MIC2230 datasheet(13/18 Pages) MICREL

13 / 18 page

Micrel, Inc.

MIC2230

April 2010

M9999-040810-C

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 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

2. 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

is another DC loss. The current required driving the

gates on and off at a constant 2.5MHz frequency and the

switching transitions make up the switching losses.

The figure above shows an efficiency curve. From no

load to 100mA, efficiency losses are dominated by

quiescent current losses, gate drive and transition

losses. By forcing the MIC2230 into Trickle Mode™

(/FPWM=High), the buck regulator significantly reduces

the required switching current by entering into a PFM

(Pulse Frequency Modulation) mode. This significantly

increases 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 threshold on

the internal MOSFETs, reducing the internal RDSON.

This improves efficiency by reducing 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;

DCR

Iout

L_Pd

2 ×

From that, the loss in efficiency due to inductor

resistance can be calculated as follows;

100

L_Pd

_Loss

Efficiency

OUT

⎥

⎦

⎤

⎢

⎣

⎡

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

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.

Trickle Mode™ Operation

Trickle Mode™ operation is achieved by clamping the

minimum peak current to approximately 150mA. This

forces a PFM mode by comparing the output voltage to

the internal reference. If the voltage is less than 0.8V,

the MIC2230 turns on the high side until the peak

inductor current reaches approximately 150mA. A

separate comparator then monitors the output voltage. If

the feedback voltage is greater than 0.8V, the high side

switch is then used as a 10µA current source, never

turning off completely. This creates a highly efficient light

load mode by increasing the time it takes for the output

capacitor to discharge, delaying the amount of switching

required and increasing light load efficiency. When the

load current is greater than approximately 100mA, the

MIC2230 automatically switches to PWM mode.

FPWM Operation

In forced PWM Mode (/FPWM=LOW) the MIC2230 is

forced to provides constant switching at 2.5MHz with

synchronous internal MOSFETs throughout the load

current. In FPWM Mode, the output ripple can be as low

as 7mV.

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