MIC2174 datasheet(15/27 Pages) MICREL

15 / 27 page

Micrel, Inc.

MIC2174/MIC2174C

September 2010

M9999-091310-C

Making the assumption that the turn-on and turn-off

transition times are equal; the transition times can be

approximated by:

HSD

OSS

ISS

(11)

where:

CISS and COSS are measured at VDS = 0

IG = gate-drive current

The total high-side MOSFET switching loss is:

HSD

)

(12)

where:

tT = Switching transition time

VD = Body diode drop (0.5V)

fSW = Switching Frequency (300kHz)

The high-side MOSFET switching losses increase with

the input voltage VHSD due to the longer turn-on time and

turn-off time. The low-side MOSFET switching losses

are negligible and can be ignored for these calculations.

Inductor Selection

Values for inductance, peak, and RMS currents are

required to select the output inductor. The input and

output voltages and the inductance value determine the

peak-to-peak inductor ripple current. Generally, higher

inductance values are used with higher input voltages.

Larger peak-to-peak ripple currents will increase the

power dissipation in the inductor and MOSFETs. Larger

output ripple currents will also require more output

capacitance to smooth out the larger ripple current.

Smaller peak-to-peak ripple currents require a larger

inductance value and therefore a larger and more

expensive inductor. A good compromise between size,

loss and cost is to set the inductor ripple current to be

equal to 20% of the maximum output current. The

inductance value is calculated by Equation 13:

OUT(max)

HSD(max)

OUT

HSD(max)

OUT

20%

)

−

(13)

where:

fSW = switching frequency, 300 kHz

20% = ratio of AC ripple current to DC output current

VHSD(max) = maximum power stage input voltage

The peak-to-peak inductor current ripple is:

)

HSD(max)

OUT

HSD(max)

OUT

L(pp)

−

(14)

The peak inductor current is equal to the average output

current plus one half of the peak-to-peak inductor current

ripple.

IL(pk) =IOUT(max) + 0.5 × ΔIL(pp)

(15)

The RMS inductor current is used to calculate the I

losses in the inductor.

ΔI

L(PP)

OUT(max)

L(RMS)

(16)

Maximizing efficiency requires both the proper selection

of core material and the minimizing of winding

resistance. The high frequency operation of the

MIC2174/MIC2174C requires the use of ferrite materials

for all but the most cost sensitive applications.

Lower cost iron powder cores may be used but the

increase in core loss will reduce the efficiency of the

power supply. This is especially noticeable at low output

power. The winding resistance decreases efficiency at

the higher output current levels. The winding resistance

must be minimized although this usually comes at the

expense of a larger inductor. The power dissipated in the

inductor is equal to the sum of the core and copper

losses. At higher output loads, the core losses are

usually insignificant and can be ignored. At lower output

currents, the core losses can be a significant contributor.

Core loss information is usually available from the

magnetics vendor. Copper loss in the inductor is

calculated by Equation 17:

PINDUCTOR(Cu) = IL(RMS)

2 × RWINDING

(17)

The resistance of the copper wire, RWINDING, increases

with the temperature. The value of the winding

resistance used should be at the operating temperature.

PWINDING(Ht) = RWINDING(20°C) × (1 + 0.0042 × (TH – T20°C))

(18)

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