Micrel, Inc.
MIC4720
current flows through the free-wheeling Schottky diode
to the output. In this case, the inductor discharge rate is
(where VD is the diode forward voltage);
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.
(
V
+ V
)
OUT
D
−
L
Discontinuous mode of operation has the advantage
over full PWM in that at light loads, the MIC4720 will skip
pulses as nessasary, reducing gate drive losses,
drastically improving light load efficiency.
The total off time can be calculated as;
1− D
2.0MHz
T
=
OFF
Efficiency Considerations
Calculating the efficiency is as simple as measuring
power out and dividing it by the power in;
P
OUT
Efficiency =
×100
P
IN
Where input power (PIN) is;
= V ×I
P
IN
IN
IN
and output power (POUT) is calculated as;
= V ×I
P
OUT
OUT
OUT
The Efficiency of the MIC4720 is determined by several
factors.
•
•
•
•
Rdson (Internal P-channel Resistance)
Diode conduction losses
Inductor Conduction losses
Switching losses
Figure 4. Off-Time
Discontinuous Operation
Rdson losses are caused by the current flowing through
the high side P-channel MOSFET. The amount of power
loss can be approximated by;
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.
2
PSW = RDSON ×IOUT × D
Where D is the duty cycle.
Since the MIC4720 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 (VF) and the output current. Diode power
losses can be approximated as follows;
PD = VF ×IOUT
×
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.
Figure 5. Discontinuous Operation
Duty cycle, or the ratio of output voltage to input voltage,
M9999-051707
11
When the inductor current (IL) has completely
May 2007
MICREL [ MICREL SEMICONDUCTOR ]
