MIC3289_0706 Datasheet, PDF (8/16 Pages) Micrel Semiconductor – 1.2MHz PWM White LED Driver with Internal Schottky Diode and True 1-Wire Digital Control

Micrel, Inc.
Functional Diagram
MIC3289
MIC3289 Block Diagram
Functional Description
The MIC3289 is a constant frequency, PWM current
mode boost regulator. It is composed of an oscillator,
slope compensation ramp generator, current amplifier,
gm error amplifier, PWM generator, bipolar output
transistor, digital interface with D/A converter and
Schottky rectifier diode. It features true one-wire digital
control that may be used to vary the brightness of the
output LEDs and to place the device into shutdown
mode. The oscillator generates a 1.2MHz clock which
triggers the PWM generator that turns on the output
transistor and resets the slope compensation ramp
generator. The current amplifier is used to measure the
switch current by amplifying the voltage signal from the
internal sense resistor. The output of the current
amplifier is summed with the output of the slope
compensation ramp generator. This summed current-
loop signal is fed to one of the inputs of the PWM
generator.
MIC3289 Block Diagram
The gm error amplifier measures the LED current
through the external sense resistor and amplifies the
error between the detected signal and the reference
voltage indicated by the digital interface. The output of
the gm error amplifier provides the voltage-loop signal
that is fed to the other input of the PWM generator.
When the current-loop signal exceeds the voltage-loop
signal, the PWM generator turns off the bipolar output
transistor. The next clock period initiates the next
switching cycle, maintaining the constant frequency
current-mode PWM control. The LED current level at
maximum brightness is set by the feedback resistor:
ILED = ^250mV
RLED
MIC3289 Digital Interface
The MIC3289 incorporates an easy to use single-wire,
serial programming interface allowing users to set LED
brightness to one of 16 levels spaced in a logarithmic
manner. In contrast to other solutions requiring a PWM
drive signal to maintain LED brightness, the MIC3289 is
“set and forget”, relieving the controlling processor of the
constant burden of supplying a drive signal. Additionally,
brightness levels can be preset so that LEDs can be
turned on at a particular brightness level.
State Diagram
The MIC3289 logic state flow is depicted in Figure 1
below. Brightness level changes are negative edge
triggered while all other state changes require a logic
high or low be applied to the DC pin for a specific length
of time.
June 2007
8
M9999-061807
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