MIC2238 Datasheet, PDF (12/16 Pages) Micrel Semiconductor – Dual Synchronous 800mA/800mA Step-Down DC/DC Regulator

Micrel, Inc.
MIC2238
Applications Information
Input Capacitor
A minimum 2.2µF ceramic is recommended on the VIN
pin for bypassing. X5R or X7R dielectrics are
recommended for the input capacitor. Y5V dielectrics,
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 MIC2238 was designed specifically for use with a
2.2µF or greater ceramic output capacitor. 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.
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 MIC2238 was designed for use with a 1µH, 2.2µH,
or 4.7µH inductor. For a better load transient response,
a 1µH inductor is recommended. For better efficiency, a
4.7µH inductor is recommended.
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 to 20% 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 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.
Compensation
The MIC2238 is an internally compensated, current
mode buck regulator. Current mode is achieved by
sampling the peak current and using the output of the
error amplifier to pulse width modulate the switch node
and maintain output voltage regulation.
The MIC2238 is designed to be stable with a 1µH, 2.2µH
or 4.7µH inductor with a 2.2µF ceramic (X5R) output
capacitor.
Feedback
The MIC2238 provides a feedback pin to adjust the
output voltage to the desired level. This pin connects
internally to an error amplifier. The error amplifier then
compares the voltage at the feedback to the internal
0.8V reference voltage and adjusts the output voltage to
maintain regulation. Calculating the resistor divider
network for the desired output is as follows;
R2 =
R1
⎜⎜⎝⎛
V_OUT
V_REF
− 1_{⎟⎟⎠⎞}
Where V_REF is 0.8V and V_OUT is the desired output
voltage.
A 100kΩ from the output to the feedback is
recommended for R1. Larger resistor values require an
additional capacitor (feed-forward) from the output to the
feedback. The large high-side resistor value and the
parasitic capacitance on the feedback pin (~10pF) can
cause an additional pole in the control loop. The
additional pole can create a phase loss at high
frequencies. This phase loss degrades transient
response by reducing phase margin. Adding feed-
forward capacitance negates the parasitic capacitive
effects of the feedback pin. A minimum 100pF capacitor
is recommended for feed forward capacitance.
Large feedback resistor values increase impedance,
making the feedback node more susceptible to noise
pick-up. A feed forward capacitor would also reduce
noise pick-up by providing a low impedance path to the
output.
Efficiency Considerations
Efficiency is defined as the amount of useful output
power, divided by the amount of power supplied.
Efficiency_%
=
⎜⎜⎝⎛
V_OUT
V_IN
× I_OUT
× I_IN
⎟⎟⎠⎞ ^{× 100}
Maintaining high efficiency serves two purposes. It
reduces power dissipation in the power supply, reducing
the need for heat sinks and thermal design
considerations and it reduces consumption of current for
battery powered applications. Reduced current draw
from a battery increases the devices operating time and
is critical in hand held devices.
There are two types of losses in switching converters;
DC losses and switching losses. DC losses are simply
the power dissipation of I²R. Power is dissipated in the
high-side switch during the on cycle. Power loss is equal
to the high side MOSFET R_DSON multiplied by the Switch
July 2007
12
M9999-071307