LTC3862IGN-PBF datasheet(29/40 Pages) LINER

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LTC3862

3862fb

additional power dissipation is important when deciding

on a diode current rating, package type, and method of

heat sinking.

To a close approximation, the power dissipated by the

diode is:

PD = ID(PEAK) • VF(PEAK) • (1 – DMAX)

The diode junction temperature is:

TJ = TA + PD • RTH(JA)

The RTH(JA) to be used in this equation normally includes

the RTH(JC) for the device plus the thermal resistance from

the board to the ambient temperature in the enclosure.

Once the proper diode has been selected and the circuit

performance has been veriﬁed, measure the temperature

of the power components using a thermal probe or infrared

camera over all operating conditions to ensure a good

thermal design.

Finally, 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.

Output Capacitor Selection

Contributions of ESR (equivalent series resistance), ESL

(equivalent series inductance) and the bulk capacitance

must be considered when choosing the correct combination

of output capacitors for a boost converter application. The

effects of these three parameters on the output voltage

ripple waveform are illustrated in Figure 22 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

ΔV.

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 ΔV. This percentage

ripple will change, depending on the requirements of the

application, and the equations provided below can easily

be modiﬁed.

One of the key beneﬁts of multi-phase operation is a reduc-

tion in the peak current supplied to the output capacitor

by the boost diodes. As a result, the ESR requirement

of the capacitor is relaxed. For a 1% contribution to the

total ripple voltage, the ESR of the output capacitor can

be determined using the following equation:

ESR

COUT

OUT

DPEAK

≤

001

.•

()

where:

DPEAK

OMAX

MAX

()

••

–

⎛

⎝⎜

⎞

⎠⎟

The factor n represents the number of phases and the factor

χ represents the percentage inductor ripple current.

APPLICATIONS INFORMATION

SW1

50V/DIV

SW2

50V/DIV

1μs/DIV

VIN = 10V

VOUT = 48V

500mA LOAD

3862 F22

VOUT

50mV/DIV

AC COUPLED

IL1 2A/DIV

IL2 2A/DIV

Figure 22. Switching Waveforms for a Boost Converter

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