Datasheet

LTC3785
12
3785fc
applicaTions inForMaTion
INDUCTOR SELECTION
The high frequency operation of the LTC3785 allows the
use of small surface mount inductors. The inductor cur-
rent ripple is typically set 20% to 40% of the maximum
inductor current. For a given ripple the inductance terms
are given as follows:
L >
V
IN(MIN)
2
• V
OUT
– V
IN(MIN)
( )
•100
f •I
OUT(MAX)
• %Ripple • V
OUT
2
, (Boost Mode)
L >
V
OUT
• V
IN(MAX)
– V
OUT
( )
•100
f •I
OUT(MAX)
• %Ripple • V
IN(MAX)
, (Buck Mode)
where:
f = Operating frequency, Hz
%Ripple = Allowable inductor current ripple, %
V
IN(MIN)
= Minimum input voltage (limit to V
OUT
/2
minimum for worst-case), V
V
IN(MAX)
= Maximum input voltage, V
V
OUT
= Output voltage, V
I
OUT(MAX)
= Maximum output load current, A
For high efficiency 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
2
R losses, and must be able to handle the
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
AND C
OUT
SELECTION
In boost mode, input current is continuous. In buck mode,
input current is discontinuous. In buck mode, the selection
of input capacitor, C
IN
, is driven by the need to filter the
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:
I
RMS
~I
OUT(MAX)
•
V
OUT
V
IN
• 1–
V
OUT
V
IN






This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
=
I
OUT(MAX)
/2. This simple worst-case condition is commonly
used for design because even significant 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
OUT
must be capable of reducing
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:
V
RIPPLE _ BOOST
=
I
OUT(MAX)
• V
OUT
– V
IN(MIN)
( )
C
OUT
• V
OUT
• f
V
RIPPLE _ BUCK
=
V
OUT
• V
IN(MAX)
– V
OUT
( )
8 •L • C
OUT
• V
IN(MAX)
• f
2
where C
OUT
= output filter capacitor, F
The steady ripple due to the voltage drop across the ESR
is given by:
 DV
BOOST,ESR
= I
L(MAX,BOOST)
• ESR
DV
BUCK,ESR
=
V
IN(MAX)
– V
OUT
( )
• V
OUT
L • f • V
IN
•ESR
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 coefficient.
Capacitors are now available with low ESR and high ripple
current ratings such as OS-CON and POSCAP.
P
OWER
N-CHANNEL MOSFET SELECTION AND
EFFICIENCY CONSIDERATIONS
The LTC3785 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
(switches B and C shown in Figure 1). Important param-