Data Sheet

ADP1851

Rev. B | Page 17 of 24

MOSFET SELECTION

The choice of MOSFET directly affects the dc-to-dc converter

and low gate charge reduces transition losses. The MOSFET should

have low thermal resistance to ensure that the power dissipated

in the MOSFET does not result in excessive MOSFET die

temperature.

The high-side MOSFET carries the load current during on time

and usually carries most of the transition losses of the converter.

Typically, the lower the on resistance of the MOSFET, the higher

the gate charge, and vice versa. Therefore, it is important to choose

a high-side MOSFET that balances the two losses. The conduction

loss of the high-side MOSFET is determined by the equation

DSON

RMS

LOAD

C

R

I

P





2

)

(

)

(

where:

The gate charging loss is approximated by the equation

SW

G

PV

G

f

Q

V

P







where:

Note that the gate charging power loss is not dissipated in the

MOSFET but rather in the ADP1851 internal drivers. This power

loss should be taken into consideration when calculating the

overall power efficiency.

The high-side MOSFET transition loss is approximated by the

equation

2

)

(

SW

F

R

LOAD

IN

T

f

t

t

I

V

P











where:

RISE

DRIVER

GSW

R

I

Q

t

_



FALL

DRIVER

GSW

F

I

Q

t

_



where:

given in the MOSFET data sheet.

ADP1851 internal gate drivers.

2

GS

GD

GSW

Q

Q

Q





where:

given in the MOSFET data sheet.

GATE

SOURCE

ON

SP

DD

RISE

DRIVER

R

R

V

V

I







_

_

GATE

SINK

ON

SP

FALL

DRIVER

R

R

V

I





_

_

where:

2.75 V and 5 V, depending on the input voltage.

this voltage can be estimated by inspecting the gate charge

graph given in the MOSFET data sheet.

given in Table 1, when charging the MOSFET.

given in Table 1, when discharging the MOSFET.

MOSFET data sheet. If an external gate resistor is added, add

The total power dissipation of the high-side MOSFET is the

sum of the conduction and transition losses:

T

C

HS

P

P

P





The synchronous rectifier, or low-side MOSFET, carries the

inductor current when the high-side MOSFET is off. The low-

side MOSFET transition loss is small and can be ignored in the

calculation. For high input voltage and low output voltage, the

low-side MOSFET carries the current most of the time. Therefore,

to achieve high efficiency, it is critical to optimize the low-side

MOSFET for low on resistance. In cases where the power loss

exceeds the MOSFET rating or lower resistance is required than

is available in a single MOSFET, connect multiple low-side

MOSFETs in parallel. The equation for low-side MOSFET

conduction power loss is

DSON

RMS

LOAD

CLS

R

I

P





2

)

(

)

(