MIC22600_09 Datasheet, PDF (12/29 Pages) Micrel Semiconductor – 1MHz, 6A Integrated Switch Synchronous Buck Regulator

Micrel, Inc.
Application Information
The MIC22600 is a 6A Synchronous step down regulator
IC with a fixed 1 MHz, voltage mode PWM control
scheme. The other features include tracking and
sequencing control for controlling multiple output power
systems, power on reset.
Component Selection
Input Capacitor
A minimum 10µF ceramic is recommended on each of
the PVIN pins for bypassing. X5R or X7R dielectrics are
recommended for the input capacitor. Y5V dielectrics is
not recommended.
Output Capacitor
The MIC22600 was designed specifically for the use of
ceramic output capacitors and 22µF is optimum output
capacitor. 22µF can be increased to 100µF to improve
transient performance. Since the MIC22600 is in 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 MIC22600.
Inductor Selection
Inductor selection is determined by the following (not
necessarily in the order of importance):
• Inductance
• Rated current value
• Size requirements
• DC resistance (DCR)
The MIC22600 is designed to use a 0.47µH 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 1.2A to the output current level. The
RMS rating should be chosen to be equal or greater than
the Current Limit of the MIC22600 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. It is important to test all
operating limits before settling on the final inductor
choice.
October 2009
MIC22600
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.
DCR is inversely proportional to size and can represent
a significant efficiency loss. Refer to the “Efficiency
Considerations” below for a more detailed description.
EN/DLY Capacitor
EN/DLY sources 1µA out of the IC to allow a startup
delay to be implemented. The delay time is simply the
time it takes 1µA to charge CDLY to 1.25V. Therefore:
T _DLY
= ^{1.24 ⋅ CDLY}
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
decreases power dissipation in the power supply,
reducing the need for heat sinks and thermal design
considerations and it decreases consumption of current
for battery powered applications. Reduced current drawn
from a battery increases the devices operating time,
particularly in hand held devices.
There are mainly two loss terms in switching converters:
conduction losses and switching losses. Conduction
losses are simply the power losses due to VI or I²R. For
example, power is dissipated in the high side switch
during the on cycle. The 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 power
consumed at 1MHz frequency and power loss due to
switching transitions add up to switching losses.
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M9999-102809-C