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

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

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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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