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LT3502/LT3502A

3502fc

Table 2

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)794-9714

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

APPLICATIONS INFORMATION

For small size, the output capacitor can be chosen

according to:

COUT =

VOUT

where COUT is in μF. However, using an output capacitor

this small results in an increased loop crossover frequency

and increased sensitivity to noise.

High performance electrolytic capacitors can be used for

the output capacitor. Low ESR is important, so choose

one that is intended for use in switching regulators. The

ESR should be speciﬁed by the supplier and should be

0.1Ω or less. Such a capacitor will be larger than a ceramic

capacitor and will have a larger capacitance, because the

capacitor must be large to achieve low ESR. Table 2 lists

several capacitor vendors.

Figure 4 shows the transient response of the LT3502A with

several output capacitor choices. The output is 3.3V. The

load current is stepped from 150mA to 400mA and back to

150mA,andtheoscilloscopetracesshowtheoutputvoltage.

The upper photo shows the recommended value. The sec-

ond photo shows the improved response (less voltage drop)

resulting from a larger output capacitor and a phase lead

capacitor. The last photo shows the response to a high

performance electrolytic capacitor. Transient performance

is improved due to the large output capacitance.

BOOST Pin Considerations

Capacitor C3 and the internal boost diode are used to

generate a boost voltage that is higher than the input

voltage. In most cases a 0.1μF capacitor will work well.

Figure 5 shows two ways to arrange the boost circuit. The

BOOST pin must be at least 2.2V above the SW pin for

best efﬁciency. For outputs of 3V and above, the standard

circuit (Figure 5a) is best. For outputs less than 3V and

above 2.5V, place a discrete Schottky diode (such as the

BAT54) in parallel with the internal diode to reduce VD. The

following equations can be used to calculate and minimize

boost capacitance in μF:

0.012/(VBD + VCATCH – VD – 2.2) for the LT3502A

0.030/(VBD + VCATCH – VD– 2.2) for the LT3502

VD is the forward drop of the boost diode, and VCATCH is

the forward drop of the catch diode (D1).

For lower output voltages the BD pin can be tied to an

external voltage source with adequate local bypassing

(Figure 5b). The above equations still apply for calculating

the optimal boost capacitor for the chosen BD voltage.

The absence of BD voltage during startup will increase

minimum voltage to start and reduce efﬁciency. You must

also be sure that the maximum voltage rating of BOOST

pin is not exceeded.

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