Switching Power Supply in Laptop Computer

①MAX786 contains two chips, 782 and 783. It uses a micro-pitch monolithic dual-CMOS integrated circuit and a surface-mounted SS0P package, which has the advantages of high integration, small size and low noise.

②Use two current-type step-down PWM controllers to generate two drive signals, which drive N-channel MOS power field-effect tubes respectively, and can output +3.3V (3A) and +5V (3A) two stable power supplies for notebook computers. Each output current can also exceed 6A, depending on the model of the power field-effect tube.

③The input voltage UI range is +5.5~30V, and the switching frequency can be set to 300kHz or 200kHz. However, when the power supply is low voltage, UI=+6.5~12V is required, the switching frequency must be 200kHz, and the maximum output current of the +5V regulated power supply is reduced to 2A, and the +3.3v regulated power supply can still output 3A current. Usually, 6 1.5V high-energy batteries or nickel-cadmium batteries can be selected in series to form a 9V low-voltage power supply.

④ There are two low-dropout, micro-power linear regulators, two precision voltage comparators/level converters, synchronous detectors, and 3.3V reference voltage sources inside. The use of synchronous detectors can greatly improve the efficiency of the power supply. It works in PWM mode when the load is large and in light load mode when the load is small. When the output current is 2A, the efficiency is as high as 95%, and the efficiency is still higher than 80% when the output is 5mA~3A. The static operating current is 420μA, and it is only 70pA in each use mode. The operating temperature range is 0~+70℃.

⑤ Due to the use of advanced current-type pulse width modulator, the capacity of the output filter capacitor is greatly reduced, and the capacitance corresponding to each ampere of output current can be reduced from 1000μF to 30μF.

⑥It has multiple functions such as soft start, overvoltage protection, overcurrent protection, etc., and works safely and reliably.

2. Working principle of MAX786

(1) Pin arrangement

MAX786 uses a miniaturized 28-pin SS0P package, and the pinout is shown in Figure 1. The pins with numbers 3 and 5 in the symbols correspond to the +3.3V and +5V switching power supply terminals, respectively.

Pin 23 V+: Connect to the 5.5~30V DC input voltage formed by the battery.

Pin 9 GND and Pin 20 POND: analog ground and power ground respectively, the two can share the same ground.

Pin 1 CS3: +3.3V current detection terminal of the switching power supply, external current detection resistor RS.

Pin 28 FB3: Feedback terminal of 3.3V switching power supply.

Pin 2 SS3: +3.3V soft-start capacitor terminal of the switching power supply.

Pin 3 ON3: +3.3V switch power on/off control terminal, should be connected to VL terminal during self-start. When ON3=0, the +3.3V switch power is turned off.

Pin 27 DH3 and Pin 24 DL3: They are respectively the high-end and low-end drive output terminals of the +3.3V switching power supply, connected to the gate of the external N-channel MOS field effect transistor.

Pin 25 BST3, Pin 17 LX5: They are respectively the boost capacitor and inductor terminals of the +3.3V switching power supply.

The functions of CS5, FB5, SS5, ON5, DH5, DL5, BST5, and LX5 are the same as above, but they correspond to +5V switching power supplies.

D1, D5, Q1, Q2: They are the in-phase input and output terminals of two sets of precision voltage comparators/level converters. The threshold voltage of the comparators is 1.65V. D1 and D2 should be connected to GND when not in use.

Pin 6 VH: Positive power supply terminal of two precision voltage comparators/level converters.

Pin 22 VL: +5V logic level output terminal, working in PWM mode, the maximum output current is 5mA, but when PWM is turned off or in each use mode, the output current capacity of this terminal increases to 25mA. VL is suitable for powering the RAM in the computer and has a power-off protection function.

Pin 10 Uref: 3.3V reference voltage output terminal, output current can reach 5mA.

Pin 11 SYNC: Switching frequency setting terminal. When this terminal is connected to GND or VL, f=200kHz; when connected to Uref, f=300kHz. You can also use an external clock of 240~350kHz to achieve synchronous operation of multiple laptops.

Pin 12 SHDN: Shutdown PWM control terminal (low level is valid). When shut down, the +3.3V power supply, +5V power supply and +3.3V reference voltage source do not work, and only the VL power supply can output (5V, 25mA).

(2) Working principle

The internal block diagram and external wiring of MAX786 are shown in Figure 2 and Figure 3 respectively. It mainly includes 300kHz/200kHz oscillator, +5V linear regulator, +3.3V bandgap reference voltage source, +3.3V PWM controller, +5V PWM controller, precision voltage comparator 1, 2/level converter 1, 2, gate circuit, protection circuit (comparator 3, 4, etc.). The switching frequency value can be set using the SYNC terminal. Selecting a 300kHz frequency can further reduce the energy storage inductance and filter capacitor value, but when using a 9V battery pack for low-voltage power supply, a ⒛0kHz frequency must be selected. The +5V voltage generated by the linear regulator is output from the YL terminal in one way; the other is output to the SYNC terminal in the other way.

Figure 2 MAX786 internal block diagram

The reference voltage source supplies power, and the +3.3V reference voltage is obtained from the Urdf terminal. In Figure 3, if SYNC and Urdf are shorted along the dotted line, the switching frequency is selected to be 300kHz.

Figure 3 MAX786 external wiring diagram

The +3.3V switching power supply consists of an internal PWM controller, external N-channel MOS power field effect transistors (N1 and N3), a detector diode (VD2) and an output filter (L1, C12, C7). Among them, the Schottky diode VD1 and L1, C12, and C7 form a step-down output circuit. C5 is a boost capacitor, which is used to increase the amplitude of the high-end (DH3) drive signal to make it greater than the battery voltage, thereby improving the output capacity of N1. The low-end (DL3) signal directly drives N3. VD2 is an external detector diode of the synchronous detector. R1 is a current detection resistor, which is used to set the limit value of the output current. C9 is a soft-start capacitor, which uses the charging process of the capacitor after power-on to gradually establish a +3.3V power supply, which can reduce the initial surge voltage. The function of internal comparators 3 and 4 is to generate a fault signal FAULT when the input (or output) voltage drops and causes UL < +4.0V or Uref < +2.8V, and shut down the +3.3V and +5V switching power supplies to protect the circuit. Comparators 1 and 2 can be used for low voltage detection.

PWM控制器有两种工作方式，一种是连续的PWM方式，适用于大负载；另一种是断续的PWM方式（也称轻载 方式），适用于负载小于满载25％的情况，此时能显著降低功耗。当SHDN=0时，脉宽调制器、基准电压源 和精密电压比较器均无输出，Q1＝Q2＝0V，但是VL电源照常工作，可向笔记本电脑的随机读写存储器RAM 提供＋5V、25mA的电源，确保RAM中的数据不至于丢失。当SHDN=1，且ON3＝ON，＝0时，芯片处于各用状 态，PWM停止工作，静态电流降至70μA。

3. Laptop switching power supply composed of MAX786

(1) Circuit of laptop switching power supply

The circuit of the notebook computer switching power supply is shown in Figure 4. The circuit is powered by 9V. If the filter capacitor C in the +5V switching power supply is increased to 660pF, the input voltage of 5.5~12V can be accepted, but the maximum output current of the +5V power supply is reduced to 2A, while the +3.3V power supply can still provide 3A current. Compared with Figure 3, the circuit adds pull-down resistors R4~R8 and switches S1~S4. When S1 is disconnected, it works normally, at this time SHDN＝0; when S1 is closed, SHDN＝1, and MAX786 is in the off state. When S2 (S3) is closed, the +3.3V (+5V) switching power supply is turned on. When S4 is closed, the switching frequency is 200kHz, and when it is disconnected, it is 300kHz. The working principle of the rest of the circuit is as described above.

Figure 4 Laptop switching power supply circuit

(2) System block diagram

The connection diagram of MAX786 and laptop is shown in Figure 5. MAX786 can provide +3.3V (or +5V) regulated power supply through power selection circuit for microprocessor, memory and peripheral devices. In addition, MAX786 can also provide power normal signal and low voltage alarm signal to microprocessor ptP, and supply a regulated power supply to RAM separately.

Figure 5. Connection diagram of MAX786 and laptop computer