LTC3780EG Linear Technology, LTC3780EG Datasheet - Page 23

• Manufacturer Part Number: LTC3780EG
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,CMOS,SSOP,24PIN,PLASTIC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Tie the PLLFLTR pin to INTV
inductance value is chosen first based on a 30% ripple
current assumption. In Buck mode, the ripple current is:
The highest value of ripple current occurs at the maximum
input voltage. In Boost mode, the ripple current is:
The highest value of ripple current occurs at V
A 6.8µH inductor will produce 13% ripple in Boost mode
(V
The R
maximum current sense voltage specification with some
accommodation for tolerances.
Select an R
Output voltage is 12V. Select R1 as 20k. R2 is:
Select R2 as 280k. Both R1 and R2 should have a tolerance
of no more than 1%.
Next, choose the MOSFET switches. A suitable choice is
the Siliconix Si4840 (R
= 150pF, θ
The maximum power dissipation of Switch A occurs
in Boost mode when Switch A stays on all the time.
Assuming a junction temperature of T
ρ
150°C
IN
R
R
P
∆
∆
A BOOST
= 6V) and 29% ripple in Buck mode (V
SENSE
2
I
I
L BUCK
L BOOST
,
SENSE
,
,
=
= 1.5, the power dissipation at V
V
OUT
JA
=
SENSE
0 8
resistor value can be calculated by using the
=
=
.
= 40°C/W).
(
=
2
•
⎛
⎜
⎝
V
f L
R
OUT
f L
•
12
V
•
5
•
I
1
IN
of 10mΩ.
OUT MAX BOOST
–
U
•
•
R
•
5
(
⎛
⎜
⎝
⎞
⎟
⎠
⎛
⎜
⎝
DS(ON)
1
– 1
– 1
2
• . • .
U
2 160
,
1 5 0 009 1 94
V
V
•
V
CC
V
OUT
OUT
= 0.009Ω (at V
IN
IN
for 400kHz operation. The
⎞
⎟
⎠
⎞
⎟
⎠
)
mV V
+ ∆
W
•
I
L BOOST
=
,
IN
IN
J
.
= 5V is:
= 150°C with
GS
IN
W
IN
= 6V), C
= 18V).
U
)
= V
•
V
OUT
OUT
RSS
/2.
Double-check the T
temperature:
The maximum power dissipation of Switch B occurs in
Buck mode. Assuming a junction temperature of T
with ρ
Double-check the T
temperature:
The maximum power dissipation of Switch C occurs in
Boost mode. Assuming a junction temperature of T
with ρ
Double-check the T
temperature:
The maximum power dissipation of Switch D occurs in
Boost mode when its duty cycle is higher than 50%.
Assuming a junction temperature of T
ρ
Double-check the T
temperature:
C
this mode, the maximum input current peak is:
100°C
IN
T
T
T
T
I
P
P
P
IN,PEAK(MAX,BUCK)
is chosen to filter the square current in Buck mode. In
J
J
J
J
C BOOST
B BUCK
D BUCK
,
,
,
= 70°C + 1.94W • 40°C/W = 147.6°C
= 70°C + 0.135W • 40°C/W = 75.4°C
= 70°C + 1.08W • 40°C/W = 113°C
= 70°C + 0.73W • 40°C/W = 99°C
80°C
110°C
= 1.35, the power dissipation at V
= 1.2, the power dissipation at V
=
=
= 1.4, the power dissipation at V
=
18 12
12
+
5
(
12 5 12
12
2 12
–
•
•
⎛
⎜
⎝
–
5
12
J
J
J
5
J
2
= 5 • (1 + 29%) = 6.5A
in the MOSFET with 70°C ambient
•
)
3
in the MOSFET at 70°C ambient
in the MOSFET at 70°C ambient
in the MOSFET at 70°C ambient
•
5
•
• •
2
5
5
5
⎞
⎟
⎠
• . • .
2
1 2 0 009 135
•
150
• .
5
1 35 0 009 0 73
2
• . • .
p
1 4 0 009
•
• .
400
=
k
J
IN
LTC3780
=
= 100°C with
= 5V is:
IN
=
1 08
IN
.
mW
= 18V is:
.
= 5V is:
J
W
J
= 110°C
23
= 80°C
W
3780f