mc33128d Freescale Semiconductor, Inc, mc33128d Datasheet - Page 7

• Manufacturer Part Number: mc33128d
• Description: Power Management Controller
• Manufacturer: Freescale Semiconductor, Inc

External Switch
switch the transmitting power amplifier (R L ) 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 V BB
Generator can be made to function as a voltage tripler. The
table in Figure 6 shows the output voltage characteristics of
the tripler circuit.
MOTOROLA ANALOG IC DEVICE DATA
A low threshold N–channel MOSFET can be used to
Load Current
(mA)
0.5
1.0
1.5
2.0
0
V BB Generator
Oscillator
Tripler Output Voltage
V CC = 3.15 V
7.96
7.48
7.24
6.99
6.62
3
Figure 6. Voltage Tripler and Switch Driver
0.22
2
V CC = 4.5 V
V BB
16
12.01
10.69
11.54
11.29
11.04
MC33128
4
8
7
5
V bat
33
4.7
4.7
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, R FB , and capacitors C 1 and C 2 is a simple
solution allowing the system designer a means to control the
ON and OFF time as well as the waveshape. Feedback
resistor R FB controls the waveshape. Capacitors C 1 and C 2
are usually of equal value.
In order to minimize adjacent channel splatter, the RF
ON/OFF
0.1
Output
Tripler
R
C 2
C 1
Load Turn ON/OFF Time
High R FB
Critical R FB
Low R FB
R FB
V bat
R L
Controlled
Turn ON/OFF
Time
7