ZXSC410/ZXSC420/ZXSC440

Document number: DS33618 Rev. 5 - 2

10 of 17

www.diodes.com

March 2013

© Diodes Incorporated

ZXSC410/ZXSC420/ZXSC440

Dimming Control using a Logic Signal

For applications where the LED current needs to be adjusted in

discrete steps a logic signal can be applied as shown in the diagram

below.

When Q1 is ‘off’, R1 sets the minimum LED current. When Q1 is ‘on’,

R2 sets the LED current that will be added to the minimum LED

current. The formula for selecting values for R1 and R2 are given

below:

MOSFET ‘off’

1

FB

)

MIN

(

LED

R

V

I



I

R2

R1

LOGIC

SIGNAL

Q1

2N7002

MOSFET ‘on’

)

MIN

(

LED

2

FB

)

MAX

(

LED

I

R

V

I





Layout Issues

Layout is critical for the circuit to function in the most efficient manner in terms of electrical efficiency, thermal considerations and noise.

For ‘step-up converters’ there are four main current loops, the input loop, power-switch loop, rectifier loop and output loop. The supply charging

the input capacitor forms the input loop. The power-switch loop is defined when Q1 is ‘on’, current flows from the input through the inductor, Q1,

rectifier loop. The output loop is formed by the output capacitor supplying the load when Q1 is switched back off.

To optimise for best performance each of these loops kept separate from each other and interconnected with short, thick traces thus minimising

lead of Q1 and ground, again minimizing stray parasitics.