As the number of consumer electronic devices continues to grow, products such as smartphones, tablets, and portable gaming consoles have become essential in daily life, offering convenience for work, communication, and entertainment. However, these devices often suffer from a common limitation: their internal lithium batteries have limited capacity, which can lead to sudden power loss and disruption of use. To address this issue, a circuit has been designed that integrates both charging and discharging functions for lithium batteries. As shown in Figure 1, the system is composed of three main components: the CN3705 lithium battery charging circuit, a 12V lithium battery, and the LM2596 lithium battery discharging circuit.
**1. Based on the CN3705 Lithium Battery Charging Circuit**
**1.1 Introduction to the CN3705 Chip**
The CN3705 is a buck-mode lithium battery charger that supports both constant current (CC) and constant voltage (CV) charging modes. It is particularly effective for deeply discharged batteries, as it initiates a trickle charge at 15% of the maximum charging current when the battery voltage is below 66.7% of the set CV voltage. During the CV phase, the charging current gradually decreases until it reaches a threshold determined by an external resistor, at which point the charging process stops. The chip operates with an input voltage range of 12V to 28V, a maximum switching frequency of 300kHz, and a maximum output current of 5A.
**1.2 Circuit Design Based on CN3705**
Figure 2 illustrates the lithium battery charging circuit built around the CN3705. This circuit uses a buck topology, with an input voltage ranging from 14V to 28V. The PWM switching frequency is set to 300kHz, and the maximum output current is 1.2A with a maximum output voltage of 12.6V. It is ideal for charging a 3S (three series) lithium battery pack consisting of 3.7V cells.
In the circuit, the P-channel MOSFET Q1, Schottky diode D2, inductor L1, and electrolytic capacitor C1 form a standard buck configuration. When selecting Q1, factors such as conversion efficiency, MOSFET power consumption, maximum temperature, on-resistance (Rds(on)), total gate charge (Qg), input voltage, and maximum charging current must be considered. The power loss in the MOSFET can be calculated using the following formula:
**1.3 Inductor Selection and Calculation**
During normal operation, the inductor current fluctuates periodically. When the MOSFET is turned on, the input voltage charges the inductor, causing the current to rise. When the MOSFET turns off, the inductor discharges into the battery, reducing the current. The ripple current increases as the inductance value decreases and as the input voltage increases. An empirical formula for calculating the inductor ripple current is as follows:
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Materials:
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Contact: phosphor bronze or brass
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Pitch: 1.27/2.0/2.54mm
Current rating: 1A, 250V AC
Contact resistance: 30M Ohms (maximum)
Insulation resistance: 3,000M Ohms, minute
Dielectric withstanding voltage: 500V AC for one minute
Operating temperature: -40 to +105 degrees Celsius
Terminated with 1.27mm pitch flat ribbon cable
Number of contacts: 06,08, 10, 12, 14, 16, 20, 24, 26, 30, 32,34, 40, 50, 60 and 64P
With RoHS mark
Used for ribbon cables
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Wire to Board
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