MP2488 – 2A, 200kHz, 55V HIGH-POWER LEDS DRIVER

MP2488 Rev. 1.0

www.MonolithicPower.com

10

6/20/2011

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the LED current ripple. Ceramic, tantalum, or low

ESR electrolytic capacitors are recommended.

Low ESR capacitors are preferred to keep the

output voltage ripple low so that the AC ripple

current through the LEDs is small. The output

voltage ripple can be estimated by:

OUT

OUT

OUT

2

IN

SOUT

VV

ΔV1

V

8f

L1 C



 



 



For most application, a 2.2uF~4.7uF ceramic

capacitor is recommended.

Compensation Components

MP2488 employs current mode control for easy

compensation and fast transient response. The

system stability and transient response are

controlled through the COMP pin. COMP pin is

the output of the internal error amplifier. A series

sets a pole-zero combination to control the

characteristics of the control system. The DC

gain of the current feedback loop is given by:

VDC

FB

CS

VEA

AR

G

A





400V/V;

is

the

current

sense

sensing resistor value.

The system has two poles of importance. One is

the

output

resistor

of

error

amplifier

transconductance, 500μA/V. The other is due to

the output capacitor and the LEDs’ AC resistor

P1

COM1

EA

1

f

2C

R







P2

OUT

LED

1

f

2C

R







The system has one zero of importance, due to

located at:

Z1

COM1

COM

1

f

2C

R







The

system

may

have

another

zero

of

importance, if the output capacitor has a large

capacitance and/or a high ESR value. The zero,

due to the ESR and capacitance of the output

capacitor, is located at:

ESR

OUT

ESR

1

f

2C

R







In this case, a third pole set by the compensation

ESR zero on the loop gain. This pole is located

at:

P3

COM2

COM

1

f

2C

R







The goal of compensation design is to shape the

converter transfer function to get a desired loop

gain and phase margin. The system crossover

frequency where the feedback loop has the unity

gain is important. Lower crossover frequencies

result in slower line and load transient responses,

while higher crossover frequencies could cause

system unstable. A good rule of thumb is to set

the crossover frequency to approximately one-

tenth of the switching frequency. To optimize the

compensation components for conditions, the

following procedure can be used.

set the desired crossover frequency. Determine

OUT

LED

C

COM

FB

EA

CS

2C

R

f

R

RG

G

 







achieve

the

desired

phase

margin.

For

applications with typical inductor values, setting

crossover frequency provides sufficient phase

following equation:

COM1

COM

C

4

C

2R

f





