Diodes AL8808 - Manual (Page 9)

NEW PROD
UC
T
AL8808
Document number
Applicatio
AL8808 Opera
In normal opera
inductor L1, and
voltage and the
This rising curre
voltage to the in
When this voltag
L1, the LEDs an
of the LEDs and
This decaying c
applied at the in
This switch-on-a
LED Current C
The LED curren
connected from
with CTRL left flo
If the CTRL pin i
For example for
When the CTRL
r: DS35648 Rev. 2
on Informat
ation
ation, when volta
d the LEDs. The
inductor L1.
ent produces a vo
put of the interna
ge reaches an in
nd the schottky d
d the schottky dio
urrent produces
nput of the intern
and-off cycle con
Control
nt is controlled b
the CTRL pin to
oating. If the CT
is driven by an e
a desired LED c
L voltage is broug
- 2
tion
age is applied at
e current ramps
oltage ramp acro
al comparator.
nternally set upp
diode D1, and ba
ode.
a falling voltage
al comparator. W
tinues to provide
by the resistor R
o an internal 5V r
RL pin is left floa
xternal voltage (
current of 660mA
ght below 0.4V, t
w
+V
IN
, the AL880
up linearly, and
oss R1. The inter
per threshold, the
ack to the supply
e at R1, which is
When this voltag
e the average LE
Figure 32.
1
(in Figure 32)
regulator. When
ating or driven ab
Where
higher than 0.5V
I
L
Where
A and V
CTRL
=2.5V
= R
SET
he output switch
9 of 20
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08 internal switc
d the ramp rate
rnal circuit of the
e internal switch
y rail, but it decay
sensed by the A
e falls to the inte
ED current set by
Typical Applica
connected betw
the CTRL pin is
bove 2.5V the no
1 R
V
I TH
LED
=
V
TH
is nominally
V and lower than
R
V
V
V
T
REF
CTRL
LED
=
e V
REF
is nomina
V or with the CTR
≈ = =
66 . 0
1 . 0
I
V
LED
TH
h is turned off wh
m
ch is turned on.
is determined b
AL8808 senses
is turned off. Th
ys, with the rate
AL8808. A voltag
ernally set lower
y the sense resist
ation Circuit
ween V
IN
and SE
s left floating it ge
ominal average o
y 100mV.
2.5V), the avera
1 R
TH
ally 2.5V
RL pin left open
Ω m 150
ich allows PWM
Current starts to
by the input volta
s the voltage acro
he inductor curre
of decay determ
ge proportional to
r threshold, the in
tor R1.
ET pins. The AL8
ets pulled up to 5
utput current in t
age LED current
the resulting res
dimming.
o flow through se
age (+V
IN
) minu
oss R1 and appli
ent continues to
mined by the forw
o the sense volt
nternal switch is
8808 has an inte
5V - increasing i
the LED(s) is def
is:
istor is:
January 201
© Diodes Incorporate
AL8808
ense resistor R1
us the LED chain
ies a proportiona
flow through R1
ward voltage drop
age across R1 is
turned on again
ernal 50k resisto
its noise rejectio
fined as:
3
ed
,
n
al
,
p
s
n.
or
n