LTC3414 datasheet(9/16 Pages) LINER

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LTC3414

3414f

CIN and COUT Selection

The input capacitance, CIN, is needed to filter the trapezoi-

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:

RMS

OUT MAX

OUT

()

–1

This formula has a maximum at VIN = 2VOUT, where

IRMS = IOUT/2. This simple worst-case condition is com-

monly 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 temperature than required. Several capacitors may

also be paralleled to meet size or height requirements in

the design. For low input voltage applications, sufficient

bulk input capacitance is needed to minimize transient

effects during output load changes.

The selection of COUT is determined by the effective series

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,

∆VOUT, is determined by:

∆≤ ∆









I ESR

OUT

The output ripple is highest at maximum input voltage

since

∆IL increases with input voltage. Multiple capacitors

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 provided that consideration is given to ripple

current ratings and long term reliability. Ceramic capaci-

tors have excellent 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,

VIN. At best, this ringing can couple to the output and be

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

current through the long wires can potentially cause a

voltage spike at VIN large enough to damage the part.

When choosing the input and output ceramic capacitors,

choose the X5R or X7R dielectric formulations. These

dielectrics have the best temperature and voltage charac-

teristics of all the ceramics for a given value and size.

Output Voltage Programming

The output voltage is set by an external resistive divider

according to the following equation:

OUT =+









The resistive divider allows pin VFB to sense a fraction of

the output voltage as shown in Figure 2.

APPLICATIO S I FOR ATIO

3414 F02

LTC3414

VFB

SGND

VOUT

Figure 2. Setting the Output Voltage

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