LTC3814-5 Datasheet, PDF (25/28 Pages) Linear Technology – 60V Current Mode Synchronous Step-Up Controller

LTC3814-5
APPLICATIONS INFORMATION
junction temperature of about 30°C above a 70°C ambient
(ρ_100°C = 1.4):
I_{IN(MAX )}
≥
190mV
1.4 • 0.009Ω
−
1
2
•
4A
= 13A
I_OUT(MAX) = I_IN(MAX) • (1-D_MAX) = 6.5A
and double-check the assumed T_J in the MOSFET:
P_TOP
=


1
1
0.5


(6.5A)²
(1.4)(0.009)
=
1.06W
T_J = 70°C + 1.06W • 20°C/W = 91°C
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
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 4.5V 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.
T_J = 70°C + 1.36W • 20°C/W = 97°C
PGOOD
V_OUT
R_OFF
403k
133k
C_OFF
100pF
20k
C_SS
1000pF
C_C2
470pF
R_FB2
1k
R_C
250k
C_C1
47pF
1 _IOFF
16
BOOST
2
3
4
5
6
LTC3814-5
V_OFF
TG
V_RNG
PGOOD
SW
I_TH
PGND
V_FB
15
14
13
12
BG
7
11
RUN/SS
INTV_CC
8
SGND
10
EXTV_CC
9
NDRV
SGND
R_FB1, 29.4k
D_B
BAS19
C_B
0.1μF
C_DRVCC
0.1μF
C_VCC
1μF
L1
5.9μH
C_IN1
68μF
20V
V_IN
C_IN2 ^12V
1μF
20V
PGND
M1
Si7848DP
M2
Si7848DP
D1
B1100
C_OUT1
330μF
35V × 2
V_OUT
24V
5A
C_OUT2
10μF
50V
PGND
38145 F14
Figure 14. 12V Input Voltage to 24V/5A
38145fb
25