LTC3785_15 Datasheet, PDF (16/20 Pages) Linear Technology – 10V, High Efficiency, Synchronous, No RSENSE Buck-Boost Controller

LTC3785
applications information
Determine the Proper Inductor Type Selection
The highest inductor current is during boost mode and
is given by:
I_L(MAX
_
AV)
=
V_OUT •I_OUT
V_IN • η
where η = estimated efficiency in this mode (use 80%).
I_L(MAX
_
AV)
=
3.3 • 3
2.7 • 0.8
=
4.6A
To limit the maximum efficiency loss of the inductor ESR
to below 5% the equation is:
ESR_{L(MAX )}
~
V_OUT •I_OUT • %Loss
I_L(MAX
_
AV
2
)
•
100
=
24mΩ
A suitable inductor for this application could be a Coiltron-
ics CD1-3R8 which has a rating DC current of 6A and ESR
of 13mΩ.
Choose a Proper MOSFET Switch
Using the same guidelines for ESR of the inductor, one
suitable MOSFET could be the Siliconix Si7940DP which
is a dual MOSFET in a surface mount package with 25mΩ
at 2.5V and a total gate charge of 12nC.
Checking the power dissipation of each switch will ensure
reliable operation since the thermal resistance of the
package is 60°C/W.
The maximum power dissipation of switch A and C oc-
curs in boost mode. Assuming a junction temperature
of T_J = 100°C with ρ_100C = 1.3, the power dissipation at
V_IN = 2.7, and using the equations from the Efficiency
Considerations section:
PA(BOOST) =


3.3
2.7
• 3^{ 2}
•
1.3 • 0.025 = 0.43W
PC(BOOST)
=
(3.3
– 2.7)
2.7²
•
3.3
•
3²
•
1.3
•
0.025
+
1•
3.3³
•
3
2.7
• 0.45 –
9
•
500 • 10³
= 0.09W
The maximum power dissipation of switch B and D occurs
in buck mode and is given by:
PB(BUCK) = ^{10 – 3.3} • 3² • 1.3 • 0.025 = 0.20W
10
PD(BOOST)
=
3.3
10
•
3²
•
1.3
•
0.025
=
0.10W
Now to double check the T_J of the package with 50°C
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_J would have to be calculated.
T_J(PKG1) = T_A + q_JA(PA + PB)

= 50 + 60 • (0.43 + 0.20) = 88°C
T_J(PKG2) = T_A + q_JA(PC + PD)

= 50 + 60 • (0.09 + 0.10) = 60°C
Set The Maximum Current Limit
The equation for setting the maximum current limit of the
IC is given by:
R_ILSET
=
6000
R_DS(ON)A •I_LIMIT
Ω
The maximum current is set 25% above I_L(PEAK) to account
for worst-case variation at 100°C = 6A.
R_ILSET
=
6000
0.025 •
6
=
42k
Choose the Input and Output Capacitance
The input capacitance should filter current ripple which is
worst-case in buck mode. Since the input current could
reach 6A, a capacitor ESR of 10mW or less will yield an
input ripple of 60mV.
The output capacitance should filter 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.
3785fc
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