MIC22950 Ver la hoja de datos (PDF) - Micrel
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MIC22950 Datasheet PDF : 22 Pages
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Micrel, Inc.
Application Information
The MIC22950 is a 10A synchronous stepdown
regulator IC with a programmable 400kHz to 2MHz
switching frequency. The control loop is a voltage-mode
PWM control scheme. Other features include tracking
and sequencing control for controlling multiple output
power systems with POR/PG output.
Component Selection
Input Capacitor
A minimum 22µF ceramic capacitor (preferable) is
recommended on each of the PVIN pins for bypassing.
X5R or X7R dielectrics are recommended for the input
capacitor. Do not use Y5V dielectrics, aside from losing
most of their capacitance over temperature and voltage,
they also become resistive at high frequencies. This
reduces their ability to localize high-frequency noise.
Output Capacitor
The MIC22950 was designed specifically for the use of
ceramic output capacitors. It is designed to work with
100µF output capacitor. This output capacitor can be
increased to improve transient performance. Since the
MIC22950 is voltage mode control loop, it relies on the
inductor and output capacitor for compensation. For this
reason, do not use excessively large output capacitors.
The output capacitor requires either an X7R or X5R
dielectric. Do not use Y5V and Z5U dielectric capacitors,
aside from the undesirable effect of their wide variation
in capacitance over temperature, become resistive at
high frequencies. Using Y5V or Z5U capacitors can
cause instability in the MIC22950.
Inductor Selection
Inductor selection will be determined by the following
(not necessarily in the order of importance):
• Inductance
• Rated current value
• Size requirements
• DC resistance (DCR)
The MIC22950 is designed for use with a 0.39µH to
2.2µH inductor.
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% loss in
inductance. Ensure that the inductor selected can handle
the maximum operating current. When the saturation
current is specified, make sure that there is enough
margin that the peak current will not saturate the
inductor. The ripple can add as much as 2A to the output
current level. The RMS rating should be chosen to be
MIC22950
equal or greater than the current limit of the MIC22950 to
prevent overheating in a fault condition. For best
electrical performance, the inductor should be placed
very close to the SW nodes of the IC. For this reason,
the heat of the inductor is somewhat coupled to the IC,
which offers some level of protection if the inductor gets
too hot. It is important to test all operating limits before
settling on the final inductor choice.
The size requirements refer to the area and height
requirements that are necessary to fit a particular
design. Please refer to the inductor dimensions on their
datasheet.
DC resistance is also important. While DCR is typically
inversely proportional to size, DCR can represent a
significant efficiency loss. Refer to the Efficiency
Considerations section for a more detailed description.
EN/DLY Capacitor
EN/DLY sources 1µA out of the IC to allow a start-up
delay to be implemented. The delay time is simply the
time it takes 1µA to charge CEN/DLY to 1.24V. Therefore:
TDLY
=
1.24 × CEN/DLY
1 × 10−6
Efficiency Considerations
Efficiency is defined as the amount of useful output
power, divided by the amount of power consumed:
Efficiency
%
=
⎜⎜⎝⎛
VOUT
VIN
× IOUT
× IIN
⎟⎟⎠⎞ × 100
Maintaining high efficiency serves two purposes. It
reduces power dissipation in the power supply, reducing
the need for heat sinks and thermal-design
considerations and it reduces consumption of current for
battery-powered applications. Reduced current draw
from a battery increases the devices operating time,
critical in hand held devices.
There are mainly two loss terms in switching converters:
Static losses and switching losses. Static losses are
simply the power losses due to V.I or I2R. For example,
power is dissipated in the high-side switch during the on
cycle. Power loss is equal to the high-side MOSFET
RDS(ON) multiplied by the RMS Switch Current squared
(ISW2). During the off cycle, the low-side N-Channel
MOSFET conducts, also dissipating power. Similarly, the
inductor’s DCR and capacitor’s ESR also contribute to
the I2R losses. Device operating current also reduces
efficiency by the product of the quiescent (operating)
current and the supply voltage. The current required to
February 2010
10
M9999-021910-B
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