MIC3291 datasheet(9/17 Pages) MICREL

9 / 17 page

Micrel, Inc.

MIC3291

September 2010

M9999-092810

(408) 944-0800

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:

LED

250mV

ILED

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.

Brightness Level

FB Voltage (mV)

250

235

220

205

190

175

160

145

130

115

100

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 (tSTART_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 tPROG_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

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