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參數(shù)資料
| 型號(hào)： | MIC4720BML |
| 廠商： | Micrel Semiconductor,Inc. |
| 英文描述： | 400 W Transient Voltage Suppressor, SOD123W (SOD2 FlatPower), Reel Pack, SMD |
| 中文描述： | 采用3mm x 3mm 2.0MHz甲集成開關(guān)降壓穩(wěn)壓器 |
| 文件頁數(shù)： | 11/21頁 |
| 文件大小： | 914K |
| 代理商： | MIC4720BML |
Micrel, Inc.
MIC4720
current flows through the free-wheeling Schottky diode
to the output. In this case, the inductor discharge rate is
(where V
D
is the diode forward voltage);
(
)
L
May 2007
11
M9999-051707
V
V
D
OUT
+
The total off time can be calculated as;
2.0MHz
D
1
T
OFF
=
Figure 4. Off-Time
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.
Figure 5. Discontinuous Operation
When the inductor current (I
L
) has completely
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 MIC4720 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;
100
P
P
Efficiency
IN
OUT
×
=
Where input power (P
IN
) is;
IN
I
IN
IN
V
P
×
=
and output power (P
OUT
) is calculated as;
OUT
OUT
V
OUT
P
I
×
=
The Efficiency of the MIC4720 is determined by several
factors.
Rdson (Internal P-channel Resistance)
Diode conduction losses
Inductor Conduction losses
Switching losses
Rdson losses are caused by the current flowing through
the high side P-channel MOSFET. The amount of power
loss can be approximated by;
D
I
R
P
2
OUT
DSON
SW
×
×
=
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 (V
F
) and the output current. Diode power
losses can be approximated as follows;
(
)
D
1
I
V
P
OUT
F
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,
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