MIC2829 Datasheet, PDF (34/52 Pages) Micrel Semiconductor – 3G/4G HEDGE/LTE PMIC with Six Buck Converters, Eleven LDOs and SIM Card Level Translation

Micrel Inc.
Although all grounds eventually connect externally, it is
important to place the capacitors close to their ideal
ground for the load to minimize parasitic inductance and
resistance. This is especially important for a PMIC with
multiple regulators. Increasing the output capacitance
will lower output ripple and improve load transient
response, but could increase solution size or cost. Both
the X7R or X5R temperature rated capacitors are
recommended. The Y5V and Z5U temperature rated
capacitors are not recommended due to their wide
variation in capacitance over temperature and increased
resistance at high frequencies.
Inductor Selection
When selecting an inductor, it is important to consider
the following factors (not necessarily in the order of
importance):
• Inductance
• Rated current value
• Size requirements
• DC resistance (DCR)
The MIC2829 was designed for use with an inductance
range from 1µH to 2.2µH. Typically, a 2.2µH inductor is
recommended for a balance of transient response,
efficiency and output ripple. For faster transient
response, a 1µH inductor will yield the best result. For
lower output ripple, a 2.2µH 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 margin so that the
peak current does not cause the inductor to saturate.
Peak current can be calculated as follows:
I _PEAK
⎡
= _⎢I_OUT
⎣
+
V_OUT
⎜⎛
⎝
1
− V_OUT /V_IN
2×f ×L
⎟⎞⎥⎤
⎠⎦
As shown by the calculation above, the peak inductor
current is inversely proportional to the switching
frequency (f) and the inductance (L); 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 Typical Application Circuit
and Bill of Materials 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.
MIC2829
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
which 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
product of the quiescent (operating) current and the
supply voltage represents another DC loss. The current
required for driving the gates on and off at the constant
switching frequency and other internal switching
transitions make up the switching losses.
100
90
80
70
60
50
40
30
20
10
0
1
DC4 Buck Efficiency
vs. Output Current
VIN = 4.3V
VIN = 3.6V
VIN = 5V
VOUT_NOM = 1.8V
L = 2.2µH
COUT = 4.7µF
10
100
OUTPUT CURRENT (mA)
1000
Figure 3. HLL Efficiency vs. Output Current
Figure 3 shows an efficiency curve. From an output
current of 1mA to 100mA, efficiency losses are
dominated by quiescent current losses, gate drive and
transition losses. By lowering the switching frequency,
the HyperLight Load™ buck regulator (DC1 to DC4) is
able to maintain high efficiency at low output currents.
Over 100mA, efficiency loss is dominated by MOSFET
R_DSON and inductor losses. Higher input supply voltages
will increase the Gate-to-Source overdrive on the
internal MOSFETs, thereby reducing the internal R_DSON.
This improves efficiency by reducing conduction losses
in the device. All but the inductor losses are inherent to
the device. For higher current levels, inductor selection
May 2010
34
M9999-051410-B