Datasheet

LT3844
20
3844fb
APPLICATIONS INFORMATION
The choice of output capacitor is also driven by the RMS
ripple current requirement. The RMS ripple current is:
II
VV
V
RMS COUT OUT MAX
OUT IN MIN
IN MIN
() ()
()
()
•=
−
At lower output voltages (<30V) it may be possible to sat-
isfy both the output ripple voltage and RMS requirements
with one or more capacitors of a single type. However, at
output voltages above 30V where capacitors with both low
ESR and high bulk capacitance are hard to fi nd, the best
approach is to use a combination of aluminum electrolytic
and ceramic capacitors. The low ESR ceramic capacitor
will minimize the ESR while the Aluminum Electrolytic
capacitor will supply the required bulk capacitance.
Boost Converter: Input Capacitor Selection
The input capacitor of a boost converter is less critical
than the output capacitor, due to the fact that the inductor
is in series with the input and the input current waveform
is continuous. The input voltage source impedance de-
termines the size of the input capacitor, which is typically
in the range of 10μF to 100μF. A low ESR capacitor is
recommended though not as critical as with the output
capacitor. The RMS input capacitor ripple current for a
boost converter is:
I
V
Lf
DC
RMS CIN
IN MIN
SW
MAX()
()
.•
•
•= 03
Please note that the input capacitor can see a very high
surge current when a battery is suddenly connected to
the input of the converter and solid tantalum capacitors
can fail catastrophically under these conditions. Be sure
to specify surge-tested capacitors.
Boost Converter: R
SENSE
Selection
The boost application in the Typical Applications section
has the location of the current sense resistor in series with
the inductor with one side referenced to V
IN
. This location
was chosen for two reasons. Firstly, the circulating current
is always monitored so in the case of an output overvoltage
or input overcurrent condition the main switch will skip
cycles to protect the circuitry. Secondly, the V
IN
node can
be considered low noise since it is heavily fi ltered and the
input current is not pulsed but continuous.
In the case where the input voltage exceeds the voltage
limits on the LT3844 Sense pins, the sense resistor can
be moved to the source of the MOSFET. In both cases the
resistor value is the calculated using the same formula.
The LT3844 current comparator has a maximum threshold
of 100mV/R
SENSE
. The current comparator threshold sets
the peak of the inductor current. Allowing adequate margin
for ripple current and external component tolerances,
R
SENSE
can be calculated as follows:
R
mV
I
SENSE
LMAX
=
70
()
Where I
L(MAX)
is the maximum average inductor current
as calculated in the Boost Converter: Inductor Selection
section.