LTC3785-1 Linear Technology, LTC3785-1 Datasheet - Page 16

• Manufacturer Part Number: LTC3785-1
• Description: Buck-Boost Controller
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
LTC3785-1
The maximum power dissipation of switch A and C oc-
curs in boost mode. Assuming a junction temperature
of T
V
Considerations section:
The maximum power dissipation of switch B and D occurs
in buck mode and is given by:
Now to double check the T
ambient. Since this is a dual NMOS package we can add
switches A + B and C + D worst case. For applications
where the MOSFETs are in separate packages each device’s
maximum T
Set The Maximum Current Limit
The equation for setting the maximum current limit of the
IC is given by:
16
IN
T
T
PA BOOST
P P C BOOST
PB BUCK
PD BOOS
R
J
J(PKG1)
J(PKG2)
= 2.7, and using the equations from the Effi ciency
ILSET
= 100°C with ρ
(
(
(
(
=
= T
= 50 + 60 • (0.43 + 0.20) = 88°C
= T
= 50 + 60 • (0.09 + 0.10) = 60°C
J
R
)
T T
would have to be calculated.
)
=
DS ON A
)
)
A
A
=
=
=
=
+ θ
+ θ
10 3 3
(
0 09
⎛
⎝ ⎜
+
(
3 3
10
6000
3 3 2 7
.
10
3 3
2 7
.
– .
JA
JA
)
1 1 3 3
. – .
100C
.
.
(PA + PB)
(PC + PD)
• .
•
W
•
3
•
I
2 7
LIMIT
2
3
.
•
= 1.3, the power dissipation at
⎞
⎠ ⎟
3
• . • .
3
2
2
1 3 0 025 0 10
2
)
•
J
• . • .
• .
2 7
Ω
• . • .
of the package with 50°C
3
3 3
.
1 3 0 025 0 43
1 3 0 025 0 20
• .
•
0 45 9 500 10
3
2
• . • .
=
– •
1 3 0 025
=
.
=
.
W
.
W
W
•
3
The maximum current is set 25% above I
for worst-case variation at 100°C = 6A.
Choose the Input and Output Capacitance
The input capacitance should fi lter current ripple which is
worst case in buck mode. Since the input current could
reach 6A, a capacitor ESR of 10mΩ or less will yield an
input ripple of 60mV.
The output capacitance should fi lter current ripple which
is worst in boost mode, but is usually dictated by the loop
response, the maximum load transient and the allowable
transient response.
PC BOARD LAYOUT CHECKLIST
The basic PC board layout requires a dedicated ground
plane layer. Also, for high current, a multilayer board
provides heat sinking for power components.
• The ground plane layer should not have any traces and
• Place C
• Use immediate vias to connect the components (includ-
• Use planes for V
• Flood all unused areas on all layers with copper. Flooding
it should be as close as possible to the layer with power
MOSFETs.
area. Place C
compact area.
ing the LTC3785-1’s GND/PGND pin) to the ground plane.
Use several large vias for each power component.
fi ltering and to keep power losses low.
with copper will reduce the temperature rise of power
components. Connect the copper areas to any DC net
(V
the following checklist should be used to ensure proper
operation of the LTC3785-1.
R
ILSET
IN
or GND). When laying out the printed circuit board,
=
IN
, switch A, switch B and D1 in one compact
0 025 6
.
6000
OUT
IN
•
, switch C, switch D and D2 in one
and V
=
42
k
OUT
to maintain good voltage
L(PEAK)
to account
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