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LTC3413

sn3413 3413fs

APPLICATIO S I FOR ATIO

The output ripple is highest at maximum input voltage

since

placed in parallel may be needed to meet the ESR and RMS

current handling requirements. Dry tantalum, special poly-

mer, aluminum electrolytic and ceramic capacitors are all

available in surface mount packages. Special polymer

capacitors offer very low ESR but have lower capacitance

density than other types. Tantalum capacitors have the

highest capacitance density but it is important to only use

types that have been surge tested for use in switching

power supplies.

Aluminum electrolytic capacitors have significantly higher

ESR, but can be used in cost-sensitive applications pro-

vided that consideration is given to ripple current ratings

and long term reliability. Ceramic capacitors have excel-

lent low ESR characteristics but can have a high voltage

coefficient and audible piezoelectric effects. The high Q of

ceramic capacitors with trace inductance can also lead to

significant ringing.

Using Ceramic Input and Output Capacitors

Higher values, lower cost ceramic capacitors are now

becoming available in smaller case sizes. Their high ripple

current, high voltage rating and low ESR make them ideal

for switching regulator applications. However, care must

be taken when these capacitors are used at the input and

output. When a ceramic capacitor is used at the input and

the power is supplied by a wall adapter through long wires,

a load step at the output can induce ringing at the input,

mistaken as loop instability. At worst, a sudden inrush of

current through the long wires can potentially cause a

When choosing the input and output ceramic capacitors,

choose the X5R or X7R dielectric formulations. These

Table 1 shows some recommended surface mount

inductors for LTC3413 applications.

Table 1. Recommended Surface Mount Inductors

Value

DCR

Manufacturer

Part Number

(

µH)

(m

Ω)

Murata

LQH55DNR47M01

0.47

13.0

Vishay/Dale

IHLP2525CZPJR47M01

0.47

4.2

Pulse

P1166.681T

0.44

6.0

Cooper

SD20-R47

0.47

20.0

dal wave current at the source of the top MOSFET. To

prevent large voltage transients from occurring, a low ESR

input capacitor sized for the maximum RMS current

should be used. The maximum RMS current is given by:

II

V

V

V

V

RMS

OUT MAX

OUT

IN

IN

OUT

=

()

–1

used for design because even significant deviations do not

offer much relief. Note that ripple current ratings from

capacitor manufacturers are often based on only 2000

hours of life which makes it advisable to further derate the

capacitor, or choose a capacitor rated at a higher tempera-

ture than required. Several capacitors may also be paral-

leled to meet size or height requirements in the design.

resistance (ESR) that is required to minimize voltage

ripple and load step transients as well as the amount of

bulk capacitance that is necessary to ensure that the

control loop is stable. Loop stability can be checked by

viewing the load transient response as described in a later

section. The output ripple,

∆≤ ∆

+









V

I ESR

fC

OUT

L

OUT

1

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