNC
8 _ITH
9
10
V_FB
PLL/LPF
TG ²⁷
SW ²⁶
SENSE^{+ 25}
SENSE^{– 21}
BGRTN ²⁰
11 _SS
12
13
SGND
SHDN
14 _UVIN
19
BG
18
DRV_CC
17
INTV_CC
EXTV_CC
16
15
NDRV
R_UV2
10k
C_C2
470pF
R_FB2
R_C
250k
1k
C_C1
47pF
SGND
R_FB1
29.4k
DB
BAS19
C_B
0.1μF
C_DRVCC
0.1μF
C_VCC
1μF
Figure 14. 12V Input Voltage to 24V/5A
C_IN1
68μF
20V
L1
5.9μH
V_IN
12V
C_IN2
1μF
20V
PGND
D1
B1100
M2
Si7848DP
M1
Si7848DP
V_OUT
24V
5A
C_OUT
330μF
35V
2x
PGND
3813 F14
3813fb
29