MIC4721 datasheet(11/19 Pages) MICREL

11 / 19 page

Micrel, Inc.

MIC4721

May 2007

M9999-052907-A

Efficiency Considerations

Calculating the efficiency is as simple as measuring

power out and dividing it by the power in.

100

OUT

Efficiency

Where input power (PIN) is:

PIN = VIN × IIN

and output power (POUT) is calculated as:

POUT = VOUT × IOUT

The Efficiency of the MIC2207 is determined by several

factors.

•

RDSON (Internal P-channel Resistance)

•

Diode conduction losses

•

Inductor Conduction losses

•

Switching losses

RDSON losses are caused by the current flowing through

the high side P-channel MOSFET. The amount of power

loss can be approximated by:

PSW = RDSON × IOUT

2 × D

Where D is the duty cycle.

Since

the

MIC4721

uses

internal

P-channel

MOSFET, RDSON losses are inversely proportional to

supply voltage. Higher supply voltage yields a higher

gate to source voltage, reducing the RDSON, thus

reducing the MOSFET conduction losses. A graph

showing typical RDSON vs. input supply voltage can be

found in the typical characteristics section of this

datasheet.

Diode conduction losses occur due to the forward

voltage drop (VF) and the output current. Diode power

losses can be approximated as follows:

PD = VF × IOUT × (1 – D)

For this reason, the low forward voltage drop Schottky

diode is the rectifier of choice. The low forward voltage

drop will help reduce diode conduction losses, and

improve efficiency. Duty cycle, or the ratio of output

voltage

input

voltage,

determines

whether

the

dominant factor in conduction losses will be the internal

MOSFET or the Schottky diode. Higher duty cycles

place the power losses on the high side switch, and

lower duty cycles place the majority of power loss on the

Schottky diode.

Inductor conduction losses (PL) can be calculated by

multiplying the DC resistance (DCR) times the square of

the output current:

PL = DCR × IOUT

Also, be aware that there are additional core losses

associated with switching current in an inductor. Since

most inductor manufacturers do not give data on the

type of material used, approximating core losses

becomes very difficult, so verify inductor temperature

rise.

Switching losses occur twice each cycle, when the

switch turns on and when the switch turns off. This is

caused by a non-ideal world where switching transitions

are

not

instantaneous,

and

neither

are

current

transitions. Figure 6 demonstrates (or exaggerates…)

how switching losses due to the transitions dissipate

power in the switch.

Figure 6. Switching Transition Losses

Normally, when the switch is on, the voltage across the

switch is low (virtually zero) and the current through the

switch is high. This equates to low power dissipation.

When the switch is off, voltage across the switch is high

and the current is zero, again with power dissipation

being low. During the transitions, the voltage across the

switch (VS-D) and the current through the switch (IS-D)

are at midpoint of their excursions and cause the

transition to be the highest instantaneous power point.

During continuous mode, these losses are the highest.

Also, with higher load currents, these losses are higher.

For discontinuous operation, the transition losses only

occur during the “off” transition since the “on” transitions

there is no current flow through the inductor.

Component Selection

Input Capacitor

A 10µF ceramic is recommended on each VIN pin for

bypassing. X5R or X7R dielectrics are recommended for

the input capacitor. Y5V dielectrics lose most of their

capacitance over temperature and voltage and are

therefore

not

recommended.

Also,

tantalum

and

electrolytic capacitors alone are not recommended

because

their

reduced

RMS

current

handling,

reliability, and higher ESR. Smaller case size capacitors

are recommended due to their lower ESL (equivalent

series

inductance).

Please

refer

layout

recommendations

for

proper

layout

the

input

capacitors.

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