ISL81601 datasheet(47/54 Pages) RENESAS

47 / 54 page

FN9299 Rev.3.1

Page 47 of 53

May 27, 2021

ISL81601

8. Component Selection Guideline

Component Selection Guideline

8.1

MOSFET Considerations

The MOSFETs are chosen for optimum efficiency given the potentially wide input voltage range and output power

requirement. Select these MOSFETs based upon rDS(ON), gate supply requirements, and thermal management

considerations.

The buck MOSFETs’ maximum operation voltage is decided by the maximum VIN voltage, and the boost

MOSFETs’ maximum operation voltage is decided by the maximum VOUT voltage. Choose the buck or boost

MOSFETs based on their maximum operation voltage with sufficient margin for safe operation.

The MOSFETs’ power dissipation is based on: conduction loss and switching loss. In Buck mode, the power loss

of the buck upper and lower MOSFETs are calculated by Equations 22 and 23. The conduction losses are the main

source of power dissipation for the lower MOSFET. Only the upper MOSFET has significant switching losses,

because the lower device turns on and off into near zero voltage. The equations assume linear voltage current

transitions and do not model power loss due to the reverse recovery of the lower MOSFET’s body diode.

In Boost mode, there is only conduction loss on the buck upper MOSFET calculated by Equation 24.

In Boost mode, the boost upper and lower MOSFETs’ power loss are calculated by Equations 25 and 26. The

conduction losses are the main component of power dissipation for the upper MOSFET. Only the lower MOSFET

has significant switching losses, because the upper device turns on and off into near zero voltage. The equations

assume linear voltage current transitions and do not model power loss due to the reverse recovery of the upper

MOSFET’s body diode.

In Buck mode, the conduction loss exists on the boost upper MOSFET calculated by Equation 27.

A large gate-charge increases the switching time, tSW, which increases the switching losses of the buck upper and

boost lower MOSFETs. Ensure that all four MOSFETs are within their maximum junction temperature at high

ambient temperature by calculating the temperature rise according to package thermal resistance specifications.

PUPPERBUCK

IOUT

 r

DS ON



 V

OUT



VIN

-----------------------------------------------------------------------

IOUT

 V

 t

 f



-----------------------------------------------------------------

(EQ. 22)

PLOWERBUCK

IOUT

 r

DS ON



 V

VOUT

–



VIN

---------------------------------------------------------------------------------------

(EQ. 23)

PUPPERBUCK

IOUT

 V

OUT



VIN



---------------------------------------------- rDS ON



(EQ. 24)

PLOWERBOOST

IOUT

 V

OUT



VIN



----------------------------------------------

VOUT VIN

–

 r

DS ON



VOUT

-----------------------------------------------------------------

IOUT

 V

OUT

 t

 f



2VIN



---------------------------------------------------------------------------

(EQ. 25)

PUPPERBOOST

IOUT

 r

DS ON



 V

OUT



VIN

-----------------------------------------------------------------------

(EQ. 26)

PUPPERBOOST

IOUT

 r

DS ON



(EQ. 27)

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