LT8331 datasheet(18/34 Pages) AD

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LT8357

Rev. 0

For more information www.analog.com

The power dissipated by the diode is:

PD = IO(MAX) • VD

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 tem-

perature 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 out-

put voltage ripple waveform for a typical boost converter

is illustrated in Figure 7.

VOUT

(AC)

tON

ΔVESR

RINGING DUE TO

TOTAL INDUCTANCE

(BOARD + CAP)

ΔVCOUT

8357 F07

tOFF

Figure 7. The Output Voltage Ripple Waveform of a

Boost Converter

The choice of components 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 the charging/discharg-

ing 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 applica-

tion, 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 fol-

lowing equation:

ESRCOUT ≤

0.01•VOUT

ID(PEAK)

APPLICATIONS INFORMATION

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

the total ripple:

COUT ≥

IO(MAX)

0.01•VOUT •f

The output capacitor in a boost regulator experiences

high RMS ripple currents, as shown in Figure 7. The RMS

ripple current rating of the output capacitor can be deter-

mined using the following equation:

IRMS(COUT) ≥IO(MAX)•

DMAX

1−DMAX

Multiple capacitors are often paralleled to meet ESR

requirements. Typically, once the ESR requirement is sat-

isfied, 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. Ceramic

capacitors should be placed from VOUT to GND as close to

the LT8357 pins as possible to suppress high frequency

switching noise on the converter output. X5R or X7R

dielectrics are preferred, as these materials retain their

capacitance over wide voltage and temperature ranges.

Many ceramic capacitors, particularly 0805 or 0603 case

sizes, have greatly reduced capacitance at the desired

operating voltage.

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 waveform

is continuous. The input voltage source impedance deter-

mines the size of the input capacitor, which is typically in

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

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

the output capacitor. Place the ceramic capacitors from

VIN to GND as close to the LT8357 pins as possible to

reduce input ripple voltage. Similar to the output capaci-

tors, X5R or X7R dielectrics and 0805 or 0603 case sizes

are also preferred for the input capacitor selection.

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