MIC5167_12 Datasheet, PDF (13/23 Pages) Micrel Semiconductor – 1MHz, 6A, Integrated Switch, High-Efficiency, Synchronous Buck DDR Memory Terminator

Micrel, Inc.
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 MIC5167 is designed for use with a 0.4µH to 4.7µH
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% 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 ripple
current can add as much as 1.2A to the output current
level. The RMS rating should be chosen to be equal or
greater than the Current Limit of the MIC5167 to prevent
overheating in a fault condition. For best electrical
performance, the inductor should be placed very close to
the SW nodes of the IC. For this reason, the heat of the
inductor is somewhat coupled to the IC, so it offers some
level of protection if the inductor gets too hot. It is
important to test all operating limits before settling on the
final inductor choice.
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”
below for a more detailed description.
Efficiency Considerations
Efficiency is defined as the amount of useful output
power, divided by the amount of power consumed.
Efficiency
%
= _{⎜⎜⎝⎛}
V_OUT
V_IN
× I_OUT
× I_IN
⎟⎟⎠⎞ ^×100
Maintaining high efficiency serves two purposes. It
decreases power dissipation in the power supply,
reducing the need for heat sinks and thermal design
considerations and it decreases consumption of current
for battery powered applications.
MIC5167
Reduced current draw from a battery increases the
devices operating time, critical in hand held devices.
There are mainly two loss terms in switching converters:
static losses and switching losses. Static losses are
simply the power losses due to VI or I²R. For example,
power is dissipated in the high-side switch during the on
cycle. Power loss is equal to the high-side MOSFET
RDS_(ON) multiplied by the RMS Switch Current squared
(I_SW2). During the off-cycle, the low-side N-Channel
MOSFET conducts, also dissipating power. Similarly, the
inductor’s DCR and capacitor’s ESR also contribute to
the I²R losses. Device operating current also reduces
efficiency by the product of the quiescent (operating)
current and the supply voltage. The current required to
drive the gates on and off at a constant 1MHz frequency
and the switching transitions make up the switching
losses.
Figure 4 shows an efficiency curve. The portion, from 0A
to 1A, efficiency losses are dominated by quiescent
current losses, gate drive, transition and core losses. In
this case, lower supply voltages yield greater efficiency
in that they require less current to drive the MOSFETs
and have reduced input power consumption.
Efficiency
95
90
85
80
VIN = 3.3V
V_TT = 1.8
75
70
0
1
2
3
4
5
6
OUTPUT CURRENT (A)
Figure 4. Efficiency Curve
The region, 1A to 6A, efficiency loss is dominated by
MOSFET R_DS(ON) and inductor DC losses. Higher input
supply voltages will increase the Gate-to-Source voltage
on the internal MOSFETs, thereby reducing the internal
R_DS(ON). This improves efficiency by decreasing DC
losses in the device. 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:
L_PD = I_OUT2 × DCR
January 2012
13
M9999-012312-B