Data Sheet

ADPL76030

analog.com

Rev.0

29 of 40

To select the power MOSFETs, the power dissipated by the device must be known. For switch A, the maximum power

dissipation happens in the boost region, when it remains on all the time. Its maximum power dissipation at maximum

output current is given by:

PA(BOOST) = (

IOUT(MAX) × VOUT

VIN

)

2

× ρT × RDS(ON)

Where, ρT is a normalization factor (unity at 25°C) accounting for the significant variation in on-resistance with

temperature, typically 0.4%/°C, as shown in Figure 45. For a maximum junction temperature of 125°C, using a value

of ρT = 1.5 is reasonable.

Switch B operates in the buck region as the synchronous rectifier. Its power dissipation at maximum output current

is given by:

PB(BUCK) = (

VIN − VOUT

VIN

)

× IOUT(MAX)

2 × ρT × R

DS(ON)

Switch C operates in the boost region as the control switch. Its power dissipation at maximum current is given by:

PC(BOOST) =

(VOUT − VIN) × VOUT

VIN

2

× IOUT(MAX)

2 × ρT × R

DS(ON) + k × VOUT 3 ×

IOUT(MAX)

VIN

× CRSS × f

Figure 45. Normalized RDS(ON) vs. Temperature

where, CRSS is usually specified by the MOSFET manufacturers. The constant k, which accounts for the loss caused by

reverse recovery current, is inversely proportional to the gate drive current and has an empirical value of 1.7.

For switch D, the maximum power dissipation happens in the boost region, when its duty cycle is higher than 50%.

Its maximum power dissipation at maximum output current is given by:

PD(BOOST) =

VOUT

VIN

× IOUT(MAX)

2 × ρT × R

DS(ON)

For the same output voltage and current, switch A has the highest power dissipation and switch B has the lowest

power dissipation, unless a short occurs at the output.

From a known power dissipated in the power MOSFET, its junction temperature can be obtained using the following

formula:

JUNCTION TEMPERATURE (°C)

1.0

1.5

0.5

0

2.0

–50

–25

0

25

50

75

100

125