LTC3785-1 Linear Technology, LTC3785-1 Datasheet - Page 12

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

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APPLICATIONS INFORMATION
LTC3785-1
INDUCTOR SELECTION
The high frequency operation of the LTC3785-1 allows
the use of small surface mount inductors. The inductor
current ripple is typically set 20% to 40% of the maximum
inductor current. For a given ripple the inductance terms
are given as follows:
where:
For high effi ciency choose an inductor with a high frequency
core material, such as ferrite, to reduce core loses. The
inductor should have low ESR (equivalent series resistance)
to reduce the I
peak inductor current without saturating. Molded chokes
or chip inductors usually do not have enough core to sup-
port the peak inductor currents in the 3A to 6A region. To
minimize radiated noise, use a toroid, pot core or shielded
bobbin inductor.
C
In boost mode, input current is continuous. In buck mode,
input current is discontinuous. In buck mode, the selection
of input capacitor, C
input square wave current. Use a low ESR capacitor, sized
to handle the maximum RMS current. For buck operation,
the maximum RMS capacitor current is given by:
12
L
L
IN
f = Operating frequency, Hz
%Ripple = Allowable inductor current ripple, %
V
minimum for worst case), V
V
V
I
I
>
>
OUT(MAX)
RMS
AND C
IN(MIN)
IN(MAX)
OUT
V
f I
IN MIN
V
•
f I
OUT
•
= Output voltage, V
~
(
OUT MAX
I
OUT MAX
OUT
OUT MAX
= Minimum input voltage (limit to V
= Maximum input voltage, V
(
•
)
= Maximum output load current, A
2
(
(
(
2
SELECTION
V
•
R losses, and must be able to handle the
IN MAX
(
)
V
(
• %
OUT
)
)
•
IN
•
% %
Ripple V
, is driven by the need to fi lter the
)
–
Ripple V
– –
V
V
V
OUT
V
IN MIN
IN
OUT
(
•
•
•
)
⎛
⎝ ⎜
IN MAX
)
•
1
)
OUT
(
100
–
•
100
V
2
V
OUT
)
IN
, (
, (
B B uck Mode
⎞
⎠ ⎟
Boost Mode
OUT
)
/2
)
This formula has a maximum at V
I
used for design because even signifi cant deviations do not
offer much relief. Note that ripple current ratings from ca-
pacitor manufacturers are often based on only 2000 hours
of life which makes it advisable to derate the capacitor.
In boost mode, the discontinuous current shifts from the
input to the output, so C
the output voltage ripple. The effects of ESR (equivalent
series resistance) and the bulk capacitance must be
considered when choosing the right capacitor for a given
output ripple voltage. The steady ripple due to charging
and discharging the bulk capacitance is given by:
where C
The steady ripple due to the voltage drop across the ESR
is given by:
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic and
ceramic capacitors are all available in surface mount
packages. Ceramic capacitors have excellent low ESR
characteristics but can have a high voltage coeffi cient.
Capacitors are now available with low ESR and high ripple
current ratings such as OS-CON and POSCAP .
POWER N-CHANNEL MOSFET SELECTION AND
EFFICIENCY CONSIDERATIONS
The LTC3785-1 requires four external N-channel power
MOSFETs, two for the top switches (switches A and D,
shown in Figure 1) and two for the bottom switches
OUT(MAX)
ΔV
ΔV
V
V
RIPPLE BOOST
RIPPLE BUCK
BOOST,ESR
BUCK ESR
OUT
/2. This simple worst-case condition is commonly
,
_
_
= output fi lter capacitor, F
=
= I
(
=
L(MAX,BOOST)
V
=
IN MAX
8 L L C
V
I
(
OUT MAX
OUT
•
OUT
L f V
•
(
)
•
• •
–
(
OUT
must be capable of reducing
C
V
V
OUT
IN MAX
OUT
)
• ESR
•
(
IN
•
IN
(
V
V
• •
)
= 2V
IN MAX
OUT
•
V
)
V
OUT
(
–
OUT
OUT
–
V
•
OUT
V
)
, where I
•
f
IN MIN
•
ESR
f
(
)
2
)
)
RMS
37851f
=