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MIC2844AYMT View Datasheet(PDF) - Micrel

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MIC2844AYMT Datasheet PDF : 14 Pages
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Micrel Inc.
Figure 4. LED Current with Brightness Level Change
MIC2844A
Start Up
Assuming the MIC2844A has been off for a long time
and no presetting brightness command is issued
(presetting is discussed in a later section), the
MIC2844A will start-up in its default mode approximately
140µs (tSTART_UP) after a logic level high is applied to the
DC pin, shown in Figure 6. In the default mode the LEDs
are turned on at the maximum brightness level of 48.
Each falling edge during the tPROG_SETUP period will cause
the default brightness level to decrease by one. This is
discussed in more detail in the Presetting Brightness
section.
Figure 5. LED Current 60° Phase Shift
Digital Dimming Interface
The MIC2844A incorporates an easy to use single-wire,
serial programming interface that allows users to set
LED brightness up to 49 different levels, as shown in the
table1.
Brightness levels 0 through 15 are logarithmically
spaced with a peak current equal to 60% of the current
programmed by RSET. Brightness level 16 is provided for
applications that want to “fade to black” with no current
flowing through the LEDs. Brightness Level 17 has the
same duty cycle as level 18, but the peak current is only
60% of the current set by RSET; therefore, the average
current is 0.1mA. Brightness levels 18 through 48 are
also logarithmically spaced, but the peak current is equal
to 100% of the current determined by RSET. Refer to
Table 1 for the translation from brightness level to
average LED duty cycle and current. The MIC2844A 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 LEDs can be turned on at any particular
brightness level.
Figure 6. Typical Start-Up Timing
Shutdown
Whenever the DC input pin is pulled low for a period
greater than or equal to tSHUTDOWN (1260µs), the
MIC2844A will be shutdown as shown in Figure 7.
Figure 7. Shutdown Timing
Once the device is shutdown, the control circuit supply is
disabled and the LEDs are turned off, drawing only
0.01µA. Brightness level information stored in the
MIC2844A prior to shutdown will be erased.
Count Up Mode/Count Down Mode
The mode of MIC2844A can be in either Count Up Mode
or Count Down Mode. The Count Down/Up Modes
determine what the falling edges of the programming
pulses will do to the brightness. In Count Up Mode,
subsequent falling edges will increase brightness while
in Count Down Mode, subsequent falling edges will
decrease brightness. By default, the MIC2844A is in
Count Down Mode when first turned on. The counting
mode can be changed to Count Up Mode, by pulling the
DC pin low for a period equal to tMODE_UP (100µs to
160µs), shown in Figure 8. The device will remain in
August 2009
9
M9999-080309-D
 

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