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LT3470I

Micropower Buck Regulator with Integrated Boost and Catch Diodes

Linear Technology 

LT3470

applications information

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 Burst Mode 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 ceramic

input capacitor combined with trace or cable inductance

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

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

age 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 ar-

range 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

VIN

C3

VIN BOOST

0.22µF

LT3470

SW

VOUT

BIAS

GND

VBOOST – VSW ≅ VOUT

MAX VBOOST ≅ VIN + VOUT

(2a)

VIN

C3

VIN BOOST

0.22µF

LT3470

BIAS

SW

VOUT

GND

VBOOST – VSW ≅ VIN

MAX VBOOST ≅ 2VIN

(2b)

3470 F02

Figure 2. Two Circuits for Generating the Boost Voltage

Table 3. Capacitor Vendors

VENDOR

PHONE

Panasonic

(714) 373-7366

Kemet

Sanyo

(864) 963-6300

(408) 749-9714

Murata

AVX

(404) 436-1300

Taiyo Yuden (864) 963-6300

URL

www.panasonic.com

www.kemet.com

www.sanyovideo.com

www.murata.com

www.avxcorp.com

www.taiyo-yuden.com

PART SERIES

Ceramic,

Polymer,

Tantalum

Ceramic,

Tantalum

Ceramic,

Polymer,

Tantalum

Ceramic

Ceramic,

Tantalum

Ceramic

COMMENTS

EEF Series

T494, T495

POSCAP

TPS Series

3470fd

11

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