MIC2811_08 Datasheet, PDF (12/17 Pages) Micrel Semiconductor – Digital Power Management IC 2MHz, 600mA DC/DC with Triple 300mA LDOs

Micrel, Inc.
MIC2811/2821
Applications Information
The MIC2811 and MIC2821 are power management ICs
with a single integrated step-down regulator and three
low dropout regulators. LDO1, LDO2, and LDO3 are
300mA low dropout regulators supplied by their own
independent input voltage pins. The supply to LDO3
(V_IN) also powers the bias circuitry and must be available
for any output to be operational. This supply requires an
external connection to DVIN. The step-down regulator is
a 2MHz 600mA PWM power supply, using small values
of L and C operating at over 90% efficiency.
DVIN/VIN/VIN1/VIN2
All four regulators, the switch mode regulator, LDO1,
LDO2, and LDO3 have their own unique input voltage
supply pin. VIN provides power to LDO3 and internal
circuitry shared by all the regulators and therefore must
be available for any of the regulators to operate properly.
DVIN and VIN must be tied together and have a
minimum input voltage of 2.7V. Inputs to LDO1 (VIN1)
and LDO2 (VIN2) can go as low as 1.65V, but should
never exceed the VIN and DVIN input voltage. Due to
the high switching speeds, a 1µF input capacitor is
recommended close to the DVIN, decoupled to the
PGND pin.
LDO1
Regulated output voltage of LDO1. Power is provided by
VIN1 and enabled through EN1. Recommended output
capacitance is 2.2µF, decoupled to the SGND pin.
LDO2
Regulated output voltage of LDO2. Power is provided by
VIN2 and enabled through EN2. Recommended output
capacitance is 2.2µF, decoupled to the SGND pin.
LDO3
Regulated output voltage of LDO3. Power is provided by
VIN and enabled through EN (MIC2811) or EN3
(MIC2821). Recommended output capacitance is 2.2µF,
decoupled to the SGND pin.
SW
The switch (SW) pin connects directly to the inductor
and provides the switching current necessary to operate
in PWM mode. Due to the high speed switching on this
pin, the switch node should be routed away from
sensitive nodes.
DC/DC Output Capacitor
The DC/DC regulator requires an output capacitor for
proper operation. Values of greater than 2.2µF improve
transient response and noise reduction at high
frequency. X7R/X5R dielectric-type ceramic capacitors
are recommended because of their superior temperature
performance. X7R-type capacitors change capacitance
by 15% over their operating temperature range and are
the most stable type of ceramic capacitors. Z5U and
Y5V dielectric capacitors change value by as much as
50% to 60% respectively over their operating
temperature ranges and for that reason are not
recommended. Larger output capacitances can be
achieved by placing tantalum or aluminum electrolytics
in parallel with the ceramic capacitor. For example, a
100µF electrolytic in parallel with a 10µF ceramic can
provide the transient and high frequency noise
performance of a 100µF ceramic at a significantly lower
cost. Specific undershoot/overshoot performance will
depend on both the values and ESR/ESL of the
capacitors.
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 MIC2811 and MIC2821 are designed for use with a
2.2µH 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%
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 inductor current can be calculated as
follows:
I_PK
= I_OUT
+
V_OUT
(1 −
V_OUT
V_IN
)
2×f ×L
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.
Efficiency Considerations
Efficiency is defined as the amount of useful output
power, divided by the amount of power supplied.
Efficiency _ % = ⎜⎛^{Vout × Iout} ⎟⎞ ×100
⎝ ^{Vin × Iin} ⎠
Maintaining high efficiency serves two purposes. It
reduces power dissipation in the power supply, reducing
the need for heat sinks and thermal design
April 2008
12
M9999-042208-A