MC33128 Просмотр технического описания (PDF) - Motorola => Freescale
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MC33128 Datasheet PDF : 8 Pages
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MC33128
Figure 6. Voltage Tripler and Switch Driver
VBB Generator
Oscillator
0.22
3
2 16
VBB
Vbat
33
0.1
4
4.7
8
4.7
7
Tripler
Output
R C2
C1
5
ON/OFF
Vbat
Controlled
Turn ON/OFF
RFB
Time
RL
Tripler Output Voltage
Load Current
(mA)
VCC = 3.15 V
VCC = 4.5 V
0
7.96
12.01
0.5
7.48
11.54
1.0
7.24
11.29
1.5
6.99
11.04
2.0
6.62
10.69
Load Turn ON/OFF Time
High RFB
Critical RFB
Low RFB
External Switch
A low threshold N–channel MOSFET can be used to
switch the transmitting power amplifier (RL) ON and OFF. To
ensure that all of the available battery voltage appears
across the load, the MOSFET must be fully enhanced over
the system’s required operating voltage range. With the
addition of two Schottky diodes and two capacitors, the VBB
Generator can be made to function as a voltage tripler. The
table in Figure 6 shows the output voltage characteristics of
the tripler circuit.
In order to minimize adjacent channel splatter, the RF
power amplifier must be turned ON and OFF in a controlled
(soft) manner. The applied voltage rise and fall time, as well
as the rate of change in rise and fall time, must be tailored to
the amplifiers characteristics. The circuit consisting of
resistors R, RFB, and capacitors C1 and C2 is a simple
solution allowing the system designer a means to control the
ON and OFF time as well as the waveshape. Feedback
resistor RFB controls the waveshape. Capacitors C1 and C2
are usually of equal value.
MOTOROLA ANALOG IC DEVICE DATA
7
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