Application of LDO in mobile phone design

With the progress of society, users have higher and higher requirements for communication convenience, which has made the mobile phone industry develop rapidly in recent years. From analog to digital, from black and white screens to color screens, from simple call functions to web surfing, video intercom, mobile TV, GPS positioning, new applications are emerging. However, as the functions of mobile phone systems become more and more complicated, the requirements for the stability of the power supply system, the supply voltage, efficiency and cost are becoming higher and higher. Corresponding system suppliers, such as MTK, TI, INFINION, NXP, etc., have also updated their own system power management units (PMUs). However, as system-level chips are updated, they are far slower than product functions. For some key devices, such as the power supply of the RF module and the PLL power supply of the GPS module, the output ripple, PSRR (power ripple rejection ratio) performance requirements are very high, these indicators will directly affect the signal reception sensitivity of the mobile phone and GPS signal reception sensitivity. Using PMU 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 operating pressure difference to adjust the output voltage to maintain it unchanged through negative feedback. According to different manufacturing processes, LDO has several types of Bipolar, BiCMOS, CMOS, performance is different, but with the increase in cost pressure, CMOS LDO has become the mainstream of the market.

The LDO is structurally a miniature on-chip feedback system. It consists of a power MOSFET, Schottky diode, sampling resistor, voltage divider, overcurrent protection, overheating protection, precision reference source, amplifier, and PG with voltage and current adjustment. (Power GOOD) and other functional circuits are integrated on a chip. Figure 1 is a typical functional diagram of CMOS LDO.

For mobile phones, it is mainly divided into three major functional units: radio frequency, baseband, and PMU. Although the PMU can meet most of the power supply requirements, for the power supply of the radio frequency part, the power supply of the camera module, the GPS, and the WIFI part, the new power supply needs, due to the speed of the PMU itself, and the consideration of cost and heat dissipation, and Unsatisfactory, 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 and output voltage difference), which can mostly meet the power supply requirements under these application conditions.

In mobile phone applications, the parameters of the LDO's PSRR, output noise, and start-up time directly affect the performance of the mobile phone. It is necessary to select appropriate parameters and consider wiring according to the actual application. In the selection of peripheral devices, we must pay attention to the following seven points:

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

2. The choice of input capacitance affects transient response performance, EMI and PSRR

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

4 Prevent the current from flowing backwards, the size of the static current

5. The circuit design should consider the suppression of input voltage overshoot (whether the choice of voltage regulator or not)

6. Wiring affects the efficiency of heat dissipation (Tdie "100 ℃"

7. Choose the appropriate start time according to the system requirements

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