ZXLD1370

ZXLD1370

Document number: DS32165 Rev. 2 - 2

21 of 33

www.diodes.com

May 2010

© Diodes Incorporated

A Product Line of

Diodes Incorporated

Buck

12

I

I

PP

LED

RMS

COUTPUT

−

−

=

Boost and Buck-boost

MAX

MAX

LED

RMS

COUTPUT

D

1

D

I

I

−

=

−

Ceramic capacitors with X7R dielectric are the best choice due to their high ripple current rating, long lifetime, and

performance over the voltage and temperature ranges.

INPUT CAPACITOR

The input capacitor can be calculated knowing the input voltage ripple

Buck

PP

IN

SW

LED

IN

V

x

f

I

x

)

D

1

(

x

D

C

−

Δ

−

=

Use D = 0.5 as worst case

Boost

PP

IN

SW

PP

L

IN

V

x

f

x

8

I

C

−

−

Δ

Δ

=

Buck-boost

PP

IN

SW

LED

IN

V

x

f

I

x

D

C

−

Δ

=

The minimum RMS current for the output capacitor is calculated as follows:

Buck

)

D

1

(

Dx

x

I

I

LED

RMS

CIN

−

=

−

use D=0.5 as worst case

Boost

12

I

I

PP

L

RMS

CIN

−

−

=

Buck-boost

)

D

1

(

D

x

I

I

LED

RMS

CIN

−

=

−

PWM OUTPUT CURRENT CONTROL & DIMMING

The ZXLD1370 has a dedicated PWM dimming input that allows a wide dimming frequency range from 100Hz to 1kHz with

up to 1000:1 resolution; however higher dimming frequencies can be used – at the expense of dimming dynamic range and

accuracy.

Typically, for a PWM frequency of 1kHz, the error on the current linearity is lower than 5%; in particular the accuracy is

better than 1% for PWM from 5% to 100%. This is shown in the graph below: