MIC22601 Datasheet, PDF (10/18 Pages) Micrel Semiconductor – 4 MHz, 6A Integrated Switch Synchronous Buck Regulator

Micrel, Inc.
MIC22601
Application Information
The MIC22601 is a 6A Synchronous step down regulator
IC with a fixed 4MHz, voltage mode PWM control
scheme. The other features include tracking and
sequencing control for controlling multiple output power
systems. Power-on-reset and easy RC compensation
are other features as well.
Component selection
Input Capacitor
A minimum 10µF ceramic is recommended on each of
the PV_IN pins for bypassing. X5R or X7R dielectrics are
recommended for the input capacitor. Y5V dielectrics,
aside from losing most of their capacitance over
temperature, they also become resistive at high
frequencies. This reduces their ability to filter out high
frequency noise.
Output Capacitor
The MIC22601 was designed specifically for the use of
ceramic output capacitors. 47µF can be increased to
improve transient performance. Since the MIC22601 is
voltage mode, the control loop 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. 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
MIC22601.
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 MIC22601 is designed for use with a 0.22µH to
4.7µH 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 the inductor selected can handle the
maximum operating current. When 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 1A to the output current level. The
RMS rating should be chosen to be equal or greater than
the Current Limit of the MIC22601 to prevent
overheating in a fault condition. For best electrical
May 2009
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 inversely
proportional to size, DCR can represent a significant
efficiency loss. Refer to the “Efficiency Considerations”
below for a more detailed description.
Enable/DLY Capacitor
Enable/DLY sources 1uA out of the IC to allow a startup
delay to be implemented. The delay time is simply the
time it takes 1uA to charge C_DLY to 1.24V. Therefore:
T_DLY
=
1.24 ⋅ C_DLY
1⋅ 10 ^-6
Efficiency considerations
Efficiency is defined as the amount of useful output
power, divided by the amount of power consumed.
Efficiency
%
=
⎜⎜⎝⎛
V_OUT
V_IN
× I_OUT
× I_IN
⎟⎟⎠⎞ ^{× 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 (during flywheel diode
conduction time) or I²R (during MOSFET conduction
time). 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 (I_SW2). 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 drive the gates on and off at a
constant 4Mhz frequency and the switching transitions
make up the switching losses.
Although one is not required, a Schottky diode rated for
2A continuous current, connected between SW and
GND can add up to 5% to efficiency. This is achieved by
preventing forward biasing of the internal MOSFET body
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M9999-050509-A