MIC22705_11 Datasheet, PDF (14/29 Pages) Micrel Semiconductor – 1MHz, 7A Integrated Switch High-Efficiency Synchronous Buck Regulator

Micrel, Inc.
Application Information
The MIC22705 is a 7A synchronous step-down regulator
IC with a fixed 1MHz, voltage-mode PWM control
scheme. The other features include tracking and
sequencing control for controlling multiple output power
systems, and power-on-reset (POR).
The MIC22705 is a voltage mode, pulse-width
modulation (PWM) regulator. By controlling the ratio of
the 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 MIC22705 will run at 100% duty
cycle.
The MIC22705 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, very-low amount of power is dissipated during the
off period.
The 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.
Component Selection
Input Capacitor
A 22µF X5R or X7R dielectrics ceramic capacitor is
recommended on each of the PVIN pins for bypassing. A
Y5V dielectrics capacitor should not be used. 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 MIC22705 was designed specifically for the use of
ceramic output capacitors. The 100µF output capacitor
can be increased to improve transient performance.
Since the MIC22705 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 MIC22705.
MIC22705
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 MIC22705 is designed for use with 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
current 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 MIC22705 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 (in such cases,
the case temperature is not the real dissipation in the
regulator), so it 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”
sub-section for a more detailed description.
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. First, 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.
March 2011
14
M9999-033111-A