Diodes AP3608E - Manual (Page 9)

EIGHT-CHANNEL CONSTANT CURRENT SINK WITH CURRENT MATCH AP3608E
9
Mar. 2011 Rev. 1. 5 BCD Semiconductor Manufacturing Limited
Data Sheet
Function Description
The AP3608E is designed for LED display application
which contains eight well-matched current sinks to
provide constant current through LED. The full scale
LED current can be adjusted from 10mA to 100mA
per channel with an external resistor. If there is some
channel unused, the channel pin should be connected
to ground. The LED bright dimming can be achieved
through PWM dimming.
The AP3608E can work with a DC/DC converter to
drive LED arrays for good performance. The device
can keep working when LED opens without damage,
and it features under voltage lockout protection and
over temperature protection. The detailed information
will be discussed in open LED self-check and
protection section.
1. LED Current Setting
The maximum LED current can be set up to 100mA
per channel by ISET pin. When the LED current is
greater than 100mA, two or more channels can be par-
alleled to achieve larger drive current. To set the
reference current I
SET
, connect a resistor R
ISET
between this pin and ground. The value of R
ISET
can
be calculated by the following formula:
This reference current is multiplied internally with a
gain (K) of 400, and then mirrored on all enabled
channels. This sets the maximum LED current,
referred to as 100% current (I
CHX_MAX
). The value
can be calculated by the following formula:
The LED current can be reduced from 100% by PWM
dimming control.
2. PWM Dimming Mode
The LED current can be adjusted by applying the
PWM signal to PWM pin. On this mode, all enabled
channels are adjusted at the same time and the
brightness can be adjusted from 1%*I
CHX_MAX
to
100%*I
CHX_MAX
(@f
Dimming
=2kHz). During the
"high level" time of the PWM signal, the LED turns
on and the 100% current flows through LED. During
the "low level" time of the PWM signal, the LED
turns off and almost no current flows through LED. So
the average current through LED is changed and the
brightness is adjusted. The external PWM signal
frequency applied to PWM pin can be allowed to
100Hz or higher.
An example for PWM dimming is shown in Figure 10.
All 8 channels are set to the maximum current
I
CHX_MAX
at the beginning. When a 50% duty cycle
PWM signal is applied to PWM pin, average current
valued 50%* I
CHX_MAX
flows through the 8 channels.
When an 80% duty cycle PWM signal is applied to
PWM pin, average current valued 80%*I
CHX_MAX
flows through the 8 channels.
ISET SET
R V 194 1 I / . =
SET MAX CH
I K I ⋅ =
X_
Figure 10. PWM Dimming Example of AP3608E
PWM
50% duty cycle
CH1...CH8
Current
80% duty cycle
0 0
I
CH_MAX
I
CH_MAX
I
CH_MAX