Diodes ZXSC410/ZXSC420/ZXSC440 - Manual (Page 10)

ZXSC410/ZXSC420/ZXSC440
Document number: DS33618 Rev. 5 - 2
10 of 17
www.diodes.com
March 2013
© Diodes Incorporated
ZXSC410/ZXSC420/ZXSC440
Application Information
(cont.)
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
V
FB
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,
R
SENSE
and to ground. When Q1 is ‘off’, the energy stored in the inductor is transferred to the output capacitor and load via D1, forming the
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
parasitic inductance, capacitance and resistance. Also the R
SENSE
resistor should be connected, with minimum trace length, between emitter
lead of Q1 and ground, again minimizing stray parasitics.