MP24830 — 4.5V–90V, PROGRAMABLE FREQUENCY WHITE LED DRIVER

MP24830 Rev. 1.0

www.MonolithicPower.com

12

10/30/2013

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(2)Compensation network for Buck

application

The DC modulator gain of the buck power stage

(from the output current to the control voltage)

is:

CS

1

DCGain _ Buck

20 R





The dominant, low frequency pole of the buck

power stage is:

P_ Buck

FB

LED

ESR

OUT

1

f

2(R

R

R

) C











The zero produced by the ESR of the output

capacitor is:

Z _ ESR

OUT

ESR

1

f

2C

* R





to derive the compensation network as follows

c

COMP

Buck

FB

p _ Buck

f

R_

gm R

DCGain _ Buck * f





That is:

c

OUT

CS

FB

LED

ESR

COMP _ Buck

FB

2f

C

20 R

(R

R

R

)

R

gm R

















Set the zero of the compensation network to

cancel the minimum pole of the Buck power

stage to get:

z_ Buck

P_ Buck

COMP _ Buck

1

C

2f

R







Set the pole of the compensation network to

cancel the ESR zero. If the ESR zero is too

p

z _ ESR

COMP _ Buck

c

COMP _ Buck

11

Cmax(

,

)

2f

R

2

5f

R







 



Selecting the Inductor

Select the inductor based on the input voltage,

the output voltage, and the LED current. Select

the inductor to make the circuit operate in

continuous current mode (CCM). Select the

inductor current rating to ensure that the

inductor

does

not

saturate

and

with

consideration to power consumption based on

the DC resistance.

(1) Selecting the Inductor for Buck-Boost

Applications

For buck-boost applications, select the inductor

based on the following equation:

IN

OUT

SW

IN

OUT

L

VV

L

f(V

V

)

I







 

of the average current of the inductor, which is:

OUT

L _ AVG

LED

IN

V

II

* (1

)

V



Select the inductor with a DC current rating that

ensurew that the inductor does not saturated at

the peak current of:

L_PK

L_ AVG

L

II

0.5 I





(2) Selecting the Inductor for Buck Applications

For buck applications, derive the inductance

value from the following equation.

SW

L

IN

OUT

IN

OUT

f

I

V

)

V

V

(

V

L













current.

Choose the inductor ripple current to around

30% to 60% of the maximum load current. The

maximum inductor peak current is calculated as:

2

I

I

I

L

LOAD

)

MAX

(

L







Selecting the Input Capacitor

The input capacitor reduces the surge current

drawn from the input supply and the switching

noise from the device. For best results, use