NCV8873

www.onsemi.com

10

LED series dimming circuit). A good starting point is to

operation should be verified empirically. Calculate the

maximum inductor value as follows:

1

* 1

V

in(min)

2

V

out

I

LED

2

The maximum average inductor current can be calculated

as follows:

I

V

V

IN(min)

The peak inductor current can be calculated as follows:

I

V

4. Select Output Capacitor

The output capacitor smoothes the output voltage and

reduces the overshoot and undershoot associated with line

transients. The steady state output ripple associated with the

output capacitors can be calculated as follows:

V

I

The capacitors must withstand an RMS ripple current as

follows:

I

I

LED

2

I

L,pk

2

3

* I

A 2.2

mF ceramic capacitor is usually sufficient for high

5. 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

3

I

L,pk

2

* I

L,avg

2

6. Select Feedback Resistors

The feedback resistor provides LED current sensing for

the feedback signal. It may be calculated as follows:

R

V

ref

I

LED

7. Select Compensator Components

Current Mode control method employed by the NCV8873

allows the use of a simple Type II compensation to optimize

the dynamic response according to system requirements. A

transconductance amplifier is used, so compensation

components must be connected between the compensation

pin and ground.

8. Select MOSFET(s)

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

the selected MOSFET must not violate the following

inequality:

Q

I

drv

The maximum RMS Current can be calculated as follows:

I

3

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 voltage:

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 or the maximum input voltage:

V

The maximum power dissipation in the diode can be

calculated as follows:

P

[V]

Low Voltage Operation

If the input voltage drops below the UVLO or MOSFET

threshold voltage, another voltage may be used to power the

device. Simply connect the voltage you would like to boost

to the inductor and connect the stable voltage to the VIN pin

of the device. In Boost configuration, the output of the

converter can be used to power the device. In some cases it

may be desirable to connect 2 sources to VIN pin, which can

be accomplished simply by connecting each of the sources

through a diode to the VIN pin.