MIC5167 datasheet(13/23 Pages) MICREL

13 / 23 page

Micrel, Inc.

MIC5167

January 2012

M9999-012312-B

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.

100

Efficiency

OUT

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

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.

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 I

2R. 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

(ISW

2). 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 I

2R 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

OUTPUT CURRENT (A)

VIN = 3.3V

VTT = 1.8

Figure 4. Efficiency Curve

The region, 1A to 6A, efficiency loss is dominated by

MOSFET RDS(ON) and inductor DC losses. Higher input

supply voltages will increase the Gate-to-Source voltage

on the internal MOSFETs, thereby reducing the internal

RDS(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:

LPD = IOUT

2 × DCR

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