CC2550-RTY1 datasheet(10/62 Pages) TI2

10 / 62 page

CC2550

SWRS039B

Page 10 of 58

Circuit Description

BIAS

XOSC_Q1 XOSC_Q2

XOSC

FREQ

SYNTH

RADIO CONTROL

RF_P

RF_N

CSn

SO (GDO1)

SCLK

GDO0 (ATEST)

RBIAS

Figure 2:

CC2550 Simplified Block Diagram

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

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.

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

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.

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.

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