1, the overall function module chart of the lithium ion battery charger. These sub module includes. Reference voltage source, reference current source, under voltage lockout module, the constant current charge amplifier, constant voltage charge amplifier, intelligent heat adjustment, clamping, oscillator amplifier, counter, battery temperature protection module and power tube substrate protection module, logic module, as well as multiple comparator module.
The overall function module chart of the lithium ion battery charger
Consider the practical application of chips, this paper design the lithium ion battery charger has the following features:
(1) in the process of charging, the temperature of the chip protection when the battery voltage to a trickle charging jump threshold voltage and stepped into the stage of constant current, constant current stage for large current charge, because this article power tube for PMOS, only the power tube between the load cell and the power, the battery voltage is low, the chip reached the maximum power dissipation. The power dissipation is:
P = (Vcc - VBAT), Icc (1)
A sharp rise in the high power dissipation will cause the temperature of the chip, so set up an intelligent thermal feedback loop. When the temperature of the chip temperature rise to thermal feedback point 105 ℃, start the thermal feedback loop, the chip temperature at 105 ℃. When the battery voltage rises further, the type (1) shows that power dissipation reduced gradually, under the small power dissipation, the temperature of the chip will gradually reduced. Exit intelligent heat adjustment work patterns, into constant current charging mode, using large current Icc to the battery, or entered the stage of constant voltage charging directly. The use of the thermal feedback loop, to maximize the rate of charge, at the same time, the user need not worry about the temperature of the chip is too high.
(2) the cost. Introduced in this paper using CMOS chip design, low cost and process is easy to implement.
(3) with the user interactive management. Chip provides multiple external users programming pin for management and use of the chip. In terms of charging current control, the user can by connecting one resistor to a chip pins programming for charging current; In terms of end charging voltage control, users can receive a pin through the chip high level or low level to set the final charge voltage of 4.1 V or 4 z V, to adapt to the use of different anode materials of lithium ion battery recharged; On the control of charging time, the user can be connected by one capacitor to chip pins 1 to programming of charging time, meet the requirements of users of different charging time. Chip design is expected to reach the features and parameters are shown in table 1.
External connection of chip pin, CHRG, FAULT, ACPR respectively and a three pin 1 k Ω resistance as well as a light emitting diode is linked together, used to indicate charging status of the chip; 4.7 u F capacitance bypass capacitors for Vcc power supply, the battery place receives a BAT pin ESR 1 Ω 1 u F bypass capacitors, without battery, hold the ripple voltage at low levels. NTC pin, a negative temperature coefficient of resistance of 10 k Ω RNTC Ω resistor in series with 4 k, the partial pressure on the RNTC as input of the NTC pin.
External connection of chip pins
2, the overall simulation results of linear lithium ion battery charger
Simulation, to shorten the simulation time, the battery is equivalent to a large capacitance CBAT, its equivalent series resistance for RESR. 2 tables for the charger chip characteristics of presetting parameters resulting from the simulation results.
To shorten the simulation time, preset voltage of 2.3 V batteries, so that the charging process can quickly by a trickle charge mode transition to constant current charging mode.
In the simulation, the RPROG Ω value is set to 3 k, a trickle charging current is 50 mA, constant current charging current is 500 mA. SEL pin grounding potential, the final battery charging voltage of 4.1 V can be known, under various working conditions, the charger can normal work. In figure 4 charging process and the curve of temperature, when the temperature is 125 ℃, the charging current is zero, this is due to the wisdom of the chip thermal adjust temperature Tc is 105 ℃, and normal operation of intelligent heat adjustment circuit of chip charging current at 125 ℃ down to zero, and remains at 2.3 V voltage of the battery.
The charger charging current and intelligent heat adjustment waveform figure
Adjust the waveform figure charger charging current and intelligent heat, when near the chip temperature of 105 ℃, intelligent heat adjustment circuit automatically start, reducing the charging current. In order to reduce the power consumption of the chip.
The worst cases, battery charging voltage simulation data finally
To ensure that even in the worst cases, the lithium ion battery charging voltage ultimately meet the requirements, all the corner of the whole circuit resistance RES_TT, RES_FF, all cornerTT RES_SS and MOSFET, FF, FS, SF, SS the intersection of the simulation, the simulation after battery charging voltage of typical case and worst case finally. Among them, the reference voltage of 2.485 V without fine-tune the simulation results, the reference voltage of 2.485 V after fine-tuning of the simulation results.
By the final simulation results indicate the battery charging voltage simulation results accord with the design requirements of the precision is as follows:
(1) after VREF fine-tuning, SEL = 0 or Vcc, VBAT can be controlled in 4.1 V or 4.2 V (1 + / - 0.4%).
(2), VREF not fine-tuning, SEL = 0 or VCC, VBAT can be controlled in 4.1 V or 4.2 V (1 + / - 0.8%).
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