Topology and application trends of ultra-low dropout linear regulators

In recent years, low dropout regulators (LDO) have been widely used in various electronic devices, especially consumer electronics with stringent requirements for power. However, with the development of lower dropout application requirements, it is increasingly difficult to meet application requirements due to the limitations of the LDO topology. Therefore, ultra-low dropout regulators (ULDO) based on a new topology came into being.

Differences between ULDO and LDO

LDO is an integrated circuit (IC) with a wide range of uses. Its advantages include simple circuit architecture, low output ripple, few and simple external components, etc. The general LDO architecture is: an error amplifier drives a P-type MOSFET, and the feedback potential is compared with the reference potential to keep the output at the correct voltage. However, when the system requires ultra-low voltage drop, low output voltage (0.8~1.8V), and high output current, it becomes quite difficult to design LDO using the traditional single power supply and P-MOSFET architecture. Therefore, the AX6610-6615 ultra-low voltage drop regulator series uses N-MOSFET as the driver. For drivers of the same size, the driving characteristics of N-MOSFET are generally better than those of P-MOSFET. However, at low input voltage, the driving characteristics of N-MOSFET are insufficient and may not be suitable for the operating voltage of the entire IC. For this reason, another set of power input is used to provide a stable operating voltage for the IC and greatly improve the driving capability of N-MOSFET. This method achieves low voltage input conversion to low voltage output and can drive large load current.

ULDO Features and Topology

The AX6610-6615 series ULDO products are designed with a dual power supply architecture. The power supply VIN is the source of output conversion, and the power supply VCNTL is the power supply used by the internal circuit. It is matched with an N-MOSFET as a driver. The reference potential of the AX6610-6615 is designed to be 0.8 volts, so the lowest output voltage can reach 0.8 volts. Its output voltage soft start time is only 1 millisecond, which can greatly reduce the current surge during startup. The input voltage VIN and VCNTL both have an undervoltage monitoring function. When the input voltage is too low, the output will stop, which can avoid malfunction of the IC end and the back-end system end. In addition, in order to ensure that the voltage obtained by the back-end system is correct, this product has a Power-OK pin to provide a back-end system alarm. The IC itself has a 90-ohm discharge resistor inside, which will be enabled when the output is turned off, and has a self-fast discharge function. The overcurrent protection circuit will limit the current when it exceeds the safe range. If the output is in a short-circuit state, the output periodic opening mechanism will be activated to prevent the output current from being in a high-voltage differential state and limit the current value (because the heat generation power is at its maximum at this time) until the short-circuit state is released. Under continuous heavy load or large cross-voltage, heat will continue to accumulate and the temperature will rise. When the maximum temperature allowable value is reached, the over-temperature protection function will be activated, and the output will be turned off to prevent the IC from burning until the temperature returns to the recovery point, at which time the output will be restarted. In addition, in terms of linear stability, load stability and power supply rejection ratio (PSRR), AX6610-6615 also has excellent performance.

Application Trends of ULDO

Under the premise of future 4C products requiring "low price, thin and light appearance, and mobile Internet access function", the power supply design scheme must abandon the following shortcomings of traditional solutions:

1. Complex external parts

2. Higher output noise and standby power consumption

3. Higher BOM cost

For this reason, Arcelite Technology has launched 1A~5A ULDO linear regulators, namely the AX6610~AX6615 series, which have extremely simplified peripheral components and ultra-low voltage dropout, specifically for UMPC/MID products. In addition, in order to meet the requirements of lightweight products and power saving, such as the power supply required for the various low voltage conversions of UMPC/MIDs equipped with Intel ATOM or VIA NANO platforms, the stable power supply solution is provided.

For example: 1.5V to 1.2V to stably supply the 1A to 5A power management solution required by 802.11/Bluetooth wireless modules or 1.8V/1.5V to 1.1V to supply the 2A to 4A dynamic power required by the system chipset PCH (Platform Controller Hub). This can reduce the external components required to a minimum and achieve lower ripple noise than traditional switching regulators, which is extremely cost-effective for customers.

Figure 1: Block diagram of the AX661x series ULDO chip architecture.

The AX6610~6615 series are linear voltage converters that can provide high current, ultra-low voltage dropout, low ripple interference, and extremely low standby power consumption (below 30μA). The input voltage can be as low as 1.1V to as high as 5V, and the output voltage can be adjusted between 0.8 and 3.3V as needed. They are mainly used in low voltage input and output linear voltage converters, notebook computers, motherboards, network systems, etc.

AX6610-6615 provides high noise immunity (PSRR up to -65dB) and stable operating voltage. The output voltage has a fast transient response and provides a Power-OK output function for convenient power monitoring. The output capacitor can be applied to solid-state capacitors (aluminum/tantalum) and ceramic capacitors (MLCC). It has complete system protection functions including soft start, under voltage lockout (UVLO), short circuit protection (SCP), over temperature protection (OTP) and over current protection (OCP) without the use of external redundant components. The above functions can be achieved and the circuit design can be minimized.

AX6610-6615 adopts SOP-8 LEP lead-free package without bottom shielding pad. Heat can be transferred to the printed circuit board by heat sink, which greatly reduces the surface temperature of components to meet the requirements of UMPC/MID applications.

Figure 2: Typical application circuit of AX6610.

AXElite Technology has just launched the AX1201~AX1204 series ULDO solutions in the third quarter of this year, which have higher withstand voltage (up to 24V) than the AX66xx series, large current output, and extremely low voltage drop characteristics. Its high withstand voltage characteristics are very suitable for the 5V or 12V low-distortion power supply requirements of UMPC/MID/Netbook, such as the audio power supply of TV/monitor or the power supply requirements of UMPC/MID charging dock.