LT3513

14

3513fa

OUTPUT CAPACITOR SELECTION

For 5V and 3.3V outputs, a 10μF 6.3V ceramic capacitor

(X5R or X7R) at the output results in very low output volt-

age ripple and good transient response. Other types and

values will also work; the following discussion explores

tradeoffs in output ripple and transient performance.

The output capacitor filters the inductor current to generate

an output with low voltage ripple. It also stores energy in

order satisfy transient loads and stabilizes the LT3513’s

control loop. Because the LT3513 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.

You can estimate output ripple with the following

equations:

V

I

fC

RIPPLE

L

OUT

=

Δ

8 ••

for ceramic capacitors, and

and aluminum)

where

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:

I

I

CRMS

L

Δ

12

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:

CL

I

V

OUT

LIM

OUT

>

⎛

⎝⎜

⎞

⎠⎟

10

2

••

The low ESR and small size of ceramic capacitors make

them the preferred type for LT3513 applications. However,

not all ceramic capacitors are the same. 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 volt-

age and at temperature extremes.

Because loop stability and transient response depend on

and X5R types. Electrolytic capacitors are also an option.

The ESRs of most aluminum electrolytic capacitors are

too large to deliver low output ripple. Tantalum and newer,

lower ESR organic electrolytic capacitors intended for

power supply use are suitable, and the manufacturers will

specify the ESR. Chose a capacitor with a low enough 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 2 lists several capacitor

vendors.

Table 2. Low ESR Surface Mount Capacitors

VENDOR

TYPE

SERIES

Taiyo Yuden

Ceramic

X5R, X7R

AVX

Ceramic

Tantalum

X5R, X7R

TPS

Kemet

Tantalum

Ta Organic

Al Organic

T491, T494, T495

T520

A700

Sanyo

Ta or Al Organic

POSCAP

Panasonic

Al Organic

SP CAP

TDK

Ceramic

X5R, X7R

DIODE SELECTION

The catch diode (D1 from Figure 1) conducts current only

during switch off time. Average forward current in normal

operation can be calculated from:

II

VV

V

D AVG

OUT

IN

OUT

IN

()

–

=

The only reason to consider a diode with a larger current

rating than necessary for nominal operation is for the

worst-case condition of shorted output. The diode current

will then increase to the typical peak switch current. Peak

reverse voltage is equal to the regulator input voltage.

OPERATION