Zxld1374, Application information – Diodes ZXLD1374 User Manual
Page 29
ZXLD1374
Document number: DS35032 Rev. 3 - 2
29 of 39
September 2012
© Diodes Incorporated
ZXLD1374
A Product Line of
Diodes Incorporated
Application Information
(cont.)
LED Current Dimming
The ZXLD1374 has 3 dimming methods for reducing the average LED current
1.
Analog dimming using the ADJ pin
2.
PWM dimming using the PWM pin
3.
Analog dimming for thermal protection using the TADJ pin.
Analog Dimming
The ZXLD1374 has a clamp on the ADJ pin to prevent over-driving of the LED current which results in the maximum voltage being applied to
internal circuitry is the reference voltage. This provides a 10:1 dynamic range of dc LED current adjustment.
The equation for DC dimming of the LED current is approximately:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
=
V
V
I
I
REF
ADJ
NOM
_
LED
DIM
_
LED
Where
I
LED_DIM
is the dimmed LED current
I
LED_NOM
is the LED current with V
ADJ
= 1.25V
One consequence of DC dimming is that as the ADJ pin voltage is reduced the sense voltage will also be reduced which has an impact on
accuracy and switching frequency especially at lower ADJ pin voltages.
0
150
300
450
600
750
0
150
300
450
600
750
0
0.25
0.5
0.75
1
1.25
ADJ VOLTAGE (V)
L
E
D CURRE
NT
(
m
A)
S
W
IT
C
H
IN
G
F
R
E
Q
UE
NCY
(k
Hz
)
Switching
Frequency
T =25°C
V
=V =12V
2 LEDs
L=33µH
R =300m
A
AUX
IN
S
Ω
Figure 36 LED Current and Switching Frequency vs. ADJ Voltage