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LTC3413

3413fc

APPLICATIONS INFORMATION

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, ΔVOUT, is determined by:

Δ≤ Δ

⎛

⎝⎜

⎞

⎠⎟

I ESR

OUT

The output ripple is highest at maximum input voltage

since

ΔIL increases with input voltage. Multiple capaci-

tors placed in parallel may be needed to meet the ESR

and RMS current handling requirements. Dry tantalum,

special polymer, aluminum electrolytic and ceramic capaci-

tors 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,

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

In most applications, VOUT is connected directly to VFB.

The output voltage will be equal to one-half of the volt-

age on the VREF pin for this case.

OUT

REF

If a different output voltage relationship is desired, an

external resistor divider from VOUT to VFB can be used.

The output voltage will then be set according to the fol-

lowing equation:

OUT

REF

⎛

⎝⎜

⎞

⎠⎟

Figure 2. Setting the Output Voltage

VOUT

3413 F02

VFB

SGND

LTC3413

Soft-Start

The RUN/SS pin provides a means to shut down the

LTC3413 as well as a timer for soft-start. Pulling the

RUN/SS pin below 0.5V places the LTC3413 in a low

quiescent current shutdown state (IQ < 1μA).

The LTC3413 contains an internal soft-start clamp that

gradually raises the clamp on ITH after the RUN/SS pin is

pulled above 2V. The full current range becomes available

on ITH after 1024 switching cycles. If a longer soft-start

period is desired, the clamp on ITH can be set externally

with a resistor and capacitor on the RUN/SS pin as shown

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