Zxld1371, Applications information – Diodes ZXLD1371 User Manual
Page 32
ZXLD1371
ZXLD1371
Document number: DS35436 Rev. 1 - 2
32 of 42
February 2012
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Applications Information
(cont.)
Input Capacitor
The input capacitor can be calculated knowing the input voltage ripple
ΔV
IN-PP
as follows:
Buck
Use D = 0.5 as worst case
Boost
Buck-boost
Use D = D
MAX
as worst case
The minimum RMS current for the output capacitor is calculated as follows:
Buck
use D=0.5 as worst case
Boost
Buck-boost
Use D=D
MAX
as worst case
LED Current Dimming
The ZXLD1371 has 3 dimming methods for reducing the average LED current
1. DC dimming using the ADJ pin
2. PWM dimming using the PWM pin
3. DC dimming for thermal protection using the TADJ pin.
DC Dimming
The ZXLD1371 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:
I
LED_DIM
=I
LED_NOM
V
ADJ
V
REF
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.
Figure 9. LED Current and switching frequency vs.
ADJ Voltage
PP
IN
SW
LED
IN
V
x
f
I
x
)
D
1
(
x
D
C
−
Δ
−
=
PP
IN
SW
PP
L
IN
V
x
f
x
8
I
C
−
−
Δ
Δ
=
PP
IN
V
x
SW
f
LED
I
x
D
IN
C
−
Δ
=
)
D
1
(
Dx
x
I
I
LED
RMS
CIN
−
=
−
12
I
I
PP
L
RMS
CIN
−
−
=
)
D
1
(
D
x
I
I
LED
RMS
CIN
−
=
−
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
CUR
RE
NT
(
m
A
)
S
W
IT
C
HING
F
RE
QUE
NCY
(k
H
z
)
Switching
Frequency
T =25°C
V
=V =12V
2 LEDs
L=33µH
R =300m
A
AUX
IN
S
Ω