NCV8877

www.onsemi.com

10

R

2859

3 must be independent of power grounds.

3. Select Current Sense Resistor

Current sensing for peak current mode control and current

limit relies on the MOSFET current signal, which is

measured with a ground referenced amplifier. The easiest

method of generating this signal is to use a current sense

resistor from the source of the MOSFET to device ground.

The sense resistor should be selected as follows:

R

V

CL

I

CL

W]

4. Select Output Inductor

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 case of load steps. A good starting point

for peak to peak ripple is around 20−40% of the inductor

After choosing a peak current ripple value, calculate the

inductor value as follows:

L

+

V

DI

The maximum average inductor current can be calculated

as follows:

I

V

V

The Peak Inductor current can be calculated as follows:

I

DI

L,max

2

5. Select Output Capacitors

The output capacitors smooth the output voltage and

reduce the overshoot and undershoot associated with line

transients. The steady state output ripple associated with the

output capacitors can be calculated as follows:

V

DI

OUT(max)

fC

OUT

)

I

OUT(max)

1

* D )

V

2fL

R

ESR

The capacitors need to survive an RMS ripple current as

follows:

I

D

WC

D

WC

)

D

WC

12

D

WC

L

R

OUT

T

SW

2

The use of parallel ceramic bypass capacitors is strongly

encouraged to help with the transient response.

6. Select Input Capacitors

The input capacitor reduces voltage ripple on the input to

the module associated with the ac component of the input

current.

I

V

IN(WC)

WC

7. Select Compensator Components

Current Mode control method employed by the NCV8877

allows the use of a simple, Type II compensation to optimize

the dynamic response according to system requirements.

8. Select MOSFET(s)

In order to ensure the gate drive voltage does not drop out

the MOSFET(s) chosen must not violate the following

inequality:

Q

I

drv

The maximum RMS Current can be calculated as follows:

I

D

D

The maximum voltage across the MOSFET will be the

maximum output voltage, which is the higher of the

maximum input voltage and the regulated output voltaged:

V

9. Select Diode

The output diode rectifies the output current. The average

current through diode will be equal to the output current:

I

Additionally, the diode must block voltage equal to the

higher of the output voltage and the maximum input voltage:

V

The maximum power dissipation in the diode can be

calculated as follows:

P