Al8807a, Application information – Diodes AL8807A User Manual
Page 12
AL8807A
Document number: DS35990 Rev. 2 - 2
12 of 20
March 2013
© Diodes Incorporated
AL8807A
Application Information
(cont.)
Soft Start
The AL8807A does not have in-built soft-start action; this can be seen in Figure 35.
Figure 35 Start up without any capacitor on CTRL Pin (V
IN
= 12V, I
LED
= 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 AL8807A
fully turns on. When the AL8807A turns on, its output switch turns causing the inductor current to increase until it reaches the upper threshold of
the sense current level and the switching process begins.
Adding an external capacitor from the CTRL pin to ground will provide a soft-start delay (see Figures 36 and 37).
Figure 36 Soft Start
Figure 37 Soft Start with 100nF Capacitor on CTRL Pin
Adding a capacitor to the CTRL pin provides a soft-start by increasing the time taken for the CTRL voltage to rise to 2.5V and by slowing down
the rate of rise of the control voltage at the input of the comparator in the hysteresis control block (refer to Figure 36). This capacitor has 2
effects:
1.
It reduces the minimum start-up current. The bigger the capacitor the lower the CTRL pin voltage will be when UVLO level is
exceeded and the output switch turns on..
2.
The rate at which the inductor/LED current is ramped up is dependent on the size of the capacitor.
As can been seen in Figure 37 adding a capacitor increases the time taken for the output to reach 90% of its final value.
There are many factors which set the initial current and ramp rate. Some practical examples are shown below with conditions
Vin 12V L=68uH 2 LEDs at 667mA,
C
DIM
Initial Current 90%
Rise Time
0nF 80mA 0.45ms
10nF 80mA 0.55ms
22nF 80mA 0.8ms
47nF 80mA 1.8ms
100nF 80mA
4.2ms
470nF 40mA
42ms