Diodes AL8400 /AL8400Q - Manual (Page 5)

AL8400/ AL8400Q
Document number: DS35115 Rev. 4 - 2
5 of 13
www.diodes.com
August 2012
© Diodes Incorporated
AL8400 /AL8400Q
Application Information
Description
The AL8400 Linear LED driver controller uses an external pass element to drive the LEDs and uses its FB pin to sense the LED current through
an external resistor R
SET
. The pass element is driven by the AL8400’s open collector OUT pin which allows the pass element to be either an
NPN transistor or N-channel MOSFET. An external pull-up resistor, R
B
, is required to be connected from the OUT pin to V
CC
. This resistor
supplies the output bias current of the AL8400 together with any current which the pass element requires.
In order to maintain the accuracy of the 200mV reference voltage on the FB pin the value of R
B
should be set so that the OUT pin sinks 1mA.
Stability
As with all ICs, for best stability a 0.1µF minimum (X7R ceramic) power supply decoupling capacitor, C
D
, connected between V
CC
and Ground
(See Figure 2) is recommended. C
D
should be placed as close to the V
CC
pin as possible < 5mm.
Figure 2 Application Circuit Using Bipolar Transistor
The AL8400 requires an output capacitor, C
L
in Figure 2, to be connected from the OUT pin to Ground. This capacitor is required to compensate
the current control loop of the AL8400.
This compensation capacitor must be placed as close to the OUT pin as possible < 5mm. If the PCB traces are too long, there is the possibility of
oscillation at about 5MHz. The capacitors C
D
and C
L
must be mounted immediately adjacent to the AL8400, with direct connections to OUT, E1,
GND and V
CC
. The limit of 5mm provides a good margin for stability.
The value of capacitor C
L
is determined from the value of the pull-up resistor R
B
so that:
C
L
x R
B
≥ 2ms
For example if R
B
= 1k
Ω, then C
L
must be 2µF or greater. The recommended capacitor type is X7R ceramic.
-50
0
50
100
150
200
1 10 100 1k 10k 100k 1M
Frequency (Hz)
Ga
in
(
d
B
)
0
45
90
135
180
225
P
h
a
s
e (
d
eg
)
Gain
Phas e
V
OUT
= 0.6V
C
OUT
= 2.2uF
-50
0
50
100
150
200
1 10 100 1k 10k 100k 1M
Frequency (Hz)
Ga
in
(
d
B
)
0
45
90
135
180
225
P
h
a
s
e (
d
eg
)
Gain
Phas e
Gain
Phas e
V
OUT
= 0.6V
C
OUT
= 2.2uF
Figure 3 Gain and Phase vs. Frequency with R
B
= 1k Ω and C
L
= 2.2µF
V
OUT
= 0.6V
R
B
= 1k Ω
C
L
= 2.2µF