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LT3844

3844fb

APPLICATIONS INFORMATION

The choice of output capacitor is also driven by the RMS

ripple current requirement. The RMS ripple current is:

RMS COUT

OUT MAX

OUT

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 ﬁnd, 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:

RMS CIN

IN MIN

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: RSENSE 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 VIN. 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 VIN node can

be considered low noise since it is heavily ﬁltered 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/RSENSE. The current comparator threshold sets

the peak of the inductor current. Allowing adequate margin

for ripple current and external component tolerances,

RSENSE can be calculated as follows:

SENSE

LMAX

()

Where IL(MAX) is the maximum average inductor current

as calculated in the Boost Converter: Inductor Selection

section.

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