MIC23201 Datasheet, PDF (14/20 Pages) Micrel Semiconductor – 2MHz PWM 2A Buck Regulator

Micrel Inc.
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 RMS
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 driving the gates on and off at a constant 2MHz
frequency and the switching transitions make up the
switching losses.
All but the inductor losses are inherent to the device. In
which case, inductor selection becomes increasingly
critical in efficiency calculations. As the inductors are
reduced in size, the DC resistance (DCR) can become
quite significant. The DCR losses can be calculated as
follows:
P_DCR = I_OUT2 x DCR
From that, the loss in efficiency due to inductor
resistance can be calculated as follows:
Efficiency
Loss


¹



V_OUT  I_OUT
V_OUT  I_OUT  P_DCR

100
Efficiency loss due to DCR is minimal at light loads and
gains significance as the load is increased. Inductor
selection becomes a trade-off between efficiency and
size in this case.
MIC23201
where:
 P_DISS is the power dissipated within the MLF
package. θ_JA is a combination of junction-to-case
thermal resistance (θ_JC) and Case-to-Ambient
thermal resistance (θ_CA), since thermal resistance of
the solder connection from the EPAD to the PCB is
negligible, so θ_JA = θ_JC + θ_CA.
 T_AMB is the operating ambient temperature.
Thermal Measurements
Measuring the IC’s case temperature is recommended to
ensure it is within its operating limits. Although this might
seem like a very elementary task, it is easy to get
erroneous results. The most common mistake is to use
the standard thermal couple that comes with a thermal
meter. This thermal couple wire gauge is large, typically
22 gauge, and behaves like a heatsink, resulting in a
lower case measurement.
Two methods of temperature measurement are using a
smaller thermal couple wire or an infrared thermometer.
If a thermal couple wire is used, it must be constructed
of 36 gauge wire or higher then (smaller wire size) to
minimize the wire heat-sinking effect. In addition, the
thermal couple tip must be covered in either thermal
grease or thermal glue to make sure that the thermal
couple junction is making good contact with the case of
the IC. Omega brand thermal couple (5SC-TT-K-36-36)
is adequate for most applications.
Wherever possible, an infrared thermometer is
recommended. The measurement spot size of most
infrared thermometers is too large for an accurate
reading on a small form factor ICs. However, an IR
thermometer from Optris has a 1mm spot size, which
makes it a good choice for measuring the hottest point
on the case. An optional stand makes it easy to hold the
beam on the IC for long periods of time.
Thermal Considerations
The MIC23201 is provided in a 3mm x 3mm MLF
package – a package that has very good thermal-
performance This package maximizes heat transfer from
the junction to the exposed pad (EP), which connects to
the ground plane. The size of the ground plane attached
to the exposed pad determines the overall thermal
resistance from the junction to the ambient air
surrounding the printed circuit board. The junction
temperature for a given ambient temperature can be
calculated using:
T_J = T_AMB + P_DISS  _JA
August 2012
14
M9999-082912-A