MIC2289 Micrel Semiconductor, MIC2289 Datasheet - Page 5

• Manufacturer Part Number: MIC2289
• Description: 2mm 2mm White LED Driver with Internal Schottky Diode and OVP
• Manufacturer: Micrel Semiconductor

Functional Diagram
Functional Description
The MIC2289 is a constant frequency, PWM current mode
boost regulator. The block diagram is shown above. The
MIC2289 is composed of an oscillator, slope compensation
ramp generator, current amplifier, g
generator, 500mA bipolar output transistor, and Schottky
rectifier diode. The oscillator generates a 1.2MHz clock. The
clock’s two functions are to trigger the PWM generator that
turns on the output transistor and to reset the slope compen-
sation 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 compensa-
tion ramp generator. This summed current-loop signal is fed
to one of the inputs of the PWM generator.
August, 2004
MIC2289
V
VIN
REF
95mV
FB
m
OVP
error amplifier, PWM
g
m
Oscillator
1.2MHz
MIC2289 Block Diagram
5
The g
external sense resistor and amplifies the error between the
detected signal and the 95mV reference voltage. The output
of the g
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, maintain-
ing the constant frequency current-mode PWM control. The
LED is set by the feedback resistor:
The Enable pin shuts down the output switching and disables
control circuitry to reduce input current to leakage levels.
Enable pin input current is zero at zero volts.
Generator
Generator
Ramp
PWM
EN
m
I
LED
m
error amplifier measures the LED current through the
error amplifier provides the voltage-loop signal that
=
95mv
R
FB
OUT
SW
GND
M9999-081104
Micrel