System Design of Car Charger Based on AP3003 PWM Buck Converter
With the continuous development of electronic technology, mobile multimedia devices such as mobile phones, MP3 and DSC are gradually becoming indispensable tools in people's lives, and the design of chargers corresponding to these products is also receiving more and more attention. 
According to the occasion of use of the charger, it can be divided into household type chargers and car chargers. Most of the chargers that come with mobile phones are household types, that is, AC input types; car chargers are a DC input type charger. Appears to diversify the charging occasions of mobile devices At present, the control chips of the more commonly used car charger solutions provided by BCD company are mainly AZ34063A / C, AZ494B / D and AP3003
The advantages of the AZ34063A / C scheme are lower cost, the disadvantages are the inaccurate current limit point, and the overheating problem is more common; the advantages of the AZ494B / D scheme are greater design flexibility, and the disadvantage is that the selection of peripheral devices is more complicated, and it is necessary to choose the appropriate power Tube, drive circuit, and loop compensation design The advantages of the AP3003 solution are fewer peripheral devices, simple design, and precise control. Although the cost is higher than the AZ34063A / C solution, its cost performance is the highest of the three solutions.
AP3003 series IC is the latest DC / DC buck converter developed by BCD There are four voltage versions of this series IC: 3.3V, 5V, 12V output voltage fixed version and ADJ output voltage adjustable version The chip integrates power tube, drive circuit, control circuit and loop
(1) Introduction to the technical indicators and design block diagram of the car charger system
The input voltage of the car charger is 12V-36V, the output voltage is 5.1V ± 0.1V, and the output current is 750mA ± 50mA When the charger is charging the battery, if the charging current is too large, it may cause the battery to heat up, shorten its life, or even damage. Therefore, a constant current (CC) function is required to achieve accurate control of the output current In order to ensure that the charger is not burned due to excessive input power during short circuit, a short circuit protection function is required
According to the above requirements, you can use AP3003 to design a car charger. The schematic block diagram is shown in Figure 1. It includes three major parts: the basic step-down circuit constructed by AP3003, a constant current and constant voltage (CC / CV) circuit, and a short circuit protection circuit.
Figure 1 Block diagram of car charger system design
(2) Basic buck circuit constructed by AP3003
This part of the circuit is the core part of the entire car charger system, which provides the necessary voltage and current for the battery during the charging process Use AP3003-ADJ to design the circuit as shown in Figure 2. The voltage divider resistors RA and RB are used to set the output voltage. The CFF capacitor can increase the phase margin of the loop and improve the system stability. The recommended value range is 10nF to 33nF
Figure 2 Basic buck circuit constructed by AP3003
(3) Constant current and constant voltage (CC / CV) circuit
The constant current and constant voltage circuit uses AS358 to sample and amplify the voltage and current signals. The circuit is shown in Figure 3 and is divided into two parts. One part is the constant current loop: the sampling resistor Rs samples the output current Io, which is amplified and amplified by AS358_1. The multiple is determined by R2 / R1 (R1 = R3, R2 = R4), the amplified signal is sent to the FB pin of AP3003 through diode D1; the other part is the constant voltage loop: the resistance RA and RB sample the output voltage Vo, after AS358_2 and Diode D2 is sent to the FB pin of AP3003 According to it, the calculation formulas (E-1) and (E-2) of constant current point and constant pressure point can be obtained:
(E-1)
(E-2)
Among them, VD1, VD2 are the forward voltages of diodes D1 and D2, VREF is the reference voltage inside AP3003, and the appropriate Rs, R2, R1, RA, RB and diode can be selected according to the design requirements, and the constant current point is 750mA 
The car charger system with a constant voltage point of 5V, the experimental test results are shown in Figure 4
Figure 3 Constant current and constant voltage (CC / CV) circuit designed with AS358
Figure 4 VI characteristic curve
(4) Short circuit protection circuit
The short circuit protection circuit uses a transistor to sample the output voltage, and judges whether a short circuit occurs according to the state change of the output voltage before and after the short circuit, thereby achieving short circuit protection Circuit shown in Figure 5 In order to indicate the short circuit or not, a light emitting diode can be added as an indicator. As shown in Figure 6, after the short circuit occurs, the light emitting diode D3 lights up. After the short circuit is eliminated, the power supply is restarted and the circuit can resume normal operation.
Figure 5 short circuit protection circuit
Figure 6 Short circuit protection circuit with indicator light
The working principle is as follows: after a short circuit occurs, the output voltage is sampled by RA and RB, and the voltage value cannot maintain the transistor Q1. Then Q1 is turned off, the capacitor C1 is charged, and the VEN connected to the AP3003 EN pin voltage rises with time High, the expression is shown as (E-3), once VEN is higher than the threshold voltage of EN pin, the whole system stops working, and the function of short circuit protection is realized
(E-3)
There are two aspects to be considered in the short-circuit protection design. The first is to avoid the short-circuit protection circuit from affecting the system startup. The selection of R1 and C1 should ensure that the short-circuit protection start time is much longer than the system startup time. The second is to select the appropriate R3 to ensure The addition of R3 will not affect the output voltage value set by RA and RB
From the above introduction and analysis, it can be seen that the car charger system designed by BCD's AP3003 and AS358 is simple in design, accurate in control, and complete in function. It is a cost-effective car charger solution with large market applications. prospect
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