MIC4722 datasheet(11/20 Pages) MICREL

11 / 20 page

Micrel, Inc.

MIC4722

September 2007

M9999-090507-B

high side switch, and lower duty cycles place the power

losses on the Schottky diode.

Inductor conduction losses (PL) can be calculated by

multiplying the DC resistance (DCR) times the square of

the output current;

OUT

DCR

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 currents. Figure 6

demonstrates 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 middle, causing 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.

MIC4722 Datasheet(PDF) 11 Page - Micrel Semiconductor