Load management and load switch design and implementation in power supplies

Preface

In recent years, many compact and light portable electronic products have been developed, such as mobile phones, digital cameras, MP3, MP4, PDA, GPS and DVD, etc. They are not only small in size, light in weight, and multi-functional, but also have a long charging interval, that is, the product saves power, thereby extending the battery life.

In order to extend the time interval between product charging, designers have adopted many methods: using lithium-ion batteries or lithium polymer batteries with the largest battery capacity per unit volume and unit weight, and increasing the battery capacity; using energy-saving circuits in circuit design; using high-efficiency low-power or micro-power products or using dedicated integrated circuits in component selection to reduce power loss; and the most important measure is to power useful circuits and cut off power to temporarily unused circuits when the product is in different working states, so as to save power to the greatest extent possible, which is the use of load management.

The power supply supplies power to each circuit, and each circuit is the "load" of the power supply. For example, the transmitting circuit and the receiving circuit are the load of the power supply, and the power amplifier circuit is also the load of the power supply. In addition, the load of the power supply also includes some devices (such as LEDs) or some other products (such as hard disks, DC motors, etc.). Multifunctional portable electronic products are composed of many functional circuits. With different working voltages, the requirements for power supplies are also different, so there are multiple power supplies. It is not easy to achieve load management. Modern load management is composed of microprocessors, power management ICs and load switches.

Power management IC composed of load management

The microprocessor implements load management by controlling the operation of the power supply, as shown in Figure 1. There are N power supplies in Figure 1, and each power supply carries a load (there are N loads). Each power supply has an enable terminal (EN, high level is valid) or a power off control terminal (SHDN, low level is valid). The I/O port of the microprocessor is connected to the EN terminal or SHDN terminal of the power supply, and the output logic level is used to control the power supply on or off, so that load management can be achieved. For example, a product has 6 power supplies. In a certain working state, power supplies 1 to 3 need to be turned on, so that loads 1 to 3 are powered on and work, and power supplies 4 to 6 are turned off. Loads 4 to 6 lose power and do not work. Just add a logic high level to the EN terminal of power supplies 1 to 3 and a low level to the EN terminal of power supplies 4 to 6.

Figure 2 is a functional structure diagram of a power management IC (TPS65021) from TI. It is powered by a lithium-ion battery and is interfaced with the device by a microprocessor via I2C. TPS65021 has internal control function circuits and 6 power supplies (3 DC/DC converters, 3 linear regulators LDO, output V1-V6 voltage). It can power mobile phones and implement load management.

There are three kinds of power supplies in digital cameras: system power supply, LCD power supply and white light LED drive power supply (backlight and flash power supply). The system power supply part is composed of 4 DC/DC converters, as shown in Figure 3. It supplies power to the CCD camera, I/F interface circuit, CPU core and telescopic lens motor respectively. The load management can be done through the microprocessor. The microprocessor can also control the LCD backlight brightness and flash drive circuit according to the working state requirements.

Load management composed of load switch IC

Load management can also be composed of a microprocessor and multiple load switches. A load switch is a power electronic switch. Its basic working principle is shown in Figure 4-a). It consists of two MOSFETs (Q1: P-MOSFET and Q2: N-MOSFET) and a resistor R. Its working principle is: when a high level is added to the ON/OFF terminal, Q2 is turned on; when Q2 is turned on, the gate of Q1 is pulled close to the ground level. If its IN is connected to the power supply voltage VIN, then -VGS≈VIN between the source and gate of Q1, then Q1 is turned on successively, and the power supply voltage can supply power to the load through the load switch; if a low level is added to the ON/OFF terminal, Q2 is turned off; the gate and source of Q1 are connected by a resistor R, so that the gate and source are at the same potential, that is, -VGS=0V, Q1 is turned off, the power supply is cut off, and the load loses power. The equivalent circuit of the load switch is shown in Figure 4-b).

The block diagram of the load management structure consisting of a microprocessor, N load switches and N loads is shown in Figure 5. Each load switch is connected between the power supply and the load, and its ON/OFF terminal is connected to the I/O port of the microprocessor. In different working states, the microprocessor outputs a logic level to the ON/OFF terminal of each load switch; the load with a high input level is powered, and the load with a low input level is de-powered. This solution can also achieve load management, and is mainly used in products where one power supply carries multiple loads.

Structure composed of power management IC and load switch

In practical applications, a load management system consisting of a power management IC and multiple load switches is often used due to the complexity of the circuit, as shown in Figure 6. There are three power supplies in Figure 6, outputting different voltages. Power supply 1 supplies power to loads 1 to 3. In order to control the power on or off of loads 1 to 3, three load switches are set between the power supply and the load. Power supply 2 and power supply 3 are controlled by the power management IC to control the power on or off of their loads.

Features of the new load switch

In recent years, with the development of portable electronic products, various advanced and multifunctional (some companies call it "all-round" or "intelligent") load switches have emerged. In summary, these load switches for load management have the following characteristics:

Increase protection function

Load switches are power devices, most of which have overheat shutdown protection. Some load switches have internal output current limiting circuits (the limiting current is fixed) or current limiting circuits that can be set to ensure that the load is not damaged by overload and that the system works safely. In addition, general load switches are equipped with output latching (UVLO) when the input voltage is too low, and output fault signals to the microprocessor when there is a fault.

Improve performance

Improving performance includes: reducing the on-resistance RDS(on) to reduce losses, currently RDS(on) = 30MΩ or so; reducing the quiescent current IQ, with the minimum IQ being only 25nA; in order to prevent a large impact current at the moment the switch is turned on, a soft start is provided (some load switches have a variety of start-up times for users to choose from or can be set by the user according to the load conditions); a discharge circuit is added when the load switch is turned off to accelerate the turn-off time; when the load switch is turned off, reverse current is prevented (flowing from the OUT end to the IN end, the diode in the switch tube can be removed); reducing the operating voltage to meet the needs of the CPU core, the minimum operating voltage is 0.8V, and the operating voltage range is wide, some are 1.2~5.5V, and some are 3~13.5V, to meet the needs of different operating voltages; the current range is 1~6A, which can meet the needs of various portable devices.

Small package size

To meet the requirements of portable and small size, load switches generally use SOT-23 package, 2mm×2mm MLP or MLF package, SC-70 package, and the smallest one uses 6-ball BGA package with a size of 1.5mm×1.0mm.

Here we briefly introduce several typical load switches.

Typical Portable Load Switch

The smallest package load switch FPF1003/4

FPF1003/4 is a load management device launched by Fairchild in April 2007. Its main features are: input voltage range 1.2 ~ 5.5V; on-resistance RDS (on), 30MΩ at VIN = 5.5V, quiescent current IQ ≤ 1μA; FPF1004 has a discharge circuit when it is turned off, and the turn-off time is 10μs; the maximum continuous switch current is 2A; the operating temperature range is -40℃ ~ +85℃, and the small size 6-ball BGA package (1.5×1.0mm2); when the switch is turned on, there is a voltage rise rate control.

The load switch is suitable for modern ultra-small portable electronic products such as PDA, mobile phone, GPS, MP3 player, digital camera, etc.

The typical application circuit of this device is shown in Figure 7. The input capacitance is generally 1μF and the output capacitance is 0.1μF (both use multilayer ceramic capacitors). When a logic high level is added to the ON terminal, the switch is turned on (when VIN=1.2V, VIH≥0.8V; when VIN=2.7~5.5V, VIH≥2V); when a logic low level is added to the ON terminal, the switch is turned off (when VIN=1.2V, VIL≤0.35V; when VIN=2.7V~5.5V, VIL≤0.8V). RDS(on) is related to VIN: when VIN=5.5V, RDS(on)≤30MΩ; when VIN=1.2V, RDS(on)≤150MΩ.

The biggest highlight of this device is that it can continuously supply 2A in such a small package and can operate at a low voltage of 1.2V. Its disadvantage is that its size is too small to integrate protection circuits such as overheating shutdown.

Load switch FPF2163/4/5 system

The FPF2163/4/5 series is a multifunctional load switch with adjustable output current launched by Fairchild in July 2007. The main features of this series are: input voltage range of 1.8~5.5V; adjustable current limit range of 0.15~1.5A; output latching (UVLO) when input voltage is lower than threshold voltage; overheat shutdown protection; power consumption is less than 2μA when switch is turned off; quiescent current is less than 7.7μA; output current limit (constant current) response is fast after overcurrent: response time is 5μS for moderate overcurrent; 30nS for short circuit; output latching and fault signal output when undervoltage lockout, overheating and overcurrent fault occur; no reverse current when switch is turned off, 2×2mm2 micro FET-6 package.

The series FPF2163, FPF2164, and FPF2165 have some functional differences, as shown in Table 1.

In addition to being used in portable electronic products such as PDAs, mobile phones, GPS, MP3 players, and digital cameras, this series of load switches can also be used in peripheral ports and hot-swappable power supplies due to the limited current setting function (with power distribution function). FPF2164 can also be used as an "electronic fuse".

The pin arrangement of FPF2163/4/5 is shown in Figure 8.

The setting of the current limit ILIM is related to RSET, and the relationship is: RSET=275.6/ILIM. The unit of RSET is ohm, and the unit of ILIM is ampere. The setting of the current limit has a certain tolerance, as shown in Table 2.

The FPF2163/4/5 series load switches have a small package size (2mm×2mm) and are fully functional, making them a good device for load management systems.