ADP1877ACPZ-R7 datasheet(21/32 Pages) AD

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ADP1877

Rev. 0 | Page 21 of 32

MOSFET SELECTION

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

performance. A MOSFET with low on resistance reduces I2R

losses, 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

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

≅

OUT

DSON

LOAD

)

(

where:

RDSON is the MOSFET on resistance.

The gate charging loss is approximated by the equation

≅

where

VPV is the gate driver supply voltage.

QG is the MOSFET total gate charge.

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

MOSFET but rather in the ADP1877 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

)

(

LOAD

≅

where:

PT is the high-side MOSFET switching loss power.

tR is the rise time in charging the high-side MOSFET.

tF is the fall time in discharging the high-side MOSFET.

tR and tF can be estimated by

RISE

DRIVER

GSW

≅

FALL

DRIVER

GSW

≅

where:

QGSW is the gate charge of the MOSFET during switching and is

given in the MOSFET data sheet.

IDRIVER_RISE and IDRIVER_FALL are the driver current put out by the

ADP1877 internal gate drivers.

If QGSW is not given in the data sheet, it can be approximated by

GSW

≅

where:

QGD and QGS are the gate-to-drain and gate-to-source charges

given in the MOSFET data sheet.

IDRIVER_RISE and IDRIVER_FALL can be estimated by

GATE

SOURCE

RISE

DRIVER

−

≅

GATE

SINK

FALL

DRIVER

≅

where:

VDD is the input supply voltage to the driver and is between 2.75 V

and 5 V, depending on the input voltage.

VSP is the switching point where the MOSFET fully conducts;

this voltage can be estimated by inspecting the gate charge

graph given in the MOSFET data sheet.

RON_SOURCE is the on resistance of theADP1877 internal driver,

given in Table 1, when charging the MOSFET.

RON_SINK is the on resistance of the ADP1877 internal driver,

given in Table 1, when discharging the MOSFET.

RGATE is the on gate resistance of MOSFET given in the

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

this external resistance to RGATE.

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

sum of conduction and transition losses:

≅

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 neglected 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

⎥

⎦

⎤

⎢

⎣

⎡

−

≅

OUT

DSON

LOAD

CLS

)

(

There is also additional power loss during the time, known as

dead time, between the turn-off of the high-side switch and the

turn-on of the low-side switch, when the body diode of the low-

side MOSFET conducts the output current. The power loss in

the body diode is given by

BODYDIODE

where:

VF is the forward voltage drop of the body diode, typically 0.7 V.

tD is the dead time in the ADP1877, typically 30 ns when

driving some medium-size MOSFETs with input capacitance of

approximately 3 nF.

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