MIC2208 MICREL [Micrel Semiconductor], MIC2208 Datasheet - Page 10

• Manufacturer Part Number: MIC2208
• Description: 3mmx3mm 1MHz 3A PWM Buck Regulator
• Manufacturer: MICREL [Micrel Semiconductor]

Therefore, peak to peak ripple current is;
Since the average peak-to-peak current is equal to
the load current. The actual peak (or highest current
the inductor will see in a steady state condition) is
equal to the output current plus half the peak-to-
peak current. For those of you who have not had
enough formulas already, here it is;
Figure 4 demonstrates the off-time. During the off-
time, the high-side internal P-channel MOSFET
turns off. Since the current in the inductor has to
discharge, the current flows through the free-
wheeling Schottky diode to the output. In this case,
the inductor discharge rate is (where V
forward voltage);
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.
September 2005
I
I
T
−
pk
pk
OFF
(
−
V
=
pk
OUT
I
OUT
=
=
1MHz
L
(
- 1
V
+
+
IN
V
D
(
D
V
-
Figure 4. Off-Time
1MHz
V
IN
)
OUT
2
-
×
V
1MHz
OUT
)
×
×
L
V
)
V
OUT
×
IN
×
V
L
V
OUT
IN
D
is the diode
10
When the inductor current (I
discharged, the voltage on the switch node rings at
the
capacitance and the inductor value. In Figure 5, it is
drawn as a DC voltage, but to see actual operation
(with ringing and all) refer to the functional
characteristics.
Discontinuous mode of operation has the advantage
over full PWM in that at light loads, the MIC2208 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 MIC2208 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;
•
•
•
•
Efficiency
P
P
IN
OUT
frequency
Rdson (Internal P-channel Resistance)
Diode conduction losses
Inductor Conduction losses
Switching losses
=
Figure 5. Discontinuous Operation
V
=
IN
V
OUT
×
=
I
IN
P
P
OUT
×
determined
IN
I
OUT
OUT
IN
×
) is;
) is calculated as;
100
L
by
) has completely
www.micrel.com
M9999-092905
the
parasitic