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LTC3813

3813fb

The required inductance can then be calculated to be:

VIN(MIN) •DMAX

f• IL

The required saturation of the inductor should be chosen

to be greater than the peak inductor current:

IL(SAT)

IO(MAX)

1 DMAX

Once the value for L is known, the type of inductor must be

selected. High efﬁciency converters generally cannot afford

the core loss found in low cost powdered iron cores, forcing

theuseofmoreexpensiveferrite,molypermalloyorKoolMμ®

cores. A variety of inductors designed for high current, low

voltage applications are available from manufacturers such

as Sumida, Panasonic, Coiltronics, Coilcraft and Toko.

Schottky Diode D1 Selection

The Schottky diode D1 shown in the front page schematic

conducts during the dead time between the conduction of

the power MOSFET switches. It is intended to prevent the

body diode of the synchronous MOSFET from turning on

and storing charge during the dead time, which can cause

a modest (about 1%) efﬁciency loss. The diode can be

rated for about one half to one ﬁfth of the full load current

since it is on for only a fraction of the duty cycle. The peak

reverse voltage that the diode must withstand is equal to

the regulator output voltage. In order for the diode to be

effective, the inductance between it and the synchronous

MOSFET must be as small as possible, mandating that

these components be placed adjacently. The diode can

be omitted if the efﬁciency loss is tolerable.

Output Capacitor Selection

In a boost converter, the output capacitor requirements

are demanding due to the fact that the current waveform

is pulsed. The choice of component(s) is driven by the

acceptable ripple voltage which is affected by the ESR,

ESL and bulk capacitance as shown in Figure 8e. The total

output ripple voltage is:

VOUT =IO(MAX)

f• COUT

ESR

1– DMAX

where the ﬁrst term is due to the bulk capacitance and

second term due to the ESR.

For many designs it is possible to choose a single capacitor

type that satisﬁes both the ESR and bulk C requirements

for the design. In certain demanding applications, however,

the ripple voltage can be improved signiﬁcantly by con-

necting two or more types of capacitors in parallel. For

example, using a low ESR ceramic capacitor can minimize

the ESR step, while an electrolytic capacitor can be used

to supply the required bulk C.

Figure 8. Switching Waveforms for a Boost Converter

APPLICATIONS INFORMATION

VIN

8a. Circuit Diagram

8b. Inductor and Input Currents

COUT

VOUT

IIN

8c. Switch Current

ISW

tON

8d. Diode and Output Currents

8e. Output Voltage Ripple Waveform

VOUT

(AC)

tOFF

ΔVESR

RINGING DUE TO

TOTAL INDUCTANCE

(BOARD + CAP)

ΔVCOUT

3813 F08

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