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LT3959

3959f

It is recommended that the peak repetitive reverse voltage

rating VRRM is higher than VOUT by a safety margin (a 10V

safety margin is usually sufficient).

The power dissipated by the diode is:

PD = IO(MAX) •VD

Where VD is diode’s forward voltage drop, and the diode

junction temperature is:

TJ = TA +PD •RθJA

The RθJA to be used in this equation normally includes the

RθJC for the device plus the thermal resistance from the

board to the ambient temperature in the enclosure. TJ must

not exceed the diode maximum junction temperature rating.

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 output

capacitors for a given output ripple voltage. The effect of

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

voltage ripple waveform for a typical boost converter is

illustrated in Figure 3.

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 ΔVESR and charging/discharging

ΔVCOUT. For the purpose of simplicity, we will choose

2% for the maximum output ripple, to be divided equally

between ΔVESR and ΔVCOUT. This percentage ripple will

change, depending on the requirements of the application,

APPLICATIONS INFORMATION

and the following equations 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:

ESRCOUT ≤

0.01• VOUT

ID(PEAK)

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

the total ripple:

COUT ≥

IO(MAX)

0.01• VOUT •ƒOSC

The output capacitor in a boost regulator experiences high

RMS ripple currents, as shown in Figure 3. The RMS ripple

current rating of the output capacitor can be determined

using the following equation:

IRMS(COUT) ≥ IO(MAX) •

DMAX

−DMAX

Multiple capacitors are often paralleled to meet ESR

requirements. Typically, once the ESR requirement is

satisfied, the capacitance is adequate for filtering and has

the required RMS current rating. Additional ceramic capaci-

tors in parallel are commonly used to reduce the effect of

parasitic inductance in the output capacitor, which reduces

high frequency switching noise on the converter output.

Boost Converter: Input Capacitor Selection

The input capacitor of a boost converter is less critical

than the output capacitor, due to the fact that the inductor

is in series with the input, and the input current wave-

form is continuous. The input voltage source impedance

determines the size of the input capacitor, which is typi-

cally in the range of 10μF to 100μF. A low ESR capacitor

is recommended, although it is not as critical as for the

output capacitor.

The RMS input capacitor ripple current for a boost

converter is:

IRMS(CIN) = 0.3 • ΔIL

VOUT

(AC)

tON

VESR

RINGING DUE TO

TOTAL INDUCTANCE

(BOARD + CAP)

VCOUT

3959 F03

tOFF

Figure 3. The Output Ripple Waveform of a Boost Converter

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