MIC3291 Datasheet, PDF (11/17 Pages) Micrel Semiconductor – 1.2MHz PWM White LED Driver with Internal Schottky Diode and Single-Wire Linear Brightness Control

Micrel, Inc.
To maintain operation at the current brightness level
simply maintain a logic level high signal at the DC pin.
MIC3291
Figure 7. Decreasing Brightness Several Levels
As mentioned, MIC3291 can be programmed to set LED
drive current to produce one of 16 distinct brightness
levels. The internal logic keeps track of the brightness
level with an up/down counter circuit. The Counter
Rollover section explains how the brightness counter
functions with continued programming edges.
Counter Rollover
The MIC3291 internal up/down counter contains
registers from 0 to 15. When the brightness level is at 0
and a programming pulse forces the brightness to step
down, then the counter will rollover to Level 15. This is
illustrated in Figure 8.
Figure 9. Up Counter Rollover
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 MIC3291’s
up/down counter mode. 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.
Figure 8. Down Counter Rollover
Similarly, when the counter mode is set to count up and
a programming pulse forces the brightness level to step
up from level 15, then the counter will rollover to level 0
as illustrated in Figure 9.
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.
September 2010
11
M9999-092810
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