MIC2619 Datasheet, PDF (8/15 Pages) Micrel Semiconductor – 1.2MHz PWM Boost Converter with OVP

Micrel, Inc.
Functional Diagram
MIC2619
MIC2619 Block Diagram
Functional Description
The MIC2619 is a constant frequency, PWM current
mode boost regulator. It is composed of an oscillator,
slope compensation ramp generator, current amplifier,
g_m error amplifier, PWM generator, and bipolar output
transistor. The oscillator generates a 1.2MHz clock
which triggers the PWM generator to turn on the output
transistor and resets the slope compensation ramp
generator. The current amplifier is used to measure
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 then 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 between the detected signal and the 1.265V
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 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.
VIN
VIN provides power to the control and reference circuitry
as well as the switch mode regulator MOSFETs. Due to
the high speed switching, a 1µF capacitor is
recommended as close as possible to the VIN and GND
pin.
EN
The enable pin provides a logic level control of the
output. In the off state, supply current of the device is
greatly reduced (typically <0.1µA). Also, in the off state,
the output drive is placed in a “tri-stated” condition,
where the bipolar output transistor is in an “off” state or
non-conducting state.
OVP
The OVP pin provides over-voltage protection on the
output of the MIC2619. When the OVP circuit is tripped,
the output voltage remains at the set OVP voltage.
Because the OVP circuit operates at a lower frequency
than the feedback circuit, output ripple will be higher
while in an OVP state. OVP requires a resistor divider
network to the output and GND to set the OVP voltage.
If the output voltage overshoots the set OVP voltage,
then the MIC2619 OVP circuit will shut off the switch;
saving itself and other sensitive circuitry downstream.
The accuracy of the OVP pin is ±5% and therefore
should be set above the output voltage to ensure noise
or other variations will not cause a false triggering of the
OVP circuit.
March 2010
8
M9999-030410-A