NCV8853_16 datasheet(10/12 Pages) ONSEMI

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NCV8853

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DESIGN METHODOLOGY

Choosing external components for the NCV8853

encompasses the following design process:

1. Define operational parameters

2. Select switching frequency

3. Select current sensor

4. Select a MOSFET

5. Select a diode

6. Select output inductor

7. Select output capacitors

8. Select compensator components

(1) Operating Parameter Definition

First, select feedback resistors to choose the output

voltage as follows:

OUT + VREF @

1 ) R2

where:

VOUT: desired output voltage

R1: upper feedback resistor (between VOUT and FB) [

R2: lower feedback resistor (between FB and GND) [

For a 5.0 V output, set R1 to 42.2 k

W and R2 to 8.06 kW.

Certain operating parameters must be defined before

proceeding with the rest of the design. These are application

dependent and include the following:

VIN: input voltage, range from minimum to maximum

with a typical value [V]

IOUT: output current, range from minimum to maximum

with an initial startup value

ICL: desired typical current limit

A number of basic calculations must be performed up front

to use in the design process, as follows:

MIN +

OUT

IN(max)

OUT

IN(typ)

MAX +

OUT

IN(min)

where:

DMIN: minimum duty cycle (ideal) [%]

VIN(max): maximum input voltage [V]

D: typical duty cycle (ideal) [%]

VIN(typ): typical input voltage [V]

DMAX: maximum duty cycle (ideal) [%]

VIN(min): minimum input voltage [V]

(2) Switching Frequency Selection

Selecting the switching frequency is a trade−off between

component size and power losses. Operation at higher

switching frequencies allows the use of smaller inductor and

capacitor values to achieve the same inductor current ripple

and output voltage ripple. However, increasing the

frequency increases the switching losses of the MOSFETs,

leading to decreased efficiency, especially noticeable at light

loads.

Typically, the switching frequency is selected to avoid

interfering with signals of known frequencies. Using the

EN/SYNC pin, the NCV8853 can be synced to frequencies

from 80% to 300% of the nominal frequency (340 kHz).

(3) Current Sensor Selection

Current sensing for peak current mode control relies on

the inductor current signal. This is translated into a voltage

via a current sense resistor, which is then measured

differentially by the current sense amplifier, generating a

single−ended output to use as a signal. The easiest means of

implementing this transresistance is through the use of a

sense resistor in series with the source of the MOSFET and

VIN. A sense resistor should be calculated as follows:

SNS +

where:

RSNS: sense resistor [

VCL: current limit threshold voltage [V]

ICL: desired cycle−by−cycle current limit [A]

(4) MOSFET Selection

The NCV8853 has been designed to work with a

P−channel

MOSFET

non−synchronous

buck

configuration. The MOSFET needs to be capable of

handling the maximum allowable current in the system, ICL.

Keep in mind that, depending on your minimum VIN signal,

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

dissipated through the MOSFET during conduction is as

follows:

MOS,on + ICL

@ D

MAX @ rDS,on

where:

PMOS,on: power through MOSFET [

ICL: cycle−by−cycle current limit [A]

rDS,on: on−resistance of the MOSFET [

To calculate the switching losses through the MOSFET, use

the following equation:

MOS,sw +

IN @ IOUT @ ton ) toff @ FSW

ton + toff +

Gate

drv

where:

PMOS, sw: MOSFET switching losses [W]

ton: time to turn on the MOSFET [s]

toff: time to turn off the MOSFET [s]

QGate: gate charge [C]

Idrv: gate drive current [A]

(5) Diode Selection

The diode must be chosen according to its maximum

current and voltage ratings, and to thermal considerations.

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