Micrel, Inc.

MIC23051

July 2009

10

M9999—070809-F

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;

From that, the loss in efficiency due to inductor resistance

can be calculated as follows;

100

L_Pd

I

V

I

V

1

_Loss

Efficiency

OUT

OUT

OUT

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.

HyperLight Load Mode™

MIC23051 uses a minimum on and off time proprietary

control loop. When the output voltage falls below the

regulation threshold, the error comparator begins a

switching cycle that turns the PMOS on and keeps it on for

the duration of the minimum-on-time. When the output

voltage is over the regulation threshold, the error

comparator turns the PMOS off for a minimum-off-time.

The NMOS acts as an ideal rectifier that conducts when

the PMOS is off. Using a NMOS switch instead of a diode

allows for lower voltage drop across the switching device

when it is on. The asynchronous switching combination

between the PMOS and the NMOS allows the control loop

to work in discontinuous mode for light load operations. In

discontinuous mode MIC23051 works in pulse frequency

modulation (PFM) to regulate the output. As the output

current increases, the switching frequency increases. This

improves the efficiency of MIC23051 during light load

currents. As the load current increases, the MIC23051

goes into continuous conduction mode (CCM) at a

constant frequency of 4MHz. The equation to calculate the

load when the MIC23051 goes into continuous conduction

mode may be approximated by the following formula:

⎟

⎠

⎞

⎜

⎝

⎛

×

×

−

=

f

2L

D

)

V

(V

I

OUT

IN

LOAD