MIC22405 Datasheet, PDF (14/30 Pages) Micrel Semiconductor – 4A Integrated Switch High-Efficiency Synchronous Buck Regulator

Micrel, Inc.
Application Information
The MIC22405 is a 4A 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 on reset (POR).
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
MIC22405 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 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 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 MIC22405 was designed specifically for use with
ceramic output capacitors. The output capacitor can be
increased from 100µF to a higher value to improve
transient performance. Since the MIC22405 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 MIC22405.
Inductor Selection
Inductor selection will be determined by the following
(not necessarily in the order of importance):
• Inductance
MIC22405
• Rated current value
• Size requirements
• DC resistance (DCR)
The MIC22405 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 so 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 MIC22405 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, 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
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 increase 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. Reduced current
demand 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 VI 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
June 2011
14
M9999-061511-A