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UC3380 датащи(PDF) 8 Page - Unisonic Technologies

номер детали UC3380
подробное описание детали  PWM STEP UP DC-DC CONTROLLER
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производитель  UTC [Unisonic Technologies]
домашняя страница  http://www.utc-ic.com
Logo UTC - Unisonic Technologies

UC3380 датащи(HTML) 8 Page - Unisonic Technologies

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UC3380
CMOS IC
UNISONICTECHNOLOGIESCO.,LTD
8 of 9
www.unisonic.com.tw
QW-R502-099.D
EXTERNAL COMPONENTS
1. Diode (D1)
The diode is the largest source of loss in DC-DC converters. The most important parameters which affect the
efficiency are the forward voltage drop UD and the reverse recovery time. The forward voltage drop creates a loss
just by having a voltage across the device while a current flowing through it. The reverse recovery time generates a
loss when the diode is reverse biased, and the current appears to actually flow backwards through the diode due to
the minority carriers being swept from the P-N junction. A Schottky diode with the following characteristics is
recommended:
*Low forward voltage: UD<0.3V
*Fast reverse recovery time/switching speed:
50nS
*Rated current: >IPK
*Reverse voltage:
U
O+UD
2. Inductor (L1)
Low inductance values supply higher output current, but also increase the ripple and reduce efficiency. Choose a
low DC-resistance inductor to minimize loss. It is necessary to choose an inductor with saturation current greater
than the peak current that the inductor will encounter in the application. Saturation occurs when the inductor’s
magnetic flux density reaches the maximum level the core can support and inductance falls.
3. Capacitor (C1, C3)
The input capacitor C1 improves the efficiency by reducing the power impedance and stabilizing the input current.
Select a C1 value according to the impedance of the power supply used. Small Equivalent Series Resistance (ESR)
Tantalum or ceramic capacitor with an appropriate value should be suitable
The output capacitor is used for smoothing the output voltage and sustaining the output voltage when the switch is
on. Select an appropriate capacitor depending on the ripple voltage that increases in case of a higher output voltage
or a higher load current. The capacitor value should be 10uF minimum. Small ESR should be used to reduce output
ripple voltage. However, the best ESR may depend on L, capacitance, wiring and applications (output load).
Therefore, fully evaluate ESR under an actual condition to determine the best value.
4. External transistor (Q1 R1 C2)
An enhancement N-channel MOSFET or a bipolar NPN transistor can be used as the external switch transistor.
*Bipolar NPN transistor
The hFE value of NPN transistor and the R1 value determine the driving capacity to increase the output
current using a bipolar transistor. 1KΩ is recommended for R1. R1 is selected from the following calculation.
Calculate
the
necessary
base
current(Ib)
from
the
bipolar
transistor
hFE
using
PK
b
FE
I
I
h
=
0.7
0.4
1
||
OUT
b
EXTH
V
R
II
=−
Since the pulse current flows through the transistor, the exact RB value should be finely tuned by the
experiment. Generally, a small RB value can increase the output current capability, but the efficiency will
decrease due to more energy is used to drive the transistor.
Moreover, a speed up capacitor, C2, should be connected in parallel with R1 to reduce switching loss and
improve efficiency. C2 can be calculated by the equation below:
1
2
21
0.7
OSC
C
Rf
π
××
×
It is due to the variation in the characteristics of the transistor used. The calculated value should be used as
the initial test value and the optimized value should be obtained by the experiment.
*Enhancement MOS FET
For enhancement N-channel MOSFET, since enhancement MOSFET is a voltage driven, it is a more efficient
switch than a BJT transistor. However, the MOSFET requires a higher voltage to turn on as compared with
BJT transistors. An enhancement N-channel MOSFET can be selected by the following guidelines:
-
Input capacitance less than 700pF.
-
Low gate threshold voltage.
-
Low on-resistance.
-
The allowable maximum current of drain should be larger than peak current of inductor.


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