MIC3291 Datasheet, PDF (9/17 Pages) Micrel Semiconductor – 1.2MHz PWM White LED Driver with Internal Schottky Diode and Single-Wire Linear Brightness Control

Micrel, Inc.
Functional Description
The MIC3291 is a constant frequency, PWM current
mode boost LED driver. 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 single-wire linear
digital control that may be used to vary the brightness of
the output LEDs. 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.
MIC3291 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
MIC3291 Digital Interface
The MIC3291 incorporates an easy to use single-wire,
serial programming interface allowing users to set LED
brightness to one of sixteen levels spaced in a linear
manner. In contrast to other solutions requiring a PWM
drive signal to maintain LED brightness, the MIC3291 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.
MIC3291
Brightness Level
FB Voltage (mV)
15
250
14
235
13
220
12
205
11
190
10
175
9
160
8
145
7
130
6
115
5
100
4
85
3
70
2
55
1
40
0
25
Table 1. Brightness Level vs. Ideal FB Voltage
Startup
Presuming no presetting brightness command is issued
(discussed in a later section), the MIC3291 will startup in
its default state approximately 140µs (t_{START_UP}) after a
logic level high has been applied and maintained at the
DC pin. In the default state the LED drive current is at
the maximum brightness level of 15 and brightness
counter is set to count down mode. Any falling edges
during the t_{PROG_SETUP} period will cause the initial
brightness level of the LEDs to be below the maximum
brightness level. This is discussed in more detail in the
Presetting Brightness section.
Figure 2. Typical Startup Timing
September 2010
9
M9999-092810
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