LTC3453 datasheet(13/20 Pages) LINER

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LT3474/LT3474-1

3474fd

APPLICATIONS INFORMATION

You can estimate output ripple with the following

equation:

RIPPLE =

ΔIL

8• f •C

OUT

() for ceramic capacitors

where ΔIL is the peak-to-peak ripple current in the inductor.

The RMS content of this ripple is very low so the RMS

current rating of the output capacitor is usually not of

concern. It can be estimated with the formula:

CRMS

() =

ΔIL

The low ESR and small size of ceramic capacitors make

them the preferred type for LT3474 applications. Not all

ceramic capacitors are the same, however. Many of the

higher value capacitors use poor dielectrics with high

temperature and voltage coefﬁcients. In particular, Y5V

and Z5U types lose a large fraction of their capacitance

with applied voltage and at temperature extremes.

Because loop stability and transient response depend on

the value of COUT, this loss may be unacceptable. Use X7R

and X5R types. Table 3 lists several capacitor vendors.

Table 3. Low-ESR Surface Mount Capacitors

VENDOR

TYPE

SERIES

Taiyo-Yuden

Ceramic

X5R, X7R

AVX

Ceramic

X5R, X7R

TDK

Ceramic

X5R, X7R

A ﬁnal caution is in order regarding the use of ceramic

capacitors at the input. A ceramic input capacitor can

combine with stray inductance to form a resonant tank

circuit. If power is applied quickly (for example by plugging

the circuit into a live power source), this tank can ring,

doubling the input voltage and damaging the LT3474. The

solution is to either clamp the input voltage or dampen the

tank circuit by adding a lossy capacitor in parallel with the

ceramic capacitor. For details, see Application Note 88.

Output Capacitor Selection

For most LEDs, a 2.2μF 6.3V ceramic capacitor (X5R or

X7R) at the output results in very low output voltage ripple

and good transient response. Other types and values will

also work; the following discusses tradeoffs in output

ripple and transient performance.

The output capacitor ﬁlters the inductor current to generate

an output with low voltage ripple. It also stores energy in

order to satisfy transient loads and stabilizes the LT3474’s

control loop. Because the LT3474 operates at a high

frequency, minimal output capacitance is necessary. In

addition, the control loop operates well with or without

the presence of output capacitor series resistance (ESR).

Ceramic capacitors, which achieve very low output ripple

and small circuit size, are therefore an option.

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