MIC2827 Datasheet, PDF (23/27 Pages) Micrel Semiconductor – Triple Output PMIC with HyperLight Load™ DCDC, two LDOs, and I2C Control

Micrel, Inc.
Application Information
The Micrel MIC2827 is a three output, programmable
Power Management IC, optimized for high efficiency
power support. The device integrates a single 500mA
PWM/PFM synchronous buck (step-down) regulator with
two Low Dropout Regulators and an I²C interface that
provides programmable Dynamic Voltage Scaling (DVS),
Power Sequencing, and individual output Enable/Disable
controls allowing the user to optimally control all three
outputs.
Input Capacitors
A 4.7µF ceramic capacitor is recommended on the DVIN
pin for bypassing. X5R or X7R dielectrics are
recommended for the input capacitor. Y5V dielectrics
lose most of their capacitance over temperature and are
therefore not recommended. Also, tantalum and
electrolytic capacitors alone are not recommended
because of their reduced RMS current handling,
reliability, and ESR increases.
An additional 0.1µF is recommended close to the DVIN
and DGND pins for high frequency filtering. Smaller case
size capacitors are recommended due to their lower
ESR and ESL.
Minimum 1.0µF ceramic capacitors are recommended
on the LDO1IN and LDO2IN pins for bypassing. Please
refer to layout recommendations for proper layout of the
input capacitors.
Output Capacitors
The MIC2827 is designed for a 2.2µF or greater ceramic
output capacitor for the DC-DC converter and 1.0µF for
the LDO regulators. Increasing the output capacitance
will lower output ripple and improve load transient
response but could increase solution size or cost. A low
equivalent series resistance (ESR) ceramic output
capacitor such as the TDK C1608X5R0J475K, size
0603, 4.7µF ceramic capacitor is recommended based
upon performance, size and cost. X5R or X7R dielectrics
are recommended for the output capacitor. Y5V
dielectrics lose most of their capacitance over
temperature and are therefore not recommended.
In addition to a 4.7µF, a small 0.1µF is recommended
close to the load for high frequency filtering. Smaller
case size capacitors are recommended due to their
lower equivalent series ESR and ESL.
Inductor
Inductor selection will be determined by the following
(not necessarily in the order of importance);
• Inductance
• Rated current value
• Size requirements
• DC resistance (DCR)
July 2009
MIC2827
The MIC2827 was designed for use with an inductance
range from 0.47µH to 4.7µH. Typically, a 1µH inductor is
recommended for a balance of transient response,
efficiency and output ripple. For faster transient
response a 0.47µH inductor may be used. For lower
output ripple, a 4.7µH is recommended.
Proper selection should ensure the inductor can handle
the maximum average and peak currents required by the
load. 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. Peak current can be calculated as follows:
I_PEAK
=
⎡
⎢^IOUT
⎣
+
V_OUT
⎜⎛
⎝
1
− V_OUT /V_IN
2×f ×L
⎟⎠⎞⎥⎦⎤
As shown by the previous calculation, the peak inductor
current is inversely proportional to the switching
frequency and the inductance; the lower the switching
frequency or the inductance the higher the peak current.
As input voltage increases, the peak current also
increases.
The size of the inductor depends on the requirements of
the application. Refer to the Application Circuit and Bill of
Material for details.
DC resistance (DCR) is also important. While DCR is
inversely proportional to size, DCR can represent a
significant efficiency loss. Refer to the Efficiency
Considerations.
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
Current squared. During the off cycle, the low side N-
channel MOSFET conducts, also dissipating power.
Device operating current also reduces efficiency. The
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M9999-072709-A