MC33170 Ver la hoja de datos (PDF) - ON Semiconductor
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MC33170 Datasheet PDF : 16 Pages
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MC33170
MC33170 application hints
The MC33170 represents a major leap toward
miniaturization and compactness of Power Amplifiers (PAs)
systems. Prior to talk about the 33170 application circuits,
let us review how a classical dual–band transmission chain
is implemented. At the beginning of the chain, the power
ramping signal is delivered by the Baseband’s Digital to
Analog Converter (DAC). Because of the digitization, a
natural discontinuity appears between the various steps the
signal is made of. As a matter of fact, this sharp transitions
create undesirable effects and need to be smoothed by an
external circuitry (figure 13).
DAC staircase
Filter action
Figure 13. DAC’s signal can be smoothed by an appropriate circuitry
The filtering action can be implemented in a various way,
but usually a 3rd order Bessel filter represents a good choice.
Actual solutions require the use of an external operational
amplifier (OPAMP) dedicated to this function.
For drain–controlled PAs, the power is directly dependent
upon the supply delivered to the device. Several methods
exist but the preferred one stays the N or P channel
modulation. In this application, the N–channel is wired in a
source–follower configuration and therefore needs an
external voltage to ensure its adequate enhancement. This
upper voltage can be obtained from a step–up converter or
directly from ON Semiconductor PAs, as with the
MRFIC0919 or MRFIC1819. To quickly charge/discharge
the MOSFET Ciss capacitor, a dedicated driver is needed,
with a voltage swing high enough to bias the N–channel
toward its specified RDSON.
Radio–Frequency PAs need stable bias levels to keep their
operating point at the right place, despite supply variations.
A Low DropOut (LDO) regulator is the obvious choice for
this purpose. Unfortunately, to keep the quiescent power at
its minimum during the GSM/DCS time–frame pauses (e.g.
no power delivered), it is important to quickly remove the
bias from the PAs. Conversely, the LDO shall be fast enough
to bias the PAs at anytime, without hampering the overall
response time. Such a task is difficult for an off–the–shelf
regulator: a specific component has to be found.
Thanks to their innovative designs, ON Semiconductor
PAs, such as the aforementioned ones, do not require any
external negative sources. However, some synchronization
signals are needed to activate the internal circuitry and
provide them with a stable operating point. This is usually
done by using external low/high power switches.
Finally, a safety system needs to be implemented to
prevent the modulation start in case the negative bias is not
established.
Gathering all these information onto a final drawing gives
birth to figure 14.
Vboost
Digital
Analog
Converter
LDO for
Bias Point
Active
3rd Order
BESSEL Filter
MOSFET Driver
Level
Translation
Vbat
Shutdown
Shutdown
900 MHz
1.8 GHz
TX
Negative
Present?
Dual RF PA
Figure 14. Actual solution to drive a two–PA configuration
http://onsemi.com
11
Power Module
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