CC2550_13 Datasheet, PDF (10/61 Pages) Texas Instruments – Low-Cost Low-Power 2.4 GHz RF Transmitter

CC2550
6 Circuit Description
RADIO CONTROL
RF_P
PA
RF_N
FREQ
SYNTH
BIAS
XOSC
RBIAS XOSC_Q1 XOSC_Q2
Figure 2: CC2550 Simplified Block Diagram
SCLK
SO (GDO1)
SI
CSn
GDO0 (ATEST)
A simplified block diagram of CC2550 is shown
in Figure 2.
The CC2550 transmitter is based on direct
synthesis of the RF frequency.
The frequency synthesizer includes a
completely on-chip LC VCO.
A crystal is to be connected to XOSC_Q1 and
XOSC_Q2. The crystal oscillator generates the
reference frequency for the synthesizer, as
well as clocks for the digital part.
A 4-wire SPI serial interface is used for
configuration and data buffer access.
The digital baseband includes support for
channel configuration, packet handling and
data buffering.
7 Application Circuit
Only a few external components are required
for using the CC2550. The recommended
application circuit is shown in Figure 3. The
external components are described in Table
12, and typical values are given in Table 13.
Bias resistor
The bias resistor R141 is used to set an
accurate bias current.
Balun and RF matching
The components between the RF_N/RF_P pins
and the point where the two signals are joined
together (C102, C112, L101, and L111) form a
balun that converts the differential RF signal
on CC2550 to a single-ended RF signal. C101
and C111 are needed for DC blocking.
Together with an appropriate LC network, the
balun components also transform the
impedance to match a 50 Ω antenna (or
cable). Suggested values are listed in Table
13.
The balun and LC filter component values and
their placement are important to keep the
performance optimized. It is highly
recommended to follow the CC2550EM
reference design ([3]).
Crystal
The crystal oscillator uses an external crystal
with two loading capacitors (C51 and C71).
See Section 22 on page 32 for details.
Power supply decoupling
The power supply must be properly decoupled
close to the supply pins. Note that decoupling
capacitors are not shown in the application
circuit. The placement and the size of the
decoupling capacitors are very important to
achieve the optimum performance. The
CC2550EM reference design ([3]) should be
followed closely.
SWRS039B
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