Diodes AL8808 - Manual (Page 12)

NEW PROD
UC
T
AL8808
Document number
Applicatio
Start-up and S
On initial power
than 0.45V (typi
the output starts
This will cause s
the upper LED c
The AL8808 doe
accuracy.
Soft start can be
rail on the CTRL
The external cap
occurs it is esse
This is achieved
the control volta
Adding this capa
dimming accura
Reducing Out
Peak to peak rip
A value of 1μF
capacitor values
Note that the ca
voltage. By add
altering the mea
r: DS35648 Rev. 2
on Informat
Soft Start
up the device w
cal). This causes
s switching the LE
some additional i
current threshold
es not have in-bu
e easily impleme
L pin will charge t
pacitor slows up
ntial that the cap
d by increasing th
ge at the input of
acitor increases
cy depending on
Figure
tput Ripple
pple current in the
will reduce the
s.
apacitor will not
ing this capacito
an current value.
- 2
tion
(cont.)
will not start switc
s a slight delay (
ED current will b
nput current to th
level by slowing
uilt soft-start acti
ented by adding a
the external capa
the ramp-up of
pacitor is large en
he time taken for
f the comparator
the time taken f
n the delay introd
39. Soft Start w
e LED(s) can be
supply ripple c
affect operating
or the current wa
w
ching until the p
(dependent on ra
uild up to the upp
I
LEDS
hat of charging th
g down the rise o
ion allowing very
an external capa
acitor up to 5V.
the CTRL pin vo
nough to keep th
r the CTRL volta
r.
for the output to
duced.
with 100nF Cap
reduced, if requ
urrent significan
g frequency or e
aveform through
12 of 20
www.diodes.co
ower supply has
amp rate of inpu
per threshold lev
1 . 0
V 5 . 2
V
CTRL
SSPK
=
he input bulk cap
f the CTRL pin v
y fast control of t
acitor from the C
oltage thereby re
he CTRL pin volta
age to rise to the
o reach 90% of i
pacitor on CTRL
ired, by shunting
ntly in the typica
efficiency, but it w
the LED(s) chan
m
s reached approx
t voltage and inp
vel:
1 R
2 . 1 1 ×
pacitance. One w
voltage – implem
the output Power
CTRL pin to groun
educing the LED
age below 2.5V d
upper (turn-off)
ts final value, th
L pin (V
IN
= 24V,
g a capacitor C2
al case. Proportio
will increase sta
nges from a trian
ximately 5.6V or
put bulk capacita
way of reducing t
enting a soft-sta
r MOSFET switc
nd. The internal
current via anal
during the ramp-
threshold and by
his delay is 25µs
I
LED
= 667mA, 1
across the LED(
onally lower ripp
art-up delay, by
ngular ramp to a
r the CTRL pin v
ance of the AL88
this additional cu
rt.
ch which improve
pull-up resistor t
log dimming. To
-up of the input v
y slowing down t
s/nF, but will imp
1 LED)
(s) as shown in F
ple can be achie
reducing the rat
a more sinusoida
January 201
© Diodes Incorporate
AL8808
voltage is greate
808 circuit). Once
urrent is to reduce
es PWM dimming
to the internal 5V
ensure soft-star
voltage.
the rate of rise o
pact on the PWM
Figure 32.
eved with highe
te of rise of LED
al version withou
3
ed
er
e
e
g
V
rt
of
M
er
D
ut