Introduction

In recent years, many small and exquisite portable electronic products have been developed, such as mobile phones, digital cameras, MP3, MP4, PDA, GPS and DVD. 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, thus extending the battery life.

In order to extend the charging interval of the product, the designer has 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 that when the product is in different working states, it supplies power to useful circuits and cuts off the power to temporarily unused circuits, so as to save power to the maximum extent, 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 LED) or some other products (such as hard disk, DC motor, etc.). Multifunctional portable electronic products are composed of many functional circuits. With different working voltages, the requirements for power supply 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. The

load management microprocessor composed of power management ICs

realizes 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 opening or closing of the power supply, so that load management can be realized. For example, a product has 6 power supplies. In a certain working state, power supply 1~3 needs to be turned on, then load 1~3 is powered on and working, power supply 4~6 is turned off, and load 4~6 loses power and does not work. Just add a logic high level to the EN terminal of power supply 1~3 and a low level to the EN terminal of power supply 4~6.


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, the -VGS between the source and the gate of Q1 is ≈ VIN, then Q1 is turned on successively, and the power supply voltage can be supplied 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 the 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 4-b).

　
The block diagram of the load management structure composed 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-energized. This solution can also achieve load management, which is mainly used in products with one power supply and multiple loads.

The structure composed of a power management IC and a load switch

is often used in actual applications due to the complexity of the circuit. The load management system composed of a power management IC and multiple load switches is shown in Figure 6. There are 3 power supplies in Figure 6, which output different voltages. Power supply 1 supplies power to loads 1~3. In order to control the power on or off of loads 1~3, 3 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 new load switches

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

Increased protection function

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

Improving performance

Improving performance includes: reducing the on-resistance RDS (on) to reduce losses. Currently, RDS (on) can be achieved to be around 30MΩ; 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 small portable 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, which is 1.5mm×1.0mm.

Here are some typical load switches.

Typical portable load switch

package The smallest 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℃, small size 6-ball BGA package (1.5×1.0mm2); when the switch is turned on, there is voltage rise rate control.

This 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 work at a low voltage of 1.2V. Its disadvantage is that it is too small to integrate protection circuits such as overheat shutdown.

Load switch FPF2163/4/5 system

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 is 1.8~5.5V; the current limit range can be set to 0.15~1.5A; when the input voltage is lower than the threshold voltage, the output is latched (UVLO); there is overheat shutdown protection; the power consumption is less than 2μA when the switch is turned off; the static current is less than 7.7μA; the output current limit (constant current) responds quickly after overcurrent: the response time is 5μS when the medium overcurrent is on; 30nS when the short circuit occurs; when undervoltage lockout, overheating and overcurrent faults occur, the output is latched and a fault signal is output; there is no reverse current when the 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.