Application of LDO series products in mobile phone design

With the progress of society, users have higher and higher requirements for communication convenience, which has led to the rapid development of the mobile phone industry in recent years. From analog to digital, from black and white screen to color screen, from simple call function to surfing the Internet, video intercom, mobile TV, GPS positioning, new applications are emerging in an endless stream. However, as the functions of mobile phone systems become more and more complex, the requirements for the stability, power supply voltage, efficiency and cost of the power supply system are also getting higher and higher. Corresponding system suppliers, such as MTK, TI, INFINION, NXP, etc., have also updated their system power management units (PMUs), but as system-level chips, the update is much slower than the update of product functions. For some key components, such as the power supply of the RF module and the PLL power supply of the GPS module, the requirements for output ripple and PSRR (power supply ripple rejection ratio) performance are very high. These indicators will directly affect the signal reception sensitivity of the mobile phone and the signal reception sensitivity of the GPS. Using PMU for power supply will increase the complexity of system design for engineers. Therefore, the application of various LDOs in mobile phones is always full of vitality.

LDO is a voltage stabilizing device that uses a low working voltage difference to adjust the output voltage through negative feedback to keep it constant. According to different manufacturing processes, LDO has several types, such as Bipolar, BiCMOS, and CMOS, with different performances. However, with the increasing cost pressure, CMOS LDO has become the mainstream in the market.

Structurally, LDO is a miniature on-chip feedback system. It is integrated with functional circuits such as power MOSFET, Schottky diode, sampling resistor, voltage divider resistor, overcurrent protection, overheat protection, precision reference source, amplifier, and PG (Power GOOD) on a chip. Figure 1 is a typical functional diagram of CMOS LDO.

For mobile phones, it is mainly divided into three functional units: RF, baseband, and PMU. Although PMU can meet most of the power supply requirements, the power supply of the RF part, the power supply of the camera module, GPS, and the newly added power supply requirements of the WIFI part cannot be met due to the update speed of the PMU itself, as well as cost and heat dissipation issues, and an additional power supply is required. SGMICRO's LDO products have extremely low quiescent current, extremely low noise, very high PSRR, and very low Dropout Voltage (input-output voltage difference), which can mostly meet the power supply requirements under these application conditions.

In mobile phone applications, the PSRR, output noise, and startup time of LDO directly affect the performance of the mobile phone. It is necessary to select appropriate parameters and consider wiring according to the actual application situation. When selecting peripheral devices, pay attention to the following seven points:

1. The selection of output capacitor affects the stability of LDO, transient response performance, and the size of output noise Vrms.

2. The selection of input capacitor affects transient response performance, EMI and PSRR.

3. The filter capacitor affects the output ripple, PSRR, transient response performance and startup time.

4. Prevent current backflow and the size of static current.

5. The circuit design should consider suppressing input voltage overshoot (whether the voltage regulator is selected or not).

6. The wiring affects the efficiency of heat dissipation (Tdie<100℃).

7. Select the appropriate startup time according to system requirements.

Figure 1: Basic architecture and simple application circuit diagram of CMOS LDO

The following parameters of LDO are particularly important in mobile phone design:

LDO stability and transient response. Due to the large dynamic changes in load current, LDO stability and transient response performance are required to be good, otherwise the system will work abnormally.

PSRR parameter. PSRR parameter directly affects the receiving sensitivity of the RF module. If used in the audio part, it can suppress EMI interference in the mobile phone and make the sound more expressive.

LDO output noise. This is directly related to the cleanliness of the output power supply.

LDO startup time. The startup time is closely related to the power-on timing of the system design and directly affects whether the system works.

LDO recommended PCB design. During the design process, the input capacitor Cin and the output capacitor Cout need to be as close to the LDO as possible. In the application of LDO, thermal design is often an easily overlooked area. It is necessary to consider the selection of appropriate packages under different power conditions. There are three common ones, SC70, SOT23 and DFN-6. Taking the power supply of the RF module as an example, the SC70 package itself allows the heat dissipation power to be usually within 0.2W:

PD=(Vin-Vout)*Iout+Vin*Ignd <0.2W

Vin=Vbattery=3.6V or above, Vout is usually 2.8V. If the current exceeds 250mA, it will cause instability. The SOT23 package allows heat dissipation of about 0.4W, which is more suitable for this part of the application. If the chip size is considered, the DFN package can be selected, which can take into account the heat dissipation requirements (PD>0.4W).

As an emerging semiconductor supplier, SG MICRO has also launched a series of LDO products: general-purpose LDO (three-terminal voltage regulator), RF LDO (PSRR can reach 73DB@1kHZ), and high-precision LDO (full load 0~300mA, full temperature range -40~125℃, accuracy 1.6%). Taking the RF LDO SGM2007 as an example, the output noise is 30μVrms, the output voltage difference is 300mV (full temperature range, full load 0-300mA), the static power consumption is as low as 77μA, the shutdown current is less than 10nA, the PSRR is 73db at 1kHz, and 78dB at 216.67Hz. The LDO SGM2007 has overheat protection and overcurrent protection functions, and the startup time is within 20μS.