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LTC3783

3783f

Boost Converter: Output Diode Selection

To maximize efficiency, a fast switching diode with low

forward drop and low reverse leakage is desired. The

output diode in a boost converter conducts current during

the switch off-time. The peak reverse voltage that the

diode must withstand is equal to the regulator output

voltage. The average forward current in normal operation

is equal to the output current, and the peak current is equal

to the peak inductor current.

D PEAK

L PEAK

OUT MAX

()

•

–

== +

⎛

⎝⎜

⎞

⎠⎟

The power dissipated by the diode is:

PD = IOUT(MAX) • VD

and the diode junction temperature is:

TJ = TA + PD • θJA

The

θJA to be used in this equation normally includes the

θJC for the device plus the thermal resistance from the

board to the ambient temperature in the enclosure.

Remember to keep the diode lead lengths short and to

observe proper switch-node layout (see Board Layout

Checklist) to avoid excessive ringing and increased

dissipation.

Boost Converter: Output Capacitor Selection

Contributions of ESR (equivalent series resistance), ESL

(equivalent series inductance) and the bulk capacitance

must be considered when choosing the correct compo-

nent for a given output ripple voltage. The effects of these

three parameters (ESR, ESL and bulk C) on the output

voltage ripple waveform are illustrated in Figure 9 for a

typical boost converter.

The choice of component(s) begins with the maximum

acceptable ripple voltage (expressed as a percentage of

the output voltage), and how this ripple should be divided

between the ESR step and the charging/discharging

∆V.

For the purpose of simplicity we will choose 2% for the

maximum output ripple, to be divided equally between the

ESR step and the charging/discharging

∆V. This percent-

age ripple will change, depending on the requirements of

the application, and the equations provided below can

easily be modified.

For a 1% contribution to the total ripple voltage, the ESR

of the output capacitor can be determined using the

following equation:

ESR

where

COUT

OUT

IN PEAK

001

.•

()

⎛

⎝⎜

⎞

⎠⎟

•

–

()

OUT MAX

MAX

For the bulk C component, which also contributes 1% to

the total ripple:

OUT

OUT MAX

OUT

()

.•

•

001

For many designs it is possible to choose a single capaci-

tor type that satisfies both the ESR and bulk C require-

ments for the design. In certain demanding applications,

however, the ripple voltage can be improved significantly

by connecting two or more types of capacitors in parallel.

For example, using a low ESR ceramic capacitor can

minimize the ESR setup, while an electrolytic capacitor

can be used to supply the required bulk C.

Once the output capacitor ESR and bulk capacitance have

been determined, the overall ripple voltage waveform

should be verified on a dedicated PC board (see Board

OPERATIO

VOUT

(AC)

∆VESR

RINGING DUE TO

TOTAL INDUCTANCE

(BOARD + CAP)

∆VCOUT

3783 F09

Figure 9. Output Ripple Voltage

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