Diodes AL8807A - Manual (Page 11)

AL8807A
Document number: DS35990 Rev. 2 - 2
11 of 20
www.diodes.com
March 2013
© Diodes Incorporated
AL8807A
Application Information
(cont.)
Analog Dimming
Further control of the LED current can be achieved by driving the CTRL pin with an external voltage lower than 2.5V; the average LED current
becomes:
SET
TH
REF
CTRL
LED
R
V
V
V
I

Where V
REF
is nominally 2.5V
The LED current decreases linearly with the CTRL voltage when
V
CTRL
≤ 2.5V, as in Figure 9 for 2 different current levels.
Note that 100% brightness setting corresponds to V
CTRL
= V
REF
,
nominally 2.5V.
If a voltage greater than 2.6V is applied to the CTRL pin an internal
clamp is activated which results in the internal reference voltage being
applied to the hysteresis control circuitry. This prevents the LED
current from being overdriven and will still set the LED current to
approximately.
SET
TH
LED
R
V
I

As the CTRL pin is reduced below 2.5V the sense voltage will proportionally decrease. This means that the time taken for the LED/Switch current
to ramp up to the upper threshold will decrease. The AL8807A, being a hysteretic converter, automatically compensates for the reduction in LED
current by reducing its lower threshold voltage and therefore its off-time. It therefore remains in continuous conduction mode maintaining a better
dimming accuracy than other peak-switch current control topologies.
A result of the reduced on- and off-times results in an increase of the switching frequency. This phenomenon can be seen in Figure 34.
Figure 34 Switching Frequency vs. V
CTRL
Ultimately at very small CTRL pin voltages the AL8807A will switch much faster than its nominal switching frequency which due to propagation
delays leads to a non-linear degrading of accuracy.
The degradation in linear dimming accuracy at small CTRL pin voltages can be improved by using larger value inductors which cause the
AL8807A to oscillate at lower frequencies.
A further cause of loss of linearity as small CTRL pin voltages is the internal offsets of the control loop; at a CTRL pin voltage of 0.25V the
nominal LED current sense voltage has been reduced to 10mV.
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
0 1 2 3 4 5
CTRL VOLTAGE (V)
Figure 33 LED Current vs. CTRL Pin Voltage
R = 100m
S

R = 150m
S

V = 12V
2 LEDs
L
IN
= 68µH
T = 25°C
A
L
E
D CUR
RE
NT
(A
)