The circuit is powered by a linear regulated power supply (+VIN, -VIN) and the supply voltage VIN should be sufficient to meet: VlN ≥ VBTA + VDs. Where VBAT is the rechargeable battery voltage and VDs is the tube voltage drop of the power FET. In the case of a charging current of 0.5A. To ensure that the N-channel MOSFET (IRF7201) is turned on, the difference between the charger input voltage and the output voltage (VIN-VBAT) should be at least 2.2V.
The charging current ICH is related to the current samples R3 (Rcs) and R4, and the relationship is: VDs = 1/3 × R4 / 80K Ω × VCTRL. In the formula, 80kΩ is the on-chip resistance, and vcTRL is the applied control voltage. When R3=0.25Ω and R4=20kΩΠ, IcH has a linear relationship with VcTRL. When VcTRL changes from 0 to 1.2V, IcH changes from 0 to 1.2A. The VcTRL control voltage input to the 5-pin is taken out by the 2.0V reference voltage output from the 7-pin via the potentiometer RP center head, and the RP can be adjusted to obtain different charging currents.
The termination of the battery charging voltage is determined by a voltage divider consisting of external resistors R1, R2. That is, VBAT = 2V × (R1/R2 + 1). R5 and C3 have the dual function of current loop compensation and high frequency ripple filtering, and R6 and C4 form an RC series voltage loop compensation network. The ADP38114 outputs 4 to 6 mA of drive current and drives the IRF7201 (vT2) via 2N3904 (VT1). When R3 detects that the rhyme charging current exceeds the limit value, the on-chip amplifier increases the output voltage of the 4-pin, and the VT1 collector voltage, that is, the VT2 gate voltage (UGS), decreases, thereby reducing and stabilizing the charging current. The tube consumption of VT2 is: (VIN-VBAT) × IcHo Therefore, the appropriate input voltage VIN should be selected according to VBAT and the MOSFET should be selected according to the tube consumption (if necessary, a suitable heat sink should be added).
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