MIC22600 datasheet(13/32 Pages) MICREL

13 / 32 page

Micrel, Inc.

MIC22600

June 2011

M9999-062411-D

Figure 2 shows an efficiency curve. The portion, from 0A

to 1A, efficiency losses are dominated by quiescent

current losses, gate drive and transition 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.

200

400

600

800

OUTPUT CURRENT (mA)

vs. Inductance

Efficiency

L = 1µH

Figure 2. 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, reducing the internal RDS(ON).

This improves efficiency by decreasing conduction loss

in the device but the inductor DCR loss is inherent to the

device. So 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

From that, the loss in efficiency due to inductor

resistance can be calculated as follows:

Efficiency Loss =

()

100

OUT

⎥

⎦

⎤

⎢

⎣

⎡

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

L = 4.7µH

Figure 3. Efficiency vs. Inductance

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.

Alternatively, under lighter loads, the ripple current

becomes a significant factor. When light load efficiencies

become more critical, a larger inductor value maybe

desired. Larger inductance reduces the peak-to-peak

inductor ripple current, which minimize losses. The

graph in Figure 3 illustrates the effects of inductance

value at light load.

Compensation

The MIC22600 has a combination of internal and

external stability compensation to simplify the circuit for

small, high efficiency designs. In such designs, voltage

mode conversion is often the optimum solution. Voltage

mode is achieved by creating an internal 1MHz ramp

signal and using the output of the error amplifier to

modulate the pulse width of the switch node, thereby

maintaining output voltage regulation. With a typical gain

bandwidth of 100-200kHz, the MIC22600 is capable of

extremely fast transient responses.

The MIC22600 is designed to be stable with a typical

application using a 1µH inductor and a 47µF ceramic

(X5R) output capacitor. These values can be varied

dependant upon the tradeoff between size, cost and

efficiency,

keeping

the

natural

frequency

(

⋅

) ideally less than 26 kHz to ensure

stability can be achieved. The minimum recommended

inductor value is 0.47µH and minimum recommended

output capacitor value is 22µF. With a larger inductor,

there is a reduced peak-to-peak current which yields a

greater efficiency at lighter loads. A larger output

capacitor will improve transient response by providing a

larger hold up reservoir of energy to the output.

MIC22600 Datasheet(PDF) 13 Page - Micrel Semiconductor

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