Switching power supply is mainly composed of two parts: power level and control level. The main task of power level is to choose different topological structure according to different application occasions and requirements, and consider the design cost of semiconductor components. The main task of the control stage is to select the appropriate control mode according to the electric signal of the circuit. At present, the switching power supply mostly adopts PWM control mode. The choice of SWITCHING power supply IC should not only consider to meet the requirements of circuit performance and reliability, but also consider its volume, weight, prolong battery life and cost.
(1) The best conditions to use LDO: when the output voltage ripple, noise is particularly small occasions, the input and output voltage difference is not large, the output current is not greater than 100mA when the use of micro power, low voltage difference (LDO) linear regulator is the most appropriate, suitable for most portable product applications.
(2) when negative power supply is used as far as possible charge pump: portable instruments often need negative power supply, because the required current is not large, the use of charge pump IC voltage reversal circuit is the most simple, if the requirements of low noise or output voltage regulator, can be used with LDO linear regulator charge pump IC.
(3) Do not pursue high precision, full function of the latest devices: switching power supply IC accuracy is generally ±2% ~ ±4%, high accuracy up to ±0.5% ~ ±1%, according to the requirements of the circuit to choose the appropriate precision, so as to reduce the cost of production. The price of the device with full function is higher, so there is no need to turn off the power function or the product without microprocessor (μP) or microcontroller (μC), there is no need to choose the device with the power off function or output power status signal, which can not only reduce the cost, and the size is smaller.
(4) don't "big horse car" : switching power supply IC is the most important three parameters, input voltage VIN, output voltage Vo and the maximum output current Iomax. Choose according to the working current of the product: it is more appropriate that the maximum working current is 70 ~ 90% of the maximum output current of the IC power supply Iomax.
A typical ultra-low standby power quasi resonant primary side feedback AC/DC converter is recommended for the 5V switching power supply IC scheme.