MIC2845 Datasheet, PDF (14/21 Pages) Micrel Semiconductor – 6 Channel High Side Current Source WLED driver

Micrel Inc.
MIC2845/46
MIC2846 Digital Dimming Interface
The MIC2846 incorporates an easy to use single-wire,
serial programming interface that allows users to set
WLED brightness up to 32 different levels, as shown in
Table 1.
Level
(0-31)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
LED Duty
Cycle (%)
100
86
72
59
45.5
36.5
29.5
22.5
18
13.5
9.5
8
6
5
4
1.6
1.6
4
5
6
8
9.5
13.5
18
22.5
29.5
36.5
45.5
59
72
86
100
Average
I_LED (mA)
12
10.32
8.6
7.1
5.5
4.4
3.5
2.7
2.2
1.6
1.1
0.96
0.72
0.6
0.48
0.192
0.32
0.8
1
1.2
1.6
1.9
2.7
3.6
4.5
5.9
7.3
9.1
11.8
14.4
17.2
20
I_PEAK (mA)
60% of I_LEDPEAK
(12mA for R_SET =
20.5k)
100% of I_LEDPEAK
(20mA for R_SET =
20.5k)
Table 1. Digital Interface Brightness Level Table
Brightness levels 0-15 is logarithmically spaced with a
peak current equal to 60% of the current set by R_SET.
Brightness levels 16-31 is also logarithmically spaced
with a peak current equal to the current determined by
R_SET. Spacing between each level is in logarithmic scale
January 2008
by design to mimic the sensitivity of the human eye.
Refer to Table 1 for the translation from brightness level
to LED duty cycle and current. The MIC2846 is designed
to receive programming pulses to increase or decrease
brightness. Once the brightness change signal is
received, the DC pin is simply pulled high to maintain the
brightness. This “set and forget” feature relieves
processor computing power by eliminating the need to
constantly send a PWM signal to the dimming pin. With
a digital control interface, brightness levels can also be
preset so that WLEDs can be turned on at any particular
brightness level.
Start Up
Assuming the MIC2846 has been off for a long time and
no presetting brightness command is issued (presetting
is discussed in a later section), the MIC2846 will start-up
in its default mode approximately 140µs (t_{START_UP}) after
a logic level high is applied to the DC pin. In the default
mode the WLEDs are turned on at the maximum
brightness level of 31. Each falling edges during the
t_{PROG_SETUP} period will cause the default brightness level
to decrease by one. This is discussed in more detail in
the Presetting Brightness section.
Figure 9. Typical Start-Up Timing
Shutdown
Whenever the DC input pin is pulled low for a period
greater than or equal to t_SHUTDOWN(1260µs), the MIC2846
will be in shutdown, shown in Figure 10.
Figure 10. Shutdown Timing
Once the device is shutdown, the control circuit supply is
disabled and the WLEDs are turned off, drawing only
0.01µA. Brightness level information stored in the
MIC2846 prior to shutdown will be erased.
Count Up Mode/Count Down Mode
The mode of MIC2846 can be in either Count Up Mode
or Count Down Mode. The Counting Modes determine
what the falling edges of the programming pulses will do
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