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LTC1871

and the RDS(ON) of the MOSFET listed in the manufacturer’s

data sheet.

The power dissipated by the MOSFET in a boost converter

is:

FET

OMAX

MAX

DS ON

MAX

OMAX

MAX

RSS









()

–

••

•

••

–

••

185

The first term in the equation above represents the I2R

losses in the device, and the second term, the switching

losses. The constant, k = 1.7, is an empirical factor in-

versely related to the gate drive current and has the dimen-

sion of 1/current.

From a known power dissipated in the power MOSFET, its

junction temperature can be obtained using the following

formula:

TJ = TA + PFET • 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 case to the ambient temperature (RTH(CA)). This value

of TJ can then be compared to the original, assumed value

used in the iterative calculation process.

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

O MAX

MAX

()

•

–

== +









The power dissipated by the diode is:

PD = IO(MAX) • VD

and 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.

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 12e 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

percentage ripple will change, depending on the require-

ments 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

COUT

IN PEAK

≤

001

.•

()

where:

IN PEAK

OMAX

MAX

()

•

–









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