MIC22600_09 datasheet(13/29 Pages) MICREL

13 / 29 page

Micrel, Inc.

MIC22600

October 2009

M9999-102809-C

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.

012345

OUTPUT CURRENT (A)

Efficiency 3V - 1.8V

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

200

400

600

800

OUTPUT CURRENT (mA)

vs. Inductance

Efficiency

L = 1µH

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 above 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.

The integration of one pole-zero pair within the control

loop greatly simplifies compensation. The optimum

values for CCOMP (in series with a 20k resistor) are shown

below.

22-47µF

47µF-

100µF

100µF-

470µF

0.47µH

0*-10pF

22pF

33pF

1µH

†-15pF

15-22pF

33pF

2.2µH

15-33pF

33-47pF

100-220pF

* VOUT > 1.2V,

† VOUT > 1V

Table1. Compensation Capacitor Selection

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