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ZXLD1371
Applications Information (cont.)
Input Capacitor
The input capacitor can be calculated knowing the input voltage ripple Δ V IN-PP as follows:
Buck
C IN =
Boost
C IN =
Buck-boost
C IN =
D x(1 ? D ) x I LED
f SW x Δ V IN ? PP
Δ I L ? PP
8 x f SW x Δ V IN ? PP
D x I LED
f SW x Δ V IN ? PP
Use D = 0.5 as worst case
Use D = D MAX as worst case
The minimum RMS current for the output capacitor is calculated as follows:
Buck
Boost
I CIN ? RMS = I LED x Dx ( 1 ? D )
use D=0.5 as worst case
I CIN ? RMS =
Buck-boost
I L ? PP
12
I CIN ? RMS = I LED x
D
( 1 ? D )
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
750
750
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
600
450
Switching
Frequency
600
450
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.
300
T A =25°C
300
150
0
V AUX =V IN =12V
2 LEDs
L=33μH
R S =300m Ω
150
0
0
0.25
0.5 0.75
1
1.25
ADJ VOLTAGE (V)
Figure 9. LED Current and switching frequency vs.
ADJ Voltage
ZXLD1371
Document number: DS35436 Rev. 1 - 2
32 of 42
www.diodes.com
February 2012
? Diodes Incorporated
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