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MIC2203 Datasheet(PDF) 8 Page - Micrel Semiconductor

Part No. MIC2203
Description  High Efficiency 1MHz Synchronous Buck Regulator
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Maker  MICREL [Micrel Semiconductor]
Homepage  http://www.micrel.com
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MIC2203 Datasheet(HTML) 8 Page - Micrel Semiconductor

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Micrel, Inc.
MIC2203
the Gate-to-Source threshold on the internal
MOSFETs, reducing the internal RDS(ON). This
improves efficiency by reducing DC losses in the
device. All but the inductor losses are inherent to the
device. In which case, inductor selection becomes
increasingly critical in efficiency calculations. As the
inductors are reduced in size, the DC resistance
(DCR) can become quite significant. The DCR
losses can be calculated as follows:
P
L = IOUT
2 ×DCR
From that, the loss in efficiency due to inductor
resistance can be calculated as follows:
Efficiency Loss = -1
V
OUT × IOUT
V
OUT × IOUT × PL
⎥×100
Efficiency loss, due to DCR, is minimal at light loads
and gains significance as the load is increased.
Inductor selection becomes a trade-off between
efficiency and size in this case.
Compensation
The MIC2203 is an internally compensated, voltage
mode buck regulator. Voltage mode is achieved by
creating an internal 1MHz ramp signal and using the
output of the error amplifier to pulse width modulate
the
switch
node,
maintaining
output
voltage
regulation. With a typical gain bandwidth of 100kHz,
the MIC2203 is capable of extremely fast transient
responses.
The MIC2203 is designed to be stable with a 10
µH
inductor and a 2.2
µF ceramic (X5R) output
capacitor.
Feedback
The MIC2203 provides a feedback pin to adjust the
output voltage to the desired level. This pin connects
internally to an error amplifier. The error amplifier
then compares the voltage at the feedback to the
internal 0.5V reference voltage and adjusts the
output voltage to maintain regulation. To calculate
the resistor divider network for the desired output is
as follows:
R2
=
R1
V
OUT
V
REF
−1
Where V
REF
is 0.5V and V
OUT
is the desired output
voltage. A 10kΩ or lower resistor value from the
output to the feedback is recommended. Larger
resistor values require an additional capacitor (feed-
forward) from the output to the feedback. The large
high
side
resistor
value
and
the
parasitic
capacitance on the feedback pin (~10pF) can cause
an additional pole in the loop. The additional pole
can create a phase loss at high frequency. This
phase loss degrades transient response by reducing
phase margin. Adding feed-forward capacitance
negates the parasitic capacitive effects of the
feedback pin.
Also, large feedback resistor values increase the
impedance, making the feedback node more
susceptible to noise pick-up. A feed-forward
capacitor would also reduce noise pick-up by
providing a low impedance path to the output.
PWM Operation
The MIC2203 is a pulse width modulation (PWM)
regulator. By controlling the ratio of on-to-off time, or
duty cycle, a regulated DC output voltage is
achieved. As load or supply voltage changes, so
does the duty cycle to maintain a constant output
voltage. In cases where the input supply runs into a
dropout condition, the MIC2203 will run at 100%
duty cycle.
The MIC2203 provides constant switching at 1MHz
with synchronous internal MOSFETs. The internal
MOSFETs include a high-side P-Channel MOSFET
from the input supply to the switch pin and an N-
Channel MOSFET from the switch pin-to-ground.
Since the low-side N-Channel MOSFET provides the
current during the off cycle, a free wheeling Schottky
diode from the switch node to ground is not required.
PWM control provides fixed frequency operation. By
maintaining
a
constant
switching
frequency,
predictable fundamental and harmonic frequencies
are achieved. Other methods of regulation, such as
burst and skip modes, have frequency spectrums
that change with load that can interfere with
sensitive communication equipment.
Synchronization
SYNC_IN allows the user to change the frequency
from 1MHz up to 1.25MHz or down to 800KHz. This
allows the ability to control the fundamental
frequency and all the resultant harmonics.
Maintaining a predictable frequency creates the
ability to either shift the harmonics away from
sensitive carrier and IF frequency bands or to
accurately filter out specific harmonic frequencies.
The SYNC_OUT function pin allows for the ability to
be able to sync up multiple MIC2203s in a “daisy-
chain”, connecting SYNC_OUT to SYNC_IN of the
other MIC2203. Synchronizing multiple MIC2203s
benefits much in the same way as syncing up one
MIC2203. All regulators will run at the same funda-
mental frequency, resulting in matched harmonic
frequencies,
simplifying
design
for
sensitive
communication equipment.
December 2004
8
M9999-120604
(408) 955-1690


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