MIC2829 datasheet(34/52 Pages) MICREL

34 / 52 page

Micrel Inc.

MIC2829

May 2010

M9999-051410-B

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:

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

−

OUT

PEAK

As shown by the calculation above, the peak inductor

current

inversely

proportional

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

the

Efficiency

Considerations.

Efficiency Considerations

Efficiency is defined as the amount of useful output

power, divided by the amount of power supplied.

100

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

OUT

Efficiency

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 I

2R. Power is dissipated in the

high side switch during the on cycle. Power loss is equal

to the high side MOSFET RDSON 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.

DC4 Buck Efficiency

vs. Output Current

100

1000

OUTPUT CURRENT (mA)

VOUT_NOM = 1.8V

L = 2.2µH

COUT = 4.7µF

VIN = 3.6V

VIN = 5V

VIN = 4.3V

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

RDSON and inductor losses. Higher input supply voltages

will increase the Gate-to-Source overdrive on the

internal MOSFETs, thereby reducing the internal RDSON.

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

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