MC33170 Datasheet, PDF (12/16 Pages) ON Semiconductor – RF Amplifier Companion Chip for Dual-Band Cellular Subscriber Terminal

MC33170
MC33170 as a Bessel filter
Thanks to its package, the MC33170 simplifies the
implementation of any filtering/driving configuration, e.g.
with either an N or P–channel MOSFET. Figure 15a details
the way to wire a 100kHz filter while driving an N–channel
MOSFET. In this application, a third order filter is achieved
by combining a first pole passive RC–filter, followed by a
second–order Sallen–Key complex pole–pair section.
V_boost
V_battery
Input
1k
19.88 k
680 pF
108.4 k
22 pF
33 pF ₁₁
+
9
-
10
MC33170
OPAMP Section
12
6
10 pF
100 k
100 k
MTSF3N02HD
To Power
Amplifier’s Drain
Figure 15a. Using the MC33170’s OPAMP to filter out the DAC discontinuities
As one can see, it is easy to select the desired gain value
via the 100k feedback resistors and accordingly tailor it to
the DAC output level. Figure 15a performs the filtering
function but also delivers the adequate sink/source current
to drive the MOSFET transistor. The two–component
section of figure 2 is reduced into a single one, saving cost
and PCB area. It also important to point out that the OPAMP
section can be totally disabled by the Common Enable pin.
Benefits of the closed loop configuration
One of the MC33170’s key applications is to make the
modulation section operating in a closed–loop
configuration. That is to say, the power chain is closed
through the feedback resistor (the 100kW network in
figure 3) and forces the output to follow the input ramp.
With N–channels, it brings several benefits:
1. The input ramp does no longer deals with the MOSFET
threshold voltage which can introduce a certain amount of
delay in the response time.
2. At low powers, the distributions between the RDS_(ON)
is automatically compensated.
With P–channels, the application does not need an
elevated voltage to ensure the channel enhancement but
maximizes the presence of the OPAMP to ensure a fully
linear chain.
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