BL8581A
DETAILED DESCRIPTION
BL8581A works with a wide range of supply
voltage, from 2.7V to 5.5V. The forward
voltage of commercial white/blue LED is in
the range of 2.9V to 3.5V at a current level of
20mA. Proper selection of the LED to match
the supply voltage can fully utilize the Li-ion
battery. For example, there is 1% ~ 3 %
power left in the Li-ion battery when its
voltage reaches 3.275V. So a LED with a
forward voltage value of 3.2V can use up to
99% of the battery power under normal
working condition. When the voltage of the
battery drops below 3.2V, the current through
the LED (hence the brightness) starts to
decrease.
Startup sequence
BL8581A starts to work only when VCC
powered on and the EN pin became high
after a delay, the delay time is at least 4us.
EN pin is not allowed to connect to VCC
directly or keep it floating.
PWM dimming control
If the frequency of the PWM signal to the EN
pin is less than 5KHz, the average LED
current is proportional to the duty cycle of the
PWM signal, and the EN works as a simple
on/off control. A high level turns on and a low
shuts down the LEDs. There is a delay time
between the input PWM signal and the output
LED current waveform, as shown in Fig.1, the
T
D
is about 20uS.
ON duty
pulse width wider than 20uS is regarded as a
signal to stop the build-in counter. The time
between two wide high levels (more than
20uS) is a counting period. When some rise
edges occur between two high level pulses of
20us, the equation as shown below can
calculate the active pulse number, called
Code_No.
Code
_
No
=
Mod
(
N
,16)
,
In the equation, N is the number of rise edges
of the input waveform and mod function
returns the remainder of N divided by 16. For
example, if N=0, 16, or other integral multiple
of 16, the mod function returns a value of 0, if
N=1, 17 or other integral multiple of 16 plus 1,
the mod function returns a value of 1, and so
on.
The relationship between LEDs’ brightness
and the Code_No is listed as the following
table.
Code No
0
1
2
3
4
5
6
7
I
SET
/20mA
0
15/15
14/15
13/15
12/15
11/15
10/15
9/15
Code No
8
9
10
11
12
13
14
15
I
SET
/20mA
8/15
7/15
6/15
5/15
4/15
3/15
2/15
1/15
EN
Td=20uS
Td=20uS
20mA
The frequency of the input pulse should be
lower than 100KHz, and the width of the high
level and low level larger than 4uS to prevent
false trigger.
Code_No=1
Code_No=4
I_LED
Fig. 1 PWM Dimming Control
Single-wire dimming control
The LEDs’ brightness can also be controlled
by the pulses applied to the EN pin, Refer to
the Fig. 2. Inside the device there is a 4-bit
counter connecting to the EN pin. The LED
current can be programmed up to 16 levels
depends on the number of rise edges of the
input waveform. A high level input (“1”) with
EN
20uS
I_LE
D
20uS Iset/20mA=15/15
Iset/20mA=12/15
Fig. 2 Single-Wire Dimming Control
www.belling.com.cn
4
BELLING [ SHANGHAI BELLING CO., LTD. ]
