NCV8852

www.onsemi.com

11

(4) MOSFET Selection

The NCV8852 has been designed to work with a

P−channel

MOSFET

in

a

non−synchronous

buck

configuration. The MOSFET needs to be capable of

it is possible to achieve 100% duty cycle. The power

dissipated through the MOSFET during conduction is as

follows:

P

2

@ D

where:

W]

W]

To calculate the switching losses through the MOSFET, use

the following equation:

P

1

2

V

Q

Gate

I

drv

where:

(5) Diode Selection

The diode must be chosen according to its maximum

current and voltage ratings, and to thermal considerations.

The maximum reverse voltage the diode sees is the

maximum input voltage (with some margin in case of

ringing on the switch node). The maximum forward current

.

(6) Output Inductor Selection

Both mechanical and electrical considerations influence

the selection of an output inductor. From a mechanical

perspective, smaller inductor values generally correspond to

smaller physical size. Since the inductor is often one of the

largest components in the power supply, a minimum

inductor value is particularly important in space−

constrained applications. From an electrical perspective, an

inductor is chosen for a set amount of current ripple and to

assure adequate transient response.

The output inductor controls the current ripple that occurs

over a switching period. A high current ripple will result in

excessive power loss and ripple current requirements. A low

current ripple will result in a poor control signal and a slow

current slew rate in the event of a load transient. A good

starting point for peak−to−peak ripple is around 10% of the

inductor current.To choose the inductor value based on the

peak−to−peak ripple current, use the following equation:

i

V

L

@ F

SW

where:

From this equation it is clear that the ripple current increases

as L decreases, emphasizing the trade−off between dynamic

response and ripple current. The peak and valley values of

the triangular current waveform are as follows:

I

i

L

2

I

i

L

2

where:

Saturation current is specified by inductor manufacturers as

the current at which the inductance value has dropped from

the nominal value, typically 10%. For stable operation, the

output inductor must be chosen so that the inductance is

close to the nominal value even at the peak output current,

saturation current sufficiently higher than the peak output

current, such that the inductance is very close to the nominal

value at the peak output current. This allows for a safety

factor and allows for more optimized compensation.

Inductor efficiency is another consideration when

selecting an output inductor. Inductor losses include dc and

ac winding losses, which are very low due to high core

resistance, and magnetic hysteresis losses, which increase

with peak−to−peak ripple current. Core losses also increase

as switching frequency increases.

Ac winding losses are based on the ac resistance of the

winding and the RMS ripple current through the inductor,

which is much lower than the dc current. The ac winding

losses are due to skin and proximity effects and are typically

much less than dc losses, but increase with frequency. Dc

winding losses account for a large percentage of output

inductor losses and are the dominant factor at switching

frequencies at or below 500 kHz. The dc winding losses in

the inductor can be calculated with the following equation:

P

2

@ R

dc

where: