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LT1500

Adaptive-Frequency Current Mode Switching Regulators

Linear Technology 

LT1500/LT1501

APPLICATIONS INFORMATION

mode. Normal operation resumes for one or more switch

cycles and the output voltage increases until the error

amplifier output falls below threshold, initiating a new

adaptive bias shutdown.

DESIGN GUIDE

Selecting Inductor Value

Inductor value is chosen as a compromise between size,

switching frequency, efficiency and maximum output cur-

rent. Larger inductor values become physically larger but

provide higher output current and give better efficiency

(because of the lower switching frequency). Low induc-

tance minimizes size but may limit output current and the

higher switching frequency reduces efficiency.

The simplest way to handle these trade-offs is to study the

graphs in the Typical Performance Characteristics sec-

tion. A few minutes with these graphs will clearly show the

trade-offs and a value can be quickly chosen that meets the

requirements of frequency, efficiency and output current.

This leaves only physical size as the final consideration.

The concern here is that for a given inductor value, smaller

size usually means higher series resistance. The graphs

showing efficiency loss vs inductor series resistance will

allow a quick estimate of the additional losses associated

with very small inductors.

One final consideration is inductor construction. Many

small inductors are “open frame ferrites” such as rods or

barrels. These geometries do not have a closed magnetic

path, so they radiate significant B fields in the vicinity of the

inductor. This can affect surrounding circuitry that is

sensitive to magnetic fields. Closed geometries such as

toroids or E-cores have very low stray B fields, but they are

larger and more expensive (naturally).

Catch Diode

The catch diode in a boost converter has an average

current equal to output current, but the peak current can

be significantly higher. Maximum reverse voltage is equal

to output voltage. A 0.5A Schottky diode like MBR0520L

works well in nearly all applications.

Input Capacitor

Input capacitors for boost regulators are less critical than

the output capacitor because the input capacitor ripple

current is a simple triwave without the higher frequency

harmonics found in the output capacitor current. Peak-to-

peak current is less than 200mA and worst-case RMS

ripple current in the input capacitor is less than 70mA.

Input capacitor series resistance (ESR) should be low

enough to keep input ripple voltage to less than 100mVP-P.

This assumes that the capacitor is an aluminum or tanta-

lum type where the capacitor reactance at the switching

frequency is small compared to the ESR.

C

≥

π

2

(f)(ESR)

A typical input capacitor is a 33µF, 6V surface mount solid

tantalum type TPS from AVX. It is a “C” case size, with

0.15Ω maximum ESR. Some caution must be used with

solid tantalum input capacitors because they can be dam-

aged with turn-on surge currents that occur when a low

impedance power source is hot-switched to the input of

the regulator. This problem is mitigated by using a capaci-

tor with a voltage rating at least twice the highest expected

input voltage. Consult with the manufacturer for additional

guidelines.

If a ceramic input capacitor is used, different design

criteria are used because these capacitors have extremely

low ESR and are chosen for a minimum number of

microfarads.

C(Ceramic)

=

1

4f

f = switching frequency

A typical unit is an AVX or Tokin 3.3µF or 4.7µF.

Output Capacitor

Output ripple voltage is determined by the impedance of

the output capacitor at the switching frequency. Solid

tantalum capacitors rated for switching applications are

recommended. These capacitors are essentially resistive

at frequencies above 50kHz, so ESR is the important factor

in determining ripple voltage. A typical unit is a 220µF, 10V

10

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