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LT3470A

3470afa

APPLICATIONS INFORMATION

Ceramic Capacitors

Ceramic capacitors are small, robust and have very low

ESR. However, ceramic capacitors can cause problems

when used with the LT3470A. 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 LT3470A’s

switching frequency depends on the load current, and at

light loads the LT3470A can excite the ceramic capacitor

at audio frequencies, generating audible noise. Since the

LT3470A operates at a lower current limit during Burst

Mode operation, the noise is typically very quiet to a ca-

sual 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 LT3470A. A ceramic

input capacitor combined with trace or cable inductance

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

LT3470A circuit is plugged into a live supply, the input volt-

age can ring to twice its nominal value, possibly exceeding

the LT3470A’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

Figure 2. Two Circuits for Generating the Boost Voltage

Table 2. Capacitor Vendors

Vendor

Phone

URL

Part Series

Comments

Panasonic

(714) 373-7366

www.panasonic.com

Ceramic,

Polymer,

Tantalum

EEF Series

Kemet

(864) 963-6300

www.kemet.com

Ceramic,

Tantalum

T494, T495

Sanyo

(408) 749-9714

www.sanyovideo.com

Ceramic,

Polymer,

Tantalum

POSCAP

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

VIN

BOOST

LT3470A

(2a)

(2b)

0.22μF

VIN

VOUT

VBOOST – VSW ≅ VOUT

MAX VBOOST ≅ VIN + VOUT

BIAS

GND

VIN

BOOST

LT3470A

BIAS

0.22μF

VIN

VOUT

3470a F02

VBOOST – VSW ≅ VIN

MAX VBOOST ≅ 2•VIN

GND

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