al8807 Diodes, Inc., al8807 Datasheet - Page 9

• Manufacturer Part Number: al8807
• Description: High Efficiency Low Emi 30v 1a Buck Led Driver
• Manufacturer: Diodes, Inc.

Applicatio
AL8807 Ope
In normal oper
sense resistor
voltage +Vin a
This rising cur
applies a prop
When this volt
to flow through
determined by
This decaying
voltage across
threshold, the
set by the sens
LED Current
The LED curre
Connected bet
For example fo
DC Dimming
Further contro
lower than 2.5V
The LED curre
levels. When t
Note that 100%
that is higher
AL8807
Document Numbe
I
LED
=
R
V
THD
SET
I
R
I
LED
LED
SET
on Informat
.
eration
and the inducto
y the forward vo
r R
rrent produces
ent is controlled
T
g
h R
tage reaches a
or a desired LE
s R
se resistor R
t Control
tween V
ent decreases
% brightness s
ration, when vo
ortional voltage
=
=
l of the LED cu
V); the average
=
=
the CTRL volta
r: DS35281 Rev. 3
internal switch
than V
current produc
=
R
V
V
1
V
1
THD
V
SET
CTRL
, inductor L
1
I
, L1, the L
LED
REF
THD
is applie
REF
IN
V
R
V
V
and
CTR
, the
THD
SET
REF
1
ED current of 66
an internally se
h is turned on a
.
urrent can be a
oltage drop of t
d by the resisto
F
age falls below
setting corresp
L1, and the LE
3 - 2
tion
e to the input o
oltage is applie
ces a falling vo
e LED current
e device will n
r L1.
EDs and the s
d at the input
L
a voltage ram
SET the nomin
linearly with th
=
. 0
0
66
1 .
2
2
5 .
5 .
EDs. The curre
of the internal c
et upper thresh
or R
≈
≈
not overdrive th
the LEDs and t
ed at +V
t of the interna
oltage at R
nal average ou
again. This sw
60mA and a de
he CTRL volta
p across R
chottky diode D
chieved by driv
becomes:
onds to V
Fig. 30 Typic
the threshold,
150
1
HIGH E
m
in Figure
Ω
IN
CTRL
, the
1
1
, w
. T
ww
witch-on-and-off
e AL8807 intern
ent ramps up li
comparator.
which is sensed
utput current in
The internal cir
age when 0.5V
L
ww.diodes.com
the schottky dio
efault voltage V
cal Applicati
ving the CTRL
FFICIENCY
D1, and back t
he LED curren
al comparator
old, the interna
30.
0.4V, the outp
= V
9 of 20
REF
, nomi
V ≤ V
put switch is tur
V
Y LOW EM
ode.
nt and will still
nal switch is tu
al switch is tur
to the supply ra
d by the AL880
inally 2.5V. Fo
cuit of the AL8
. When this v
f cycle continu
nearly, and the
on Circuit
the LED(s) is d
pin with an ext
CTRL
CTRL
=2.5V the
≤ 2.5
MI 30V 1A B
ues to provide t
urned on. Curre
8807 senses th
rned off. The in
e resulting resis
rned off which a
e ramp rate is
ail, but it decay
07. A voltage p
defined as:
ternal voltage (
oltage falls to
V, as in figure
r any voltage a
set the LED
BUCK LED
e 2 for 4 differe
he voltage acro
nductor current
(higher than 0.4
ent starts to flo
ys, with the rat
proportional to
the average LE
stor is:
allows PWM di
applied on the
current to app
determined by
the internally
AL
© Diodes
Nove
L8807
proximately
ow through
oss R
te of decay
t continues
ent current
y the input
ED current
4V and
imming.
mber 2011
s Incorporated
DRIVER
the sense
CTRL pin
set lower
1
and