LT3694 datasheet(13/28 Pages) LINER

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LT3694/LT3694-1

36941fa

quality (under damped) tank circuit. If the LT3694 circuit

is plugged into a live supply, the input voltage can ring to

twice its nominal value, possibly exceeding the LT3694’s

maximum input voltage rating. See Linear Technology

Application Note 88 for more details.

Output Capacitor Selection

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 stabilize the LT3694’s

control loop. Because the LT3694 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.

Output ripple can be estimated with the following

equations:

RIPPLE =

ΔI

8• f • C

OUT

; Ceramic

RIPPLE =ΔIL • ESR

; Electrolytic

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:

C(RMS) =

ΔI

Another constraint on the output capacitor is that it must

have greater energy storage than the inductor; if the stored

energy in the inductor transfers to the output, the resulting

voltage step should be small compared to the regulation

voltage. For a 5% overshoot, this requirement indicates:

OUT > 10 • L •

LIM

OUT

⎛

⎝⎜

⎞

⎠⎟

The low ESR and small size of ceramic capacitors make

them the preferred type for LT3694 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 instead.

Electrolytic capacitors are also an option. The ESRs of

most aluminum electrolytic capacitors are too large to

deliver low output ripple. Surge rated tantalum capacitors

or low ESR, organic, electrolytic capacitors intended for

power supply use are suitable. Choose a capacitor with a

sufficiently low ESR for the required output ripple. Because

the volume of the capacitor determines its ESR, both the

size and the value will be larger than a ceramic capacitor

that would give similar ripple performance. One benefit

is that the larger capacitance may give better transient

response for large changes in load current. Table 3 lists

several capacitor vendors.

Table 3. Low ESR Surface Mount Capacitors

SERIES

TYPE

MANUFACTURER

Ceramic

Taiyo Yuden

www.t-yuden.com

TPM, TPS

Ceramic, Tantalum

AVX

www.avx.com

T494, T495,

T510, T520,

T525, T530,

A700

Ceramic, Tantalum,

Tantalum Organic Polymer,

Aluminum Organic Polymer

Kemet

www.kemet.com

POSCAP,

OS-CON

Tantalum Organic Polymer,

Aluminum Organic Polymer

Sanyo

www.sanyo.com

SP-CAP

Ceramic,

Aluminum Organic Polymer

Panasonic

www.panasonic.com

Ceramic

TDK

www.tdk.com

APPLICATIONS INFORMATION

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