MIC22205 Datasheet, PDF (14/29 Pages) Micrel Semiconductor – 2A, Integrated, Switch, High-Efficiency

Micrel, Inc.
Application Information
The MIC22205 is a 2A, synchronous voltage mode,
PWM step down regulator IC with a programmable
frequency range from 300kHz to 4MHz. Other features
include tracking and sequencing control for controlling
multiple output power systems and power good (PG).
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
MIC22205 will run at 100% duty cycle.
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 very low amount of power is
dissipated during the off period.
The PWM control technique also provides adjustable
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 10µF X5R or X7R dielectrics ceramic capacitor is
recommended on each of the PVIN pins for bypassing. A
Y5V dielectric 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 MIC22205 was designed specifically for use with
ceramic output capacitors. The output capacitor can be
increased from 47µF to a higher value to improve
transient performance. The MIC22205 operates in
voltage mode, so 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 a wide variation in capacitance
over temperature, become resistive at high frequencies.
Using Y5V or Z5U capacitors can cause instability in the
MIC22205.
MIC22205
Inductor Selection
Inductor selection will be determined by the following
(not in order of importance):
• Inductance
• Rated current value
• Size requirements
• DC resistance (DCR)
The MIC22205 is designed for use with a 0.47µH to
4.7µH inductor.
Maximum current ratings of the inductor are generally
given using 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 so the
peak current will not saturate the inductor. The ripple
current can add as much as 1.2A to the output current
level. Choose an RMS rating that is equal to or greater
than the current limit of the MIC22205 to prevent
overheating in a fault condition. For best electrical
performance, place the inductor very close to the SW
nodes of the IC. The heat of the inductor is somewhat
coupled to the IC, so it offers some level of protection if
the inductor gets too hot (In such cases IC case
temperature is not a true indication of IC
dissipation). It is important to test all operating limits
before settling on the final inductor choice.
The size requirements refer to the area and height
necessary to fit a particular design. Please refer to the
inductor dimensions on the manufacturer’s datasheet.
DC resistance is also important. While DCR is inversely
proportional to size, DCR increase can represent a
significant efficiency loss. Refer to the “Efficiency
Considerations” section below 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
August 2011
14
M9999-082511-A