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LTC1871-7

18717fc

APPLICATIONS INFORMATION

rating for the inductor should be checked at the minimum

input voltage (which results in the highest inductor cur-

rent) and maximum output current.

SEPIC Converter: Power MOSFET Selection

Important parameters for the power MOSFET include the

drain-to-source breakdown voltage (BVDSS), the threshold

voltage (VGS(TH)), the on-resistance (RDS(ON)) versus gate-

to-source voltage, the gate-to-source and gate-to-drain

charges (QGS and QGD, respectively), the maximum drain

current (ID(MAX)) and the MOSFET’s thermal resistances

(RTH(JC) and RTH(JA)).

The gate drive voltage is set by the 7V INTVCC low dropout

regulator. Consequently, 6V rated threshold MOSFETs are

required in most LTC1871-7 applications.

The maximum voltage that the MOSFET switch must

sustain during the off-time in a SEPIC converter is equal

to the sum of the input and output voltages (VO + VIN).

As a result, careful attention must be paid to the BVDSS

speciﬁcations for the MOSFETs relative to the maximum

actual switch voltage in the application. Many logic-level

devices are limited to 30V or less. Check the switching

waveforms directly across the drain and source terminals

of the power MOSFET to ensure the VDS remains below

the maximum rating for the device.

Sense Resistor Selection

During the MOSFET’s on-time, the control circuit limits

the maximum voltage drop across the power MOSFET to

about 150mV (at low duty cycle). The peak inductor cur-

rent is therefore limited to 150mV/RSENSE. The relationship

between the maximum load current, duty cycle and the

sense resistor is:

RSENSE

VSENSE(MAX)

IO(MAX)

•

VO + VD

VIN(MIN)

+ 1

The VSENSE(MAX) term is typically 150mV at low duty

cycle and is reduced to about 100mV at a duty cycle of

92% due to slope compensation, as shown in Figure 11.

The constant ‘χ’ in the denominator represents the ripple

current in the inductors relative to their maximum cur-

rent. For example, if 30% ripple current is chosen, then

χ = 0.30.

Calculating Power MOSFET Switching and Conduction

Losses and Junction Temperatures

In order to calculate the junction temperature of the

power MOSFET, the power dissipated by the device must

be known. This power dissipation is a function of the

duty cycle, the load current and the junction temperature

itself. As a result, some iterative calculation is normally

required to determine a reasonably accurate value. Since

the controller is using the MOSFET as both a switching

and a sensing element, care should be taken to ensure

that the converter is capable of delivering the required

load current over all operating conditions (load, line and

temperature) and for the worst-case speciﬁcations for

VSENSE(MAX) and the RDS(ON) of the MOSFET listed in the

manufacturer’s data sheet.

The power dissipated by the MOSFET in a SEPIC converter

is:

PFET = IO(MAX) •

1–D

•RDS(ON) •D• T

+ k• VIN + VO

()2 •IO(MAX) • D

1–D

•CRSS •f

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 inversely

related to the gate drive current and has the dimension

of 1/current.

The ρT term accounts for the temperature coefﬁcient of

the RDS(ON) of the MOSFET, which is typically 0.4%/°C.

Figure 12 illustrates the variation of normalized RDS(ON)

over temperature for a typical power MOSFET.

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