MIC3289_0706 Datasheet, PDF (11/16 Pages) Micrel Semiconductor – 1.2MHz PWM White LED Driver with Internal Schottky Diode and True 1-Wire Digital Control

Micrel, Inc.
DC:
Edge Ignored
UP
COUNT
15
10
10 11 12 13 14 15 0 1 2 3
4
OUTPUT
LEVEL
0
Figure 9. Up Counter Roll-over
One-Step Brightness Changes
For applications where a keypad button press is to be
translated into a brightness level change, the following
method of decreasing the brightness level may be
useful. This “One-Step” brightness change procedure
relieves the user from keeping track of the MIC3289’s
up/down counter state. It combines a counter mode
change with a programming pulse, therefore a one-step
decrease in brightness is assured no matter what the
previous up/down counter mode was.
MIC3289
Figure 11. One-Step Brightness Increase
Presetting Brightness
The brightness level can be preset before the MIC3289
begins to drive the LEDs by sending a series of
programming edges via the DC pin during the t_{START_UP}
(140µs) period and within 35µs to 50µs after the DC pin
is pulled high. The 15µs timeframe between 35µs and
50µs is the t_{PROG_SETUP} period. The MIC3289 does not
drive current into the load until DC pin is kept high for
t_{START_UP} (140µs) after presetting has concluded in order
to grant the user sufficient time to preset LED
brightness. The first presetting pulse edge must occur
somewhere between the timeframe of 35µs to 50µs after
DC pin is first pulled HIGH otherwise the MIC3289 may
continue to start at the full (default) brightness level.
Figure 10. One-Step Brightness Decrease
This method is quite simple and the only requirement is
that the first DC low period be equal to the t_{MODE_DOWN}
(420µs to 500µs) and immediately followed by a falling
edge within t_{PROG_HIGH} (1µs to 32µs) as shown in Figure
10 for One-Step Brightness Decrease. Similarly a one-
step increase can be assured by first generating a DC
down pulse whose period is equal to the t_{MODE_UP} (100µs
to 160µs) and immediately followed by a falling edge
within t_{PROG_HIGH} (1µs to 32µs). Figure 11 illustrates the
proper timing for execution of a One-Step Brightness
Increase.
Figure 12. Presetting Timing
Figure 12 shows the correct presetting sequence to set
the MIC3289 brightness to level 6 prior to start up. The
sequence is initiated by driving the DC pin low for a
period exceeding t_SHUTDOWN (1260µs) to insure that the
part has entered the power saving shutdown state
erasing all brightness level state and mode setting. Then
the DC pin is driven high and the first presetting pulse
edge is entered within the t_{PROG_SETUP} window. Notice
that when using the presetting feature the first
programming pulse is not ignored. This is because the
counter’s default mode is Count Down and a Mode
Change cannot be performed in the presetting mode.
(Note that the same timing requirements of standard
brightness programming also apply during presetting
brightness.)
June 2007
11
M9999-061807
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