MIC4782 Datasheet, PDF (12/23 Pages) Micrel Semiconductor – 1.8 MHz Dual 2A Integrated Switch Buck Regulator

Micrel, Inc.
R_DSON 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 (V_F) and the output current. Diode power
losses can be approximated as follows;
P_D = V_F × I_OUT × (1 − D)
For this reason, the Schottky diode is the rectifier of
choice. Using the lowest forward voltage drop will help
reduce diode conduction losses, and improve efficiency.
Duty cycle, or the ratio of output voltage-to-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 power
losses on the Schottky diode.
Inductor conduction losses (P_L) can be calculated by
multiplying the DC resistance (DCR) times the square of
the output current;
P_L = DCR × I_OUT ²
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.
MIC4782
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 (V_S-D) and the current through the switch (I_S-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.
August 2009
12
M9999-081709-D