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LT8330

Low IQ Boost/SEPIC/ Inverting Converter with 1A, 60V Switch

Linear Technology 

LT8330

Applications Information

Switching Frequency and Inductor Selection

The LT8330 switches at 2MHz, allowing small value induc-

tors to be used. 0.68µH to 10µH will usually suffice. Choose

an inductor that can handle at least 1.4A without saturating,

and ensure that the inductor has a low DCR (copper-wire

resistance) to minimize I2R power losses. Note that in

some applications, the current handling requirements of

the inductor can be lower, such as in the SEPIC topology

where each inductor only carries one-half of the total

switch current. For better efficiency, use similar valued

inductors with a larger volume. Many different sizes and

shapes are available from various manufacturers. Choose

a core material that has low losses at 2MHz, such as a

ferrite core. The final value chosen for the inductor should

not allow peak inductor currents to exceed 1A in steady

state at maximum load. Due to tolerances, be sure to ac-

count for minimum possible inductance value, switching

frequency and converter efficiency.

Table 1. Inductor Manufacturers

Sumida

(847) 956-0666

TDK

(847) 803-6100

Murata

(714) 852-2001

Coilcraft

(847) 639-6400

Würth

(605) 886-4385

www.sumida.com

www.tdk.com

www.murata.com

www.coilcraft.com

www.we-online.com

Input Capacitor

Bypass the input of the LT8330 circuit with a ceramic ca-

pacitor of X7R or X5R type placed as close as possible to

the VIN and GND pins. Y5V types have poor performance

over temperature and applied voltage, and should not be

used. A 4.7µF to 10µF ceramic capacitor is adequate to

bypass the LT8330 and will easily handle the ripple cur-

rent. If the input power source has high impedance, or

there is significant inductance due to long wires or cables,

additional bulk capacitance may be necessary. This can

be provided with a low performance electrolytic capacitor.

A precaution regarding the ceramic input capacitor con-

cerns the maximum input voltage rating of the LT8330.

A ceramic input capacitor combined with trace or cable

inductance forms a high quality (under damped) tank cir-

cuit. If the LT8330 circuit is plugged into a live supply, the

input voltage can ring to twice its nominal value, possibly

exceeding the LT8330’s voltage rating. This situation is

easily avoided (see Application Note 88).

Output Capacitor and Output Ripple

Low ESR (equivalent series resistance) capacitors should

be used at the output to minimize the output ripple voltage.

Multilayer ceramic capacitors are an excellent choice, as

they are small and have extremely low ESR. Use X5R or

X7R types. This choice will provide low output ripple and

good transient response. A 4.7µF to 15µF output capacitor

is sufficient for most applications, but systems with very

low output currents may need only a 1µF or 2.2µF output

capacitor. Solid tantalum or OS-CON capacitor can be

used, but they will occupy more board area than a ceramic

and will have a higher ESR. Always use a capacitor with a

sufficient voltage rating.

Compensation

The LT8330 is internally compensated. The decision to

use either low ESR (ceramic) capacitors or the higher

ESR (tantalum or OS-CON) capacitors, for the output

capacitor, can affect the stability of the overall system.

The ESR of any capacitor, along with the capacitance

itself, contributes a zero to the system. For the tantalum

and OS-CON capacitors, this zero is located at a lower

frequency due to the higher value of the ESR, while the

zero of a ceramic capacitor is at a much higher frequency

and can generally be ignored.

A phase lead zero can be intentionally introduced by placing

a capacitor in parallel with the resistor between VOUT and

FBX. By choosing the appropriate values for the resistor and

capacitor, the zero frequency can be designed to improve

the phase margin of the overall converter. The typical target

value for the zero frequency is between 30kHz to 60kHz.

A practical approach to compensation is to start with one

of the circuits in this data sheet that is similar to your ap-

plication. Optimize performance by adjusting the output

capacitor and/or the feed forward capacitor (connected

across the feedback resistor from output to FBX pin).

For more information www.linear.com/LT8330

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