MIC2253_11 Datasheet, PDF (7/13 Pages) Micrel Semiconductor – 3.5A 1MHz High Efficiency Boost Regulator with OVP and Softstart

Micrel, Inc.
Functional Diagram
MIC2253
Functional Description
The MIC2253 is a constant frequency, pulse-width-
modulated (PWM) peak current-mode step-up regulator.
The device’s simplified control scheme is illustrated in the
block diagram above. A reference voltage is fed into the
PWM engine where the duty cycle output of the constant
frequency PWM engine is computed from the error, or
difference, between the REF and FB voltages. The PWM
engine encompasses the necessary circuit blocks to
implement a current-mode boost switching power supply.
The necessary circuit blocks include, but are not limited to,
an oscillator/ramp generator, slope compensation ramp
generator, g_m error amplifier, current amplifier, PWM
comparator, and drive logic for the internal 3.5A bipolar
power transistor.
Inside the PWM engine, the oscillator functions as a trigger
for the PWM comparator that turns on the bipolar power
transistor and resets the slope compensation ramp
generator. The current amplifier is used to measure the
power transistor’s current by amplifying the voltage signal
from the sense resistor connected to the emitter of the
bipolar power transistor. The output of the current amplifier
is summed with the output of the slope compensation ramp
generator where the result is connected to one of the inputs
of the PWM comparator.
The g_m error amplifier measures the feedback voltage
through the external resistor and amplifies the error
between the detected voltage signal from the feedback
and the internal 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 comparator. When the
current loop signal exceeds the voltage loop signal the
PWM comparator turns off the power transistor. The next
oscillator/clock period initiates the next switching cycle,
maintaining the constant frequency current-mode PWM
control. The enable pin shuts down the output switching
and disables control circuitry to reduce input current-to-
leakage levels. Enable pin input current is approximately
zero, at zero volts.
DC-to-DC PWM Boost Conversion
The MIC2253 is a constant-frequency boost converter. It
can convert a low DC input voltage to a high DC output
voltage. Figure 1 shows a typical circuit. Boost regulation
is achieved by turning on an internal switch, which draws
current through the inductor. When the switch turns off,
the inductor’s magnetic field collapses. This causes the
current to be discharged into the output capacitor
through an external Schottky diode. The Functional
Characteristics show Input Voltage ripple, Output
Voltage ripple, SW Voltage, and Inductor Current for
300mA load current. Regulation is achieved by
modulating the pulse width i.e., pulse-width modulation
(PWM).
April 2011
7
M9999-042011-C