10

LT3470

3470f

Ceramic Capacitors

Ceramic capacitors are small, robust and have very low

ESR. However, ceramic capacitors can cause problems

when used with the LT3470. Not all ceramic capacitors are

suitable. X5R and X7R types are stable over temperature

and applied voltage and give dependable service. Other

types, including Y5V and Z5U have very large temperature

and voltage coefficients of capacitance. In an application

circuit they may have only a small fraction of their nominal

capacitance resulting in much higher output voltage ripple

than expected.

Ceramic capacitors are piezoelectric. The LT3470’s switch-

ing frequency depends on the load current, and at light

loads the LT3470 can excite the ceramic capacitor at audio

frequencies, generating audible noise. Since the LT3470

operates at a lower current limit during BurstMode opera-

tion, the noise is typically very quiet to a casual ear. If this

audible noise is unacceptable, use a high performance

electrolytic capacitor at the output. The input capacitor can

be a parallel combination of a 2.2µF ceramic capacitor and

a low cost electrolytic capacitor.

A final precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LT3470. A ce-

ramic input capacitor combined with trace or cable induc-

tance forms a high quality (under damped) tank circuit. If

the LT3470 circuit is plugged into a live supply, the input

voltage can ring to twice its nominal value, possibly

exceeding the LT3470’s rating. This situation is easily

avoided; see the Hot Plugging Safely section.

BOOST and BIAS Pin Considerations

Capacitor C3 and the internal boost Schottky diode (see

Block Diagram) are used to generate a boost voltage that

is higher than the input voltage. In most cases a 0.22µF

capacitor will work well. Figure 2 shows two ways to

arrange the boost circuit. The BOOST pin must be more

than 2.5V above the SW pin for best efficiency. For outputs

of 3.3V and above, the standard circuit (Figure 2a) is best.

For outputs between 2.5V and 3V, use a 0.47µF. For lower

output voltages the boost diode can be tied to the input

Table 3. Capacitor Vendors

Vendor

Phone

URL

Part Series

Comments

Panasonic

(714) 373-7366

www.panasonic.com

Ceramic,

Polymer,

EEF Series

Tantalum

Kemet

(864) 963-6300

www.kemet.com

Ceramic,

Tantalum

T494, T495

Sanyo

(408) 749-9714

www.sanyovideo.com

Ceramic,

Polymer,

POSCAP

Tantalum

Murata

(404) 436-1300

www.murata.com

Ceramic

AVX

www.avxcorp.com

Ceramic,

Tantalum

TPS Series

Taiyo Yuden

(864) 963-6300

www.taiyo-yuden.com

Ceramic

BOOST

LT3470

(2a)

(2b)

SW

36

5

7

4

C3

0.22µF

BIAS

GND

BOOST

LT3470

SW

BIAS

36

5

7

4

C3

0.22µF

3470 F02

GND

Figure 2. Two Circuits for Generating the Boost Voltage

APPLICATIO S I FOR ATIO