MIC2224 Datasheet, PDF (10/13 Pages) Micrel Semiconductor – High Efficiency 2MHz PWM Buck Converter with DAC Input and 40mOHM Bypass Switch

Micrel
MIC2224
Functional Description
VIN
VIN provides power to the MOSFETs for the switch
mode regulator section, along with the current
limiting sensing. Due to the high switching speeds, a
1µF capacitor is recommended close to VIN and the
power ground (PGND) pin for bypassing.
AVIN
Analog V_IN (AVIN) provides power to the internal
reference and control section. AVIN and VIN must
be tied together. Careful layout should be
considered to ensure high frequency switching noise
caused by VIN is reduced before reaching AVIN.
DAC
The DAC pin is the control pin that sets the output
voltage. The Output voltage is 3X the voltage set on
the DAC pin. When 1.2V or greater is applied to the
DAC pin, the MIC2224 enters bypass mode. In
bypass mode, the input supply is connected to the
output through a 40mOhm P-Channel MOSFET.
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 <1µA). Also, in the off
state, the output drive and bypass switch are placed
in a "tri-stated" condition, where both the high side
P-channel Mosfet and the low-side N-channel are in
an off or non-conducting state. Do not drive the
enable pin above the supply voltage.
VOUT
The VOUT pin connects the internal bypass drain
and the feedback signal to the output. The bypass
applies the input voltage through a low resistance
(40mOhm typical) P-Channel MOSFET switch.
The feedback signal provides the control path to set
the output at 3X the DAC voltage.
SW
The switch (SW) pin connects directly to the inductor
and provides the switching current necessary to
operate in PWM mode. Due to the high speed
switching on this pin, the switch node should be
routed away from sensitive nodes.
PGND
Power ground (PGND) is the ground path for the
high current PWM mode. The current loop for the
power ground should be as small as possible and
separate from the Analog ground (AGND) loop.
AGND
Signal ground (AGND) is the ground path for the
biasing and control circuitry. The current loop for the
signal ground should be separate from the Power
ground (PGND) loop.
Applications Information
The MIC2224 is a 600mA PWM power supply that
utilizes a dynamically adjustable output voltage for
powering RF power amplifiers. By dynamically
adjusting the output power as necessary, battery life
can be dramatically improved in battery powered RF
applications. For instances where high power is
required, the MIC2224 also has a bypass mode.
Bypass mode is enabled by driving the DAC pin
above 1.2V. This bypasses the input supply directly
to the output through a highly efficient, short circuit
protected, 40mOhm P channel MOSFET.
Input Capacitor
A minimum 1µF ceramic is recommended on the
VIN pin for bypassing. X5R or X7R dielectrics are
recommended for the input capacitor. Y5V
dielectrics lose most of their capacitance over
temperature and are therefore not recommended.
A minimum 1µF is recommended close to the VIN
and PGND pins for high frequency filtering. Smaller
case size capacitors are recommended due to their
lower ESR and ESL.
Output Capacitor
The MIC2224 is optimized for a 1µF ceramic output
capacitor. The MIC2224 utilizes type III internal
compensation and utilizes internal high frequency
zeros to compensate for the double pole roll off of
the LC filter. For this reason, larger output capacitors
can create instabilities. X5R or X7R dielectrics are
recommended for the output capacitor. Y5V
dielectrics lose most of their capacitance over
temperature and are therefore not recommended.
In addition to a 1µF, a small 10nF is recommended
close to the load for high frequency filtering. Smaller
case size capacitors are recommended due to there
lower ESR and ESL.
Inductor Selection
The MIC2224 is designed for use with a 4.7µH
inductor. Proper selection should ensure the
inductor can handle the maximum average and peak
currents required by the load. Maximum current
ratings of the inductor are generally given in two
methods; permissible DC current and saturation
current. Permissible DC current can be rated either
for a 40°C temperature rise or a 10% to 20% loss in
January 2005
10
M9999-012705
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