LT3513

16

3513fa

Inductor Selection

Table 1 lists several inductor vendors and types that are

suitable to use with the LT3513. Consult each manufacturer

for detailed information and for their entire selection of

related parts. Use ferrite core inductors to obtain the best

efficiency, as core losses at frequencies above 1MHz are

much lower for ferrite cores than for powdered-iron units.

A 10μH to 22μH inductor will be the best choice for most

LT3513 step-up and charge pump designs. Choose an

inductor that can carry the entire switch current without

saturating. For inverting and SEPIC regulators, a coupled

inductor, or two separate inductors is an option. When

using coupled inductors, choose one that can handle

at least the switch current without saturating. If using

uncoupled inductors, each inductor need only handle ap-

proximately one-half of the total switch current. A 4.7μH

to 15μH coupled inductor or two 10μH to 22μH uncoupled

inductors will usually be the best choice for most LT3513

inverting and SEPIC designs.

Output Capacitor Selection

Use low ESR (equivalent series resistance) capacitors at

the output to minimize the output ripple voltage. Multilayer

ceramic capacitors are an excellent choice, as they have

an extremely low ESR and are available in very small pack-

ages. X7R dielectrics are preferred, followed by X5R, as

these materials retain their capacitance over wide voltage

and temperature ranges. A 10μF to 22μF output capaci-

tor is sufficient for most LT3513 applications. Even less

also work, but they will occupy more board area and will

have a higher ESR than a ceramic capacitor. Always use

a capacitor with a sufficient voltage rating.

Diode Selection

A Schottky diode is recommended for use with the

LT3513 switcher 2 and switcher 4. The Schottky diode for

switcher 3 is integrated inside the LT3513. Choose diodes

for switcher 2 and switcher 4 rated to handle an average

current greater than the load current and rated to handle

the maximum diode voltage. The average diode current in

the step-up and SEPIC is equal to the load current. Each of

the two diodes in the charge pump configurations carries

an average diode current equal to the load current. The

ground connected diode in the charge pump is integrated

into the LT3513. The maximum diode voltage in the step-

The maximum diode voltage in the SEPIC and inverting

Input Capacitor Selection

Bypass the input of the LT3513 circuit with a 4.7μF or higher

ceramic capacitor of X7R or X5R type. A lower value or

a less expensive Y5V type will work if there is additional

bypassing provided by bulk electrolytic capacitors or if the

input source impedance is low. The following paragraphs

describe the input capacitor considerations in more detail.

Step-down regulators draw current from the input sup-

ply in pulses with very fast rise and fall times. The input

capacitor is required to reduce the resulting voltage ripple

at the LT3513 input and to force this switching current

into a tight local loop, minimizing EMI. The input capaci-

tor must have low impedance at the switching frequency

to do this effectively and it must have an adequate ripple

current rating. The input capacitor RMS current can be

calculated from the step-down output voltage and current,

and the input voltage:

CI

VV

V

V

I

IN RMS

OUT

OUT

IN

OUT

IN

OUT

()

–

=

()

<

2

ripple current contribution from the other channels will

be minimal. Considering that the maximum load current

from switcher 1 is ~3A, RMS ripple current will always be

less than 1.5A. The high frequency of the LT3513 reduces

the energy storage requirements of the input capacitor, so

that the capacitance required is less than 10μF. The com-

bination of small size and low impedance (low equivalent

series resistance or ESR) of ceramic capacitors makes

OPERATION