Design of power supply for handheld device based on SP6641

1 Introduction

At present, various electronic products and portable instruments powered by batteries tend to be designed with low voltage, low power consumption and miniaturization, and their power management systems all require DC/DC converters. The core of this converter is output accuracy, conversion efficiency, start-up voltage, etc. Here we introduce a boost PFM control DC/DC converter-SP6641 with simple structure, small size, high efficiency and low power consumption, and give a detailed application of this device in the power management of psychological assessment handheld data acquisition system.

2 SP6641 Device Introduction

SP6641 is a handheld low-power, low-voltage, step-up DC/DC converter that can provide a stable DC power supply for the microcontroller system when the input voltage dynamic range is large. It has extremely low quiescent current, high battery conversion efficiency, and high cost performance. It is mostly used in battery-powered devices. The quiescent current of SP6641 is 10μA, with a 0.3Ω N-channel charging switch, a minimum startup voltage of 0.9V, and a 0.33A or 1.0A inductive current limit module. The SP6641 device adopts a 5-pin SOT-23 chip package, and its pin configuration is shown in Figure 1.

2.1 Performance characteristics

The performance characteristics of the device are as follows: extremely low quiescent current of 10μA; wide input voltage range of 0.9~4.5V; IOUT can reach 90mA at 1.3V input (SP6641A-3.3V); IOUT can reach 500mA at 2.6V input (SP6641B-3.3V); IOUT can reach 100mA at 2.0V input (SP6641A-5.0V); IOUT can reach 500mA at 3.3V input (SP6641B-5.0V); fixed output voltage of 3.3V or 5.0V; power conversion efficiency up to 87%; 0.3Ω NFET RDSon; 0.33A inductive current protection module (SP6641A); 1A inductive current protection module (SP6641B); logic shutdown control; SOT-23-5 package.

2.2 Usage

Different versions of SP6641 have different powers and fixed output voltages (3.3 V, 5.0 V). You can choose according to the actual use situation when applying. SP6641 provides two versions with different powers, such as SP6641A and SP6641B. The pins of these two are the same, but the maximum output current provided is different, and the peripheral components are also different. When the input voltage drops, the output current also drops. When VBATT is 1.3 V, the output current is 90 mA (SP6641A) and 200 mA (SP6641B). When a different output power supply voltage is required, just select the version with a different suffix. Figure 2 is the schematic diagram of the SP6641B power supply circuit.

3 Application of SP6641B in Handheld Devices

3.1 Handheld device design

When designing, first determine the corresponding power supply device according to the required fixed output voltage and maximum operating current. Then determine the power supply circuit and peripheral components based on the power supply device. The handheld device uses the low-power microcontroller (MCU) MSP430F123A as the main controller, scans the keyboard response consisting of 0 to 9 numbers, and then realizes wireless data transmission through the LSD-RF1100-A433 wireless module. The principle structure block diagram of the handheld device is shown in Figure 3.

The LSD-RF1100-A433 wireless module is a radio frequency module based on TI's CC1100. The operating voltage of the module is 3 to 3.6 V, and the operating current is less than 30 mA. The operating voltage of the single-chip microcomputer MSP430F123 is 1.8 to 3.6 V, and the operating current is less than 2 mA. Considering the convenience of actual use, two ordinary No. 5 alkaline batteries are used as the power supply. To ensure the normal operation of the system, it is necessary to design a stable 3.3 V power supply circuit with a supply current of not less than 50 mA, and to ensure a stable output of 3.3 V when the battery voltage is 1.5 V. In order to extend the working time of the handheld device, the SP6641B with a boost function is used here to design a power supply circuit to provide a long-term stable 3.3 V power supply for the handheld device.

3.2 Power supply device selection

Since the handheld device only needs a fixed voltage of 3.3 V and the maximum operating current is about 50 mA, SP6641B is selected as the power supply device. When VBATT = 1.3 V, the output current is 200 mA. If the power supply is required to have a lower voltage ripple, a filter circuit can be added.

3.3 Selection of peripheral circuit components

The data sheet of SP6641B provides the parameters of the required peripheral circuit components: C1=C2=100μF (POSCAP), L1=10μH (CDRH5D2B), VD1 is ZHCS2000, C3=1μF ceramic capacitor. The stability of the output voltage of the power supply device depends not only on the quality of the power supply device, but also on the reasonable value and quality of the peripheral circuit components. The selection of external component values ​​is mainly determined by the output voltage, maximum load current and maximum and minimum input voltages. First determine the inductance value, and then select the diode, capacitor and other values ​​in turn.

(1) Inductor selection Inductor is the main factor that determines the stability of the power supply system. The main parameters of the inductor include: inductance peak current and internal resistance. The resistance of the inductor coil should be less than 0.5Ω. In order to reduce noise radiation, a shielded inductor should be selected. Inductor L1 is the energy storage element of the entire circuit, and an appropriate power element must be selected. Therefore, the inductor is selected from the German Wiirth Elektronik company, the model is the WE-PD series shielded power inductor, with IDC=1.83 A and RDC=0.072Ω. At this time, the conversion voltage of the power supply circuit is stable and the conversion efficiency is high. When the input voltage is lower than 1.2 V, it still provides a stable 3.3 V output.

(2) Diode selection During operation, the forward voltage drop of diode VD1 has a great influence on the efficiency of the switching power supply. To improve the efficiency of the converter, while ensuring the normal operation of the diode, a Schottky diode with a small forward voltage drop and a short reverse recovery time should be selected. Its output voltage and peak switching current should also be considered. Therefore, 1N4148 is selected here.

(3) Capacitor selection When designing, the output filter capacitor should be selected based on the principle of minimum equivalent series resistance (ESR). At the same time, the capacitor should be placed as close to SP6641B as possible. Therefore, the component parameters are selected here: C1 and C2 are 100μF and 330μF tantalum capacitors respectively, and C3 is a 1μF ceramic capacitor. During debugging, if capacitor C3 is omitted, the power supply voltage will be very unstable. Therefore, when designing, it should be placed as close to SP6641B as possible.

3.4 Test Results

After testing, the current of the handheld device is only 31.5 mA when it is dynamic (the wireless module is in the transmitting and receiving state); the current is only 20μA when it is static.

(1) Two-cell alkaline battery input Used alkaline batteries are used to test the power supply voltage regulation value under different input voltage conditions. Due to the discreteness of alkaline battery voltage, only a few discrete voltage values ​​need to be measured. When Vin is 3.1 V, 2.96 V, 2.83 V, 2.66 V, 2.4 V, and 2.14 V, Vout is 3.3 V and the system works normally; but when Vin is 2.14 V, Vout is 3.3 V in static state, and Vin drops to 1.7 V in dynamic state.

(2) When Vin>1.34 V is input from one alkaline battery, Vout=3.3 V and the system works normally. When Vin=1.2 V or 1.07 V, Vout is 3.3 V and the system cannot work normally. The test found that Vin dropped to 0.8 V or 0.6 V when the system was working.

The above test results show that the minimum startup voltage of SP6641 can reach 0.9 V, but if it is powered by alkaline batteries, the battery itself will consume more energy. Using a single battery will affect the power supply time, while using two alkaline batteries will have a smaller impact.

4 Conclusion

The application effect of handheld device power design based on SP6641B is good. Since the SP6641 device is small in size, has stable output voltage, high conversion efficiency, can easily work in energy-saving mode, and has high cost performance, it has broad application prospects in the field of portable battery-powered devices.