Micrel, Inc.
MIC3291
Shutdown
Whenever a logic level LOW is applied to the DC input
pin for a period greater than or equal to
t
SHUTDOWN
(1260µs), the MIC3291 will be in power saving
shutdown mode.
Figure 5. Mode Change to Count Down
Figure 3. Shutdown Timing
Once the device is shutdown, the boost supply is
disabled and the LEDs are turned off. Brightness level
information stored in the MIC3291 prior to shutdown will
be lost.
Programming Pulse Counter Modes
At power up the MIC3291 defaults to count down mode.
In count down mode, the brightness level is decreased
one level by applying two programming pulses, as
shown in Figure 6. The counting mode can be changed
to count up mode by pulling the DC pin LOW for a period
equal to t
MODE_UP
(100µs to 160µs). The device will
remain in count up mode until its state is changed to
count down mode or by disabling the MIC3291.
Programming the Brightness Level
MIC3291 is designed to start driving the output LEDs
(t
START_UP
) in 140µs at the maximum brightness level of
15. After startup, the internal control logic is ready to
decrease the LED brightness upon receiving
programming pulses (negative edges applied to DC pin).
Since MIC3291 starts in count down mode, the
brightness level is decreased one level by applying two
programming pulses, as shown in Figure 6. Each
programming pulse has a high (t
PROG_HIGH
) and a low
(t
PROG_LOW
) pulse width that must be between 1µs to
32µs. Note that n+1 number of pulses are needed to
decrease brightness by n level(s) since the first clock
pulse is ignored. Ignoring the first clock pulse is
necessary so mode change (t
MODE_UP
, t
MODE_DOWN
) pulses
do not result in adjustments to the brightness level. The
MIC3291 internal circuit can be changed from count
down mode to count up mode and vice versa. The user
may elect to send a mode change as shown in Figure 4
to set the MIC3291 to step up the brightness level with
subsequent programming pulses. For proper operation,
ensure the DC pin has remained HIGH for at least
t
DELAY
(140µs) before issuing a mode change command.
Figure 4. Mode Change to Count up
To change the state back to count down mode, pull the
DC pin LOW for a period equal to t
MODE_DOWN
(420µs to
500µs). Now the internal circuitry will remain in count
down mode until changed to count up mode as
described previously.
Figure 6. Brightness Programming Pulses
Multiple brightness levels can be changed together with
multiple programming pulses, as shown in Figure 7.
When issuing multiple brightness level adjustment
commands to the DC pin, ensure both t
PROG_LOW
and
t
PROG_HIGH
are within 1µs and 32µs.
September 2010
10
M9999-092810
(408) 944-0800
MICREL [ MICREL SEMICONDUCTOR ]
