ST AN2760 Application note

AN2760
Application note
Using clock distribution circuits in smart phone system design
Introduction
As smart phones become more and more popular in the market, additional features such as A-GPS, Bluetooth, WLAN and DVB-H are now included in the cell phone design. In all of these application modules, there is a common need for a master clock which is typically an external crystal oscillator. With the master clock the data transmitted or received is modulated to the proper frequency. In this application note, conventional clock trees which use discrete crystals for different modules or discrete buffer solutions are compared to the clock solution using the integrated STCD1020/1030/1040 series circuits from STMicroelectronics (referenced as STCD10x0 throughout this application note, please refer to the STCD1020, STCD1030, STCD1040 datasheet). The benefits of using STCD10x0 are illustrated and technical hints are given to help cell phone system designers use the STCD10x0 clock distribution solution.
July 2008 Rev 1 1/14
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Contents AN2760
Contents
1 Conventional cell phone clock solutions compared to the STCD10x0
integrated solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Conventional clock solution - dedicated crystals for dedicated modules . . 4
1.2 Conventional clock solution - discrete buffer solution . . . . . . . . . . . . . . . . . 4
1.3 Integrated clock solution - STCD10x0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Integrated STCD10x0 clock solution in MID application . . . . . . . . . . . . . . 7
1.5 Integrated STCD10x0 clock solution in multimode RF front-end . . . . . . . . 7
2 Cell phone system design with STCD10x0 clock solution . . . . . . . . . . 8
2.1 Preparation to use STCD10x0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.1 Setting a common clock frequency for each application . . . . . . . . . . . . . 8
2.1.2 Choosing the right clock source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2 Power supply VCC of STCD10x0 and decoupling . . . . . . . . . . . . . . . . . . . 9
2.3 Connection of the MCLK pin to external clock source . . . . . . . . . . . . . . . . 9
2.4 Connecting STCD10x0 outputs to application modules . . . . . . . . . . . . . . 10
2.5 Power-down sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
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AN2760 List of figures
List of figures
Figure 1. Conventional clock solution - dedicated crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 2. Conventional clock solution - discrete buffer solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 3. Typical application circuit using STCD1040 in mobile phone or MID application . . . . . . . . 6
Figure 4. Typical application circuit using STCD1040 in dual-mode mobile RF front-end . . . . . . . . . 7
Figure 5. Connection of the DC-CUT capacitor and bias. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 6. Power-down possibility (incorrect sequence) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
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Conventional cell phone clock solutions compared to the STCD10x0 integrated solution AN2760
1 Conventional cell phone clock solutions compared to
the STCD10x0 integrated solution

1.1 Conventional clock solution - dedicated crystals for dedicated modules

The easiest design of the clock tree in smart phones is to use dedicated crystals for different application modules. This is illustrated in Figure 1.

Figure 1. Conventional clock solution - dedicated crystals

TCXO
TCX
XO
XO
XTAL
XTAL
Although the solution in Figure 1 is simple, an obvious drawback is the cost. Normally WLAN and Bluetooth can accept the master clock in several different frequencies. For example, 13, 16, 16.8, 19.2, 26, 3.6, 38.4 and 52 MHz are all accepted by WLAN, Bluetooth, and FM radio. Different modules recommend different master clock frequencies and those recommended values are not always the same. For example, 19.2 MHz is recommended for Bluetooth by some chip vendors while 26 MHz is recommended for WLAN. Due to the design risk and time-to-market, cell phone vendors are not willing to use the same crystal to serve all application modules although this is possible.
A typical external clock source (typically 10 pF load capability) is always short of fan out capability if several application modules are directly connected to its output.
WLAN
WLA
Bluetooth
Media
Media
ai14013

1.2 Conventional clock solution - discrete buffer solution

Since the cost of a TCXO (temperature compensated crystal oscillator) is extremely high and some of the common frequencies can be used for different applications, cell phone designers have tried to use a discrete buffer solution (with emitter followers) to solve the insufficient fan out issue. A two-stage discrete buffer is used as shown in Figure 2. The first stage could solve the problem of the fan out of the clock source (normally TCXO) and also provide a good isolation of input channel to output channels. The second stage distributes the master clock to different application modules and also provides good isolation between different output channels.
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AN2760 Conventional cell phone clock solutions compared to the STCD10x0 integrated solution

Figure 2. Conventional clock solution - discrete buffer solution

2.8V
R627
R601
C603
V601
CLK_OUT
C602
CLK_IN
Clock
Source
C602
R603
R602
In Figure 2 only one clock source (TCXO) is used in order to greatly reduce the cost, but there are several drawbacks. The most important issue is the PCB area. This solution includes 5 discrete buffers and each buffer consists of many discrete components such as a bipolar transistor, resistors and capacitors. The large PCB area and the difficulty in routing always become problematic for the system designers. The second issue is the large quiescent current (the static current consumed by the buffer itself) and the inability of the output buffers to be shut down (constantly conduct quiescent current) to save current even when the connected application module does not require the clock.

1.3 Integrated clock solution - STCD10x0

In order to resolve the drawbacks of conventional clock solutions and facilitate cell phone system design, STMicroelectronics has introduced an integrated clock solution with STCD10x0 series clock distribution circuits. A typical application is given in Figure 3. The user should note that both sine wave and square wave clock source can be used and the STCD10x0 can distribute to the outputs with unity gain clocks.
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Conventional cell phone clock solutions compared to the STCD10x0 integrated solution AN2760
Figure 3. Typical application circuit using STCD1040 in mobile phone or MID
application
BT_External_Req
Internal_Req
Bluetooth
WLAN
GPS
Other device
ai14015
VCTCXO
MCLK
V
CC
V
CC
STCD1040
GND
EN4
CLK4
EN3 CLK3
EN2 CLK2
EN1 CLK1
Note: The output DC cut capacitors should only be used on the sine wave clock transmission (not
necessary for square wave clock source).
An external clock source (VCTCXO: voltage controlled TCXO) can be connected to the STCD10x0 directly since a DC-CUT capacitor has been integrated into the STCD10x0. Each of the output channels can be enabled/disabled individually so that there is no extra current consumption when there is no clock request for this channel. In order to match different application requirements, two voltage versions (1.8 V and 2.8 V) can be selected by the users. Four-channel outputs with 10 pF capacitive loads for each output consume only
2.8 mA quiescent current. A comparison of the STCD1040 and a discrete buffer solution is given in Tab le 1 .

Table 1. Conventional discrete buffer and integrated STCD10x0 clock solution

Parameter Conventional discrete buffer STCD10x0
PCB area/cost Large Small (up to 60% less PCB area)
Quiescent current High Low (at least 30% less current)
Output enable Always on Can be disabled individually
Input DC-CUT cap Required Not required
System cost High Low
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AN2760 Conventional cell phone clock solutions compared to the STCD10x0 integrated solution

1.4 Integrated STCD10x0 clock solution in MID application

Mobile Internet Device (MID) represents a new category of small and truly mobile devices which integrate the advantages of both UMPC (Ultra-Mobile PC) and mobile phones. This application has recently gained much attention and the trend to incorporate many types of wireless connection technologies such as WLAN, BT and GPS into MIDs. The STCD10x0 solution provides significant benefit to the MID clock solution, not only in system cost but also in PCB area. The connection of the STCD10x0 clock solution in MIDs is also illustrated in Figure 3.
1.5 Integrated STCD10x0 clock solution in multimode RF front­end
In multimode mobile RF front-end, multiple reference clocks are typically used for the transceivers. Figure 4 gives an example of TD-SCDMA/GSM dual-mode mobile RF front­end clock application. Alternatively, the clock from VCTCXO could be distributed by STCD10x0 to the TD-SCDMA transmitter and receiver and GSM transceiver separately.
Figure 4. Typical application circuit using STCD1040 in dual-mode mobile RF front-
end
VCTCXO
MCLK
V
CC
V
CC
STCD1040
GND
EN4
CLK4
EN3
CLK3
EN2
CLK2
EN1
CLK1
Mode selection
TD-SCDMA transmitter
TD-SCDMA receiver
GSM transceiver
ai13954
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Cell phone system design with STCD10x0 clock solution AN2760

2 Cell phone system design with STCD10x0 clock
solution

2.1 Preparation to use STCD10x0

2.1.1 Setting a common clock frequency for each application

In each of the applications (WLAN, GPS, BT, FM radios etc.), the default reference clock frequencies are not the same. The user should configure the internal register or external pins of each application module to make sure they accept the same clock frequency. Once this is done, the common clock source signal can be distributed by the STCD10x0.
Ta bl e 2 gives the user an example of different methods to configure each application module
to accept the same common reference clock frequency. CS-101560 is the GPS chip from CSR, GSC3KT is the Bluetooth chip from Sirf, STLC4550 and STLC9000 are WLAN chips from STMicroelectronics. Normally, 19.2 MHz, 26 MHz and 38.4 MHz are commonly accepted and the user can configure the internal registers, or external pins, or just directly connect to ensure they run at the same common frequency.
Table 2. Configuring common reference clocks for GPS, BT or WLAM
Part
CS-101560 (CSR)
GSC3KT (Sirf)
STLC4550 (ST)
STLC9000 (ST)
Common
ref. freq.
19.2 MHz
26 MHz Set register "PSKEY_ANA_FREQ (0x1fe)" value to "26000".
38.4 MHz
19.2 MHz
26 MHz Set pins "ED[7]:JTCK:JTDI" to be "110".
38.4 MHz Set pins "ED[7]:JTCK:JTDI" to be "000".
19.2 MHz
26 MHz Set pins "MODE(4:0)" to be "11100".
38.4 MHz Set pins "MODE(4:0)" to be "11010".
19.2 MHz
26 MHz
38.4 MHz
Ref. clock
setting method
Internal register
Pin configure
Pin configure
Automatic
detection
How to configure
Set registers "PSKEY_ANA_FREQ (0x1fe)" and "PSKEY PLLX_FREQ_REF (0xabc)" value to "19200".
Set registers "PSKEY_ANA_FREQ (0x1fe)" and "PSKEY PLLX_FREQ_REF (0xabc)" value to "38400".
Set pins "ED[7]:JTCK:JTDI" to be "010".
Set pins "MODE(4:0)" to be "00100".
An integrated automatic detection algorithm detects the system clock frequencies
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AN2760 Cell phone system design with STCD10x0 clock solution

2.1.2 Choosing the right clock source

Phase noise is a critical specification in reference clocks. The total phase noise of the clock tree should be obtained by summing the additive phase noise of the STCD10x0 and the phase noise of the clock source (TCXO) in terms of power as illustrated in Equation 1:
Equation 1
PN
C
T
10
PN
X
10
PN1010PN
<+=
A
where:
PN
is the total phase noise in dBc/Hz
T
PN
is the additive phase noise of STCD10x0 and
C
PN
is the phase noise of clock source
X
Make sure the total phase noise is kept within the phase noise requirement of each application PN
. The user should choose the right TCXO with proper phase noise to meet
A
the requirement.

2.2 Power supply VCC of STCD10x0 and decoupling

The STCD10x0 is designed with ultra-low added phase noise so that the device can also be used for master clocks in RF front-end transmitters and receivers for cell phones. A very clean power supply ( as specified 30 µVrms supply noise from 300 Hz to 50 kHz in the datasheet) is needed. Normally a TCXO (or VCTCXO) is used as the master clock source. It is strongly recommended to share the same power supply for STCD10x0 with the TCXO since the supply for the TCXO is always an ultra-low noise, high PSRR, LDO. The LDO can be shut down when the TCXO (also STCD10x0) is not needed.
A 1 µF capacitor is highly recommended to be placed as close as possible to the V the STCD10x0 on the PC board to minimize the noise of the V performance of the phase noise.
and guarantee the
CC
CC
pin of

2.3 Connection of the MCLK pin to external clock source

If the output of the clock source voltage level is within the supply rails of the STCD10x0, the output of the source clock should be connected directly to the MCLK of the STCD10x0. The direct connection of the source clock is the most common case, allowing a DC-CUT capacitor to be saved on the PCB.
The input clock voltage level of the STCD10x0 cannot exceed the supply rails when it is directly connected to the source clock. If it is necessary to connect a source clock with a voltage level exceeding the supply rails of the clock distribution circuits in the application, the user needs to connect a DC-CUT capacitor serially as shown in Figure 5. A voltage divider formed by a resistor string is also needed to set a proper DC bias for the clock input which can keep the clock voltage within the supply rails of STCD10x0. The proper DC voltage is around half of the supply.
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Cell phone system design with STCD10x0 clock solution AN2760

Figure 5. Connection of the DC-CUT capacitor and bias

V
CC
R
1
VCTCXO
OUT
0.1μF
DC-CUT
MCLK
R
2
STCD1020
STCD1030
STCD1040
ai13957

2.4 Connecting STCD10x0 outputs to application modules

The output channels of the STCD10x0 are biased at half of VCC internally by design. 1µF DC-CUT capacitors are required at each of the outputs in most applications.

2.5 Power-down sequence

If the STCD10x0 shares the same power supply with TCXO (clock source), it is suggested to power up the TCXO before enabling each of the clock outputs since normally it will take several milliseconds for the TCXO to stabilize its output.
If the STCD10x0 uses different power supplies with TCXO, it is not recommended to power down V (when no clock output is enabled, the STCD10x0 enters standby mode). If shutdown of the STCD10x0 is preferred, the following power-down steps must be followed:
1. Shut down clock source TCXO output
2. Disable EN pins, pull low
3. Shut down STCD10x0 power V
If the power-down sequence is not strictly followed as above, the user may find the clock output waveform viewed at the condition of TCXO active with output, V shutdown, and EN pins active. This possibility is illustrated in
of STCD10x0 since it only consumes standby current which is no more than 1µA
CC
CC
of STCD10x0
CC
Figure 6.
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AN2760 Cell phone system design with STCD10x0 clock solution

Figure 6. Power-down possibility (incorrect sequence)

In the STCD10x0 chip design, there are ESD diodes for the EN pins to connect to VCC and GND for ESD protection. If V
is powered down (disconnect to supply) and an active high
CC
signal is present on the EN pin (for example, 2.8 V), the ESD diode which connects the EN pin to V
will conduct and pull VCC up to 2.1V. When the external TCXO is active and
CC
sends a clock output to MCLK of STCD10x0, a clock waveform should be viewed at the output as illustrated in
Figure 6 which could cause a problem to the application.
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Conclusion AN2760

3 Conclusion

As smart phones become more and more powerful with versatile application features, more and more clocks are needed in the cell phone system design. Conventional clock solutions with dedicated crystals or discrete buffers cannot meet the requirement of the design. Using the STCD10x0 in cell phone system design is a very cost-effective, easy solution and shortens the design and layout cycle which allows for faster time-to-market.
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AN2760 Revision history

4 Revision history

Table 3. Document revision history

Date Revision Changes
31-Jul-2008 1 Initial release.
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AN2760
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