MIC22400_10 Datasheet, PDF (12/28 Pages) Micrel Semiconductor – 4A Integrated Switch Synchronous Buck Regulator with Frequency Programmable up to 4MHz

Micrel, Inc.
Application Information
The MIC22400 is a 4A synchronous step-down regulator
IC with an adjustable switching frequency from 800kHz
to 4MHz, 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 22µF ceramic is recommended on each of
the PVIN pins for bypassing. X5R or X7R dielectrics are
recommended for the input capacitor. Do not use Y5V
dielectrics. They lose most of their capacitance over
temperature and become resistive at high frequencies.
This reduces their ability to filter out high frequency
noise.
Output Capacitor
The MIC22400 was designed specifically for the use of
ceramic output capacitors. A 100µF can be increased to
improve transient performance. Since the MIC22400 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 MIC22400.
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 MIC22400 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
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 MIC22400 to prevent overheating
in a fault condition.
December 2010
MIC22400
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. 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”
section for a more detailed description.
EN/DLY Capacitor
EN/DLY pin 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 C_EN/DLY to 1.25V. Therefore:
T_EN/DLY
=
1.24 ⋅ C_EN/DLY
1.10 ^−6
CF Capacitor
Adding a capacitor to this pin can adjust switching
frequency from 800kHz to 4MHz. CF sources 400µA out
of the IC to charge the CF capacitor to set up the
switching frequency. The switch period is simply the time
it takes 400µA to charge CF to 1.0V. Therefore:
Capacitor CF
56pF
68pF
82pF
100pF
150pF
180pF
220pF
270pF
330pF
390pF
470pF
Frequency
4.4MHz
4MHz
3.4MHz
2.8MHz
2.1MHz
1.7MHz
1.4MHz
1.2MHz
1.1MHz
1.05MHz
1MHz
Table 1. CF vs. Frequency
It is necessary to connect the CF capacitor between the
CF pin and power ground.
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M9999-120310-F