AL8807AMP-13 Diodes Inc., AL8807AMP-13 Datasheet - Page 12

• Manufacturer Part Number: AL8807AMP-13
• Description: LED Lighting Drivers High EFF DIMM 36V 1A / 1.3A Buck LED
• Manufacturer: Diodes Inc.

Specifications of AL8807AMP-13

• Rohs: yes
• Input Voltage: 6 V to 36 V
• Operating Frequency: 1 MHz
• Maximum Supply Current: 1 A
• Mounting Style: SMD/SMT
• Package / Case: MSOP-8EP

Soft Start
The AL8807A does not have in-built soft-start action; this can be seen in Figure 35.
As can been seen in Figure 37 adding a capacitor increases the time taken for the output to reach 90% of its final value.
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:
Application Information
At power–up V
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).
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,
AL8807A
Document number: DS35990 Rev. 1 - 2
CTRL
GND
1.
2.
C
DIM
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..
The rate at which the inductor/LED current is ramped up is dependent on the size of the capacitor.
50k
IN
rises exponentially, due to the bulk capacitor, the internal reference will reach 2.5V before V
Figure 36 Soft Start
2.5V
V
REF
Dimming
Analog
Figure 35 Start up without any capacitor on CTRL Pin (V
AL8807A
V
2.55V
CTRLH
(cont.)
Hysteresis
100nF
470nF
LED Current
Control
10nF
22nF
47nF
200mA/div
C
0nF
DIM
Initial Current 90%
2V/div
V
CTRL
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80mA
80mA
80mA
80mA
80mA
40mA
12 of 20
Figure 37 Soft Start with 100nF Capacitor on CTRL Pin
IN
LED Current
200mA/div
= 12V, I
Rise Time
2V/div
0.45ms
0.55ms
V
0.8ms
1.8ms
4.2ms
42ms
IN
2ms/div
LED
2V/div
V
= 667mA, 2 LEDs)
CTRL
IN
reaches the Under-Voltage Lock-
2V/div
V
AL8807A
IN
2ms/div
© Diodes Incorporated
September 2012