Diodes AL8807B - Manual (Page 14)

AL88070B
Document number: DS36191 Rev. 1 - 2
14 of 18
www.diodes.com
March 2014
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AL8807B
Applications Information
(cont.)
Figure 39, shows the power derating of the AL8807BMP on an FR4 51x51mm PCB with 2oz copper standing in still air. Changing the PCB
dimensions, material, amount of metal associated with the thermal and other PCB components will change the AL8807BMP’s junction-ambient
thermal impedance.
Figure 39 Derating Curve for Different PCB
Soft-Start
The AL8807B does not have in-built soft-start action; this can be seen in Figure 40.
Figure 40 LED Current Start-up (VIN = 12V, ILED = 667mA, 2 LEDs)
At power–up V
IN
rises exponentially, due to the bulk capacitor, the internal reference will reach 2.5V before V
IN
reaches the Under-Voltage Lock-
Out turn-on threshold at around 5.6V. This causes the CTRL pin voltage to rise and reaches 2.5V – 100% LED current - before the AL8807B
fully turns on. When the AL8807B turns on, its output switch turns on causing the inductor current to increase until it reaches the upper threshold
of the sense current level and the switching process begins.
As the CTRL pin only has PWM functionality, placing a capacitor on the CTRL pin will have no effect on the ramp-up of the LED current; the
capacitor will just delay the ramp-up of the LED current and delay/extend the ramp-down of the LED current.
If some form of extra soft-start is required then the AL8807, AL8807A or PAM2861 should be considered.
MSOP-8EP
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