New prod uc t al8807b, Application information, Al8807b – Diodes AL8807B User Manual
Page 9: 6 ~ 36v
AL88070B
Document number: DS36191 Rev. 1 - 2
9 of 18
March 2014
© Diodes Incorporated
NEW PROD
UC
T
AL8807B
Application Information
The AL8807B is a hysteretic (also known as equal ripple) LED driver with integrated power switch. It is available in two packages that provide a
PCB area-power dissipation capability compromise. It is recommended that at higher LED currents/smaller PCBs that the MSOP-8EP version is
used to maximize the allowable LED current over a wider ambient temperature range.
AL8807B Operation
In normal operation, when voltage is applied at +V
IN
, the AL8807B internal switch is turned on. Current starts to flow through sense resistor R
1
,
inductor L1, and the LEDs. The current ramps up linearly, and the ramp rate is determined by the input voltage +Vin and the inductor L1.
This rising current produces a voltage ramp across R
1
. The internal circuit of the AL8807B senses the voltage across R
1
and applies a
proportional voltage to the input of the internal comparator.
When this voltage reaches an internally set upper threshold, the internal switch is turned off. The inductor current continues to flow through R
1
, L1,
the LEDs and the Schottky diode D1, and back to the supply rail, but it decays, with the rate of decay determined by the forward voltage drop of
the LEDs and the Schottky diode.
This decaying current produces a falling voltage at R
1
, which is sensed by the AL8807B. A voltage proportional to the sense voltage across R
1
is
applied at the input of the internal comparator. When this voltage falls to the internally set lower threshold, the internal switch is turned on again.
This switch-on-and-off cycle continues to provide the average LED current set by the sense resistor R
1
.
LED Current Control
The LED current is controlled by the resistor R1 in Figure 31.
Figure 31 Typical Application Circuit
Figure 32 Typical Operating Waveform (C2 not fitted)
Connected between V
IN
and SET the nominal average output current in the LED(s) is defined as:
1
R
V
I
THD
LED
For example for a desired LED current of 660mA and a default voltage V
CTRL
=2.5V the resulting resistor is:
m
150
66
.
0
1
.
0
I
V
1
R
LED
THD
AL8807B
V
IN
SE
T
SW
CT
RL
GND
C2
R1
C1
D1
L1
PWM Dimming
Input:
V
IN
:
6 ~ 36V
V
IN
= 12V
T
A
=25ºC
2 LEDs
20ns/div
No C2
Ch2: 2V/div
Ch4: 100mA/div
Ch4: LED Current
Ch2: SW Pin