mic4723yml Micrel Semiconductor, mic4723yml Datasheet - Page 10

• Manufacturer Part Number: mic4723yml
• Description: Mic4723 3a 2mhz Integrated Switch Buck Regulator
• Manufacturer: Micrel Semiconductor

The total off time can be calculated as;
Discontinuous Operation
Discontinuous operation is when the inductor current
discharges to zero during the off cycle. Figure 5
demonstrates the switch voltage and inductor currents
during discontinuous operation.
Micrel, Inc.
January 2008
−
T
OFF
(
V
Figure 5. Discontinuous Operation
OUT
=
L
2.0MHz
+
1
V
Figure 4. Off-Time
−
D
D
)
10
When
discharged, the voltage on the switch node rings at the
frequency determined by the parasitic capacitance and
the inductor value. In Figure 5, it is drawn as a DC
voltage, but to see actual operation (with ringing) refer to
the functional characteristics.
Discontinuous mode of operation has the advantage
over full PWM in that at light loads, the MIC4723 will skip
pulses as nessasary, reducing gate drive losses,
drastically improving light load efficiency.
Efficiency Considerations
Calculating the efficiency is as simple as measuring
power out and dividing it by the power in;
Where input power (P
and output power (P
The Efficiency of the MIC4723 is determined by several
factors.
Rdson losses are caused by the current flowing through
the high side P-channel MOSFET. The amount of power
loss can be approximated by;
Where D is the duty cycle.
Since the MIC4723 uses an 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, 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 (V
losses can be approximated as follows;
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
•
•
•
•
Rdson (Internal P-channel Resistance)
Diode conduction losses
Inductor Conduction losses
Switching losses
the
Efficiency
P
P
P
P
IN
OUT
SW
D
=
=
inductor
=
V
V
=
F
R
IN
F
V
) and the output current. Diode power
×
DSON
OUT
×
=
I
OUT
I
OUT
IN
P
IN
P
OUT
×
) is;
IN
) is calculated as;
×
×
I
current
I
OUT
OUT
(
1
×
−
100
D
2
)
×
D
(IL)
has
M9999-011808-D
completely
MIC4723