Power supply circuit switching and lithium battery charging circuit design

Currently, the charging management IC on the market is designed based on the charging characteristics of the rechargeable battery. Rechargeable Batteries are divided into Ni-MH batteries and lithium batteries based on different charging media. Because the lithium battery has no memory effect, lithium battery is currently used in various handheld devices and portable electronic products. Due to the charging characteristics of lithium battery. The charging process is generally divided into three processes:
1. the streaming charging phase. (When the battery is in transient discharge mode and the voltage is low)
Lithium Battery usually drops below 3.0v after transient discharge. Some physical changes occur in the internal media of the lithium battery, resulting in bad charging characteristics and reduced capacity. At this stage, the lithium battery can only be slowly charged by a small stream, and the dielectric inside the lithium battery slowly recovers to normal state.
2. Constant Current charging phase. (The battery is restored from overlay to normal)
After the streaming charging phase, the dielectric inside the battery can withstand a large amount of charging current, so the external can charge the lithium battery by a large amount of current, in order to shorten the charging time. The charging current in this phase is generally determined by a pin external to the charging management IC and a resistor. The resistance value is calculated based on the formula on the datasheet of the Charging management IC. 3. Constant Pressure Charging phase (already filled with more than 85% of the charge, and is slowly being supplemented)
When the capacitance of the lithium battery reaches 85% (approx.), it must enter the slow charge phase again. Increase the voltage slowly. The maximum voltage of the lithium battery is 4.2 V.
Generally, the lithium battery has a bat pin output, which is connected to the lithium battery end. This pin is also the lithium battery voltage detection pin. The lithium battery charging management IC detects this pin to determine the status of the battery.
In the actual portable product circuit design, because the battery charging process is required, the product must be properly applied. Therefore, the following circuit is the correct method in the design:

Figure 1 a210 Power Supply Diagram

The external voltage 5 V is sent to switch sw2 through D2, and the charging management IC mcp73831 is sent to the lithium battery. The left-side voltage of sw2 is 5 V-0.7 V = 4.3 v. The voltage of the lithium battery is less than 4.3 V at the left of sw2, regardless of whether it is fully charged or not. So d1 is the deadline. The charging management IC is used to charge the lithium battery normally.

If the diode D2 and D1 are not added, and the latter-level ldos rt9193 is directly connected to the bat pin output, a false positive error will occur when the charging IC is powered on. 5 V external power supply will appear, but the lithium battery will not charge, charging management ic led light is not indicated. The back-level load regulator does not get the normal input voltage (the input voltage is very small ). In this case, as longThe voltage input pin of the Charging management IC is directly connected to the bat pin through a short circuit. All the statuses are normal, the charging can be performed, and the back-level load regulator works normally..

This is becauseWhen the charging management IC is powered on, it needs to detect the status of batThe input pin of the ldos is also connected to the branch connecting the bat and the lithium battery cathode, which will affect the working status of the bat pin, causing the charging management IC to enter the streaming charging phase. Connect the bat pin and the voltage input of the Charging management IC to short-circuit, so that the voltage of the bat pin is forcibly increased, so that the charging management IC determines that the lithium battery enters the constant current charging stage, so the output of large current. It can drive post-load LVS and so on.

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In addition, to improve the power utilization efficiency, D1 and D2 should choose diodes with low voltage drop. Such as diode, diode, and switch. In the design that requires battery switching, the diode with a forward voltage drop of 10mV and no reverse leakage current is a luxury of the designer ". So far, however, it is still the best choice for the tolerance diode. Its Forward Voltage Drop is between 300mV and 500mV. However, some battery Switching circuits cannot meet the design requirements even if the selection of the diode is used. For an efficient voltage converter, the energy saved may be completely wasted by the forward voltage drop of the diode. In order to effectively save battery energy in a low-voltage system, a power-Transistor Switch should be selected to replace the diode. Using sot encapsulation, the on-resistance is only a few dozen Euro-class Enis, in the current level of portable products can ignore its on-going pressure drop.

Determine whether a system needs to use a MOSFET to switch the power supply. It is best to compare the diode conduction voltage drop, the transistor conduction voltage drop and the battery voltage, and regard the ratio of the voltage drop to the battery voltage as the efficiency loss. For example, if a Forward Voltage Drop of 350mV is used to switch the Li + Battery (nominal value of 3.6 V), the loss is 9.7%, if the two AA cells are used to switch (nominal value: 2.7 V), the loss is 13%. In a low-cost design, these losses may be acceptable. However, when using an efficient DC-DC, it is necessary to weigh the cost of the DC-DC and the cost of improving the efficiency brought by the upgrade of the diode to the MOs.

The selection of do not use the tolerance diode and MOSFET, but also take into account the discharge characteristics of the battery used on the product. The discharge characteristics of lithium battery are as follows:

 

It can be seen that when the lithium battery consumes 90% of the power at normal temperature, the voltage will remain at about V, and a good LVS device will be selected. the output voltage will remain stable at 3.3 V at 3.v.

 

According to the actual test of the ldos rt9193, when the load resistance is 50 ohm and the load current is 60mA, the relationship between the input voltage and the output voltage is shown in the following table:

Input voltage	Output Voltage
2.8 v	2.65 v
3.4 V	3.0 V
4.0 V	3.3 V

It can be seen that even if the lithium battery consumes 90% of the power, the output end of the LDC can still stably output 3.3v. after analyzing the power supply circuit of Figure 1 a210 and adding the silicon diode D1, input voltage = 3.5 --- 0.7 V = 2.8 v. in this way, as long as the module burning program can work around V, the silicon diode can also be used in this circuit.

However, in terms of circuit performance, the use of diode or diode is the best choice.

The specific circuit design needs to be considered based on the operating voltage range and characteristics of other circuits of the product, and the cost.

 

Power supply circuit switching and lithium battery charging circuit design