MIC2288 Datasheet, PDF (6/12 Pages) Micrel Semiconductor – 1A 1.2 MHZ PWM BOOST CONVERTER IN THIN SOT 23 AND 2 X MLF

MIC2288
Functional Diagram
VIN FB OVP*
EN
OVP*
g_m
V_REF
1.24V
PWM
Generator
Σ
CA
1.2MHz
Oscillator
Ramp
Generator
*OVP available on MLF^TM package option only.
Figure 1. MIC2288 Block Diagram
SW
GND
Micrel
Functional Description
The MIC2288 is a constant frequency, PWM current mode
boost regulator. The block diagram is shown in Figure 1. The
MIC2288 is composed of an oscillator, slope compensation
ramp generator, current amplifier, g_m error amplifier, PWM
generator, and a 1A bipolar output transistor. 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 g_m error amplifier measures the feedback voltage through
the external feedback resistors and amplifies the error be-
tween the detected signal and the 1.24V reference voltage.
The output of the g_m 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 sig-
nal, 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 con-
trol.
M9999-021904
6
February 2004