MIC2289C_07 Datasheet, PDF (5/9 Pages) Micrel Semiconductor – White LED Driver Internal Schottky Diode and OVP

Micrel, Inc.
Functional Diagram
VIN FB
EN
OUT
MIC2289C
OVP
g_m
V_REF
95mV
PWM
Generator
S
1.2MHz
Oscillator
Ramp
Generator
MIC2289C Block Diagram
SW
GND
Functional Description
The MIC2289C is a constant frequency, PWM current
mode boost regulator. The block diagram is shown
above. The MIC2289C is composed of an oscillator,
slope compensation ramp generator, current amplifier,
gm error amplifier, PWM 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
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.
The gm error amplifier measures the LED current
through the external sense resistor and amplifies the
error between the detected signal and the 95mV
reference voltage. 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 is set by the feedback resistor:
I_LED
=
95mW
R _FB
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.
August 2007
5
M9999-081507-B