MIC2845 Datasheet, PDF (15/21 Pages) Micrel Semiconductor – 6 Channel High Side Current Source WLED driver

Micrel Inc.
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 MIC2846 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 t_{MODE_UP} (100µs to 160µs), shown in
Figure 11. The device will remain in Count Up Mode until
its mode is changed to Count Down Mode or by
disabling the MIC2846 to reset the mode back to default.
Figure 11. Mode Change to Count Up
To change the mode back to Count Down Mode, pull the
DC pin low for a period equal to t_{MODE_DOWN} (420µs to
500µs), shown in Figure 12. Now the internal circuitry
will remain in Count Down Mode until changed to Count
Up as described previously.
MIC2845/46
tPROG_LOW
tPROG_HIGH
DC
LEVEL n
BRIGHTNESS
LEVEL
PULSE
IGNORED
LEVEL n + 1
LEVEL n - 1
Figure 13. Brightness Programming Pulses
Multiple brightness levels can be changed as shown in
Figure 14. When issuing multiple brightness level
adjustments to the DC pin, ensure both t_{PROG_LOW} and
t_{PROG_HIGH} are within 1µs and 32µs.
To maintain operation at the current brightness level
simply maintain a logic level high at the DC pin.
Figure 12. Mode Change to Count Down
Programming the Brightness Level
MIC2846 is designed to start driving the WLEDs 140µs
(t_{START_UP}) after the DC pin is first pulled high at the
maximum brightness level of 31. After start up, the
internal control logic is ready to decrease the WLED
brightness upon receiving programming pulses (negative
edges applied to DC pin). Since MIC2846 starts in Count
Down Mode, the brightness level is decreased one level
by applying two programming pulses, as shown in Figure
13. Note that the extra pulse is needed to decrease
brightness because the first edge is ignored. Anytime the
first falling edge happens greater than 32µs after a Mode
Change, it will be ignored. Ignoring the first falling edge
is necessary in order that Mode Change (t_{MODE_UP},
t_{MODE_DOWN}) pulses do not result in adjustments to the
brightness level. Each programming pulse has a high
(t_{PROG_HIGH}) and a low (t_{PROG_LOW}) pulse width that must
be between 1µs to 32µs. The MIC2846 will remember
the brightness level and mode it was changed to. For
proper operation, ensure that the DC pin has remained
high for at least t_DELAY(140µs) before issuing a mode
change command.
Figure 14. Decreasing Brightness Several Levels
As mentioned, MIC2846 can be programmed to set
WLED drive current to produce one of 32 distinct
brightness levels. The internal logic keeps track of the
brightness level with an Up/Down counter circuit. The
following section explains how the brightness counter
functions with continued programming edges.
One-Step Brightness Changes
The “One-Step” brightness change procedure relieves
the user from keeping track of the MIC2846’s up/down
counter mode. It combines a Mode Change with a
programming edge; therefore, regardless of the previous
Count Mode, it will change the brightness level by one.
January 2008
15
M9999-010808-A