MIC22705 datasheet(15/28 Pages) MICREL

15 / 28 page

Micrel, Inc.

MIC22705

December 2010

M9999-121710-A

There are mainly two loss terms in switching converters:

static losses and switching losses. Static losses are

simply the power losses due to VI or I

2R. For example,

power is dissipated in the high side switch during the on

cycle. Power loss is equal to the high-side MOSFET

RDS(ON) multiplied by the RMS switch current squared

(ISW

2). During the off-cycle, the low-side N-Channel

MOSFET conducts, also dissipating power. Similarly, the

inductor’s DCR and capacitor’s ESR also contribute to

the I

2R losses. Device operating current also reduces

efficiency by the product of the quiescent (operating)

current and the supply voltage. The current required to

drive the gates on and off at a constant 1MHz frequency

and the switching transitions make up the switching

losses.

Figure 2 illustrates an efficiency curve. The portion, from

0A to 0.4A, efficiency losses are dominated by quiescent

current losses, gate drive, transition and core 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.

Efficiency (VIN = 3.3V)

vs. Output Current

100

123

456

OUTPUT CURRENT (A)

VIN = 3.3V

IOUT = 1.8V

Figure 2. Efficiency Curve

The region, 1A to 7A, 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, thereby reducing the internal

RDS(ON). This improves efficiency by decreasing 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:

LPD = IOUT

2 × DCR

From that, the loss in efficiency due to inductor

resistance can be calculated as follows:

()

100

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.

Alternatively, under lighter loads, the ripple current due

to the inductance becomes a significant factor. When

light load efficiencies become more critical, a larger

inductor value maybe desired. Larger inductances

reduce the peak-to-peak inductor ripple current, which

minimize losses.

Compensation

The MIC22705 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 100kHz

− 200kHz, the MIC22705 is

capable of extremely fast transient responses.

The MIC22705 is designed to be stable with a typical

application using a 1µH inductor and a 100µF ceramic

(X5R) output capacitor. These values can be varied

dependent 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. The tradeoff between

changing these values is that 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.

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