Battery Charging on USB （2）

Published:2011/8/14 23:29:00 Author:Phyllis From:SeekIC

By Harry Baggen

Professional products

Various semiconductor manufacturers have now developed special products for charging batteries using a USB port. Some of them are intended lo be used for charging lithium-ion batteries, but that’s not particularly interesting for DIY circuits, due to the difficulty of obtaining that type of battery.

Maxim has developed an idea for powering a circuit from a single NiMH cell that can be recharged from the USB port. Figure 3 shows such a design using two Moxim ICs. The MAX1692 (Ul) is a DC/DC step-down converter with current limiting. It charges the NiMH penlight cell (Bl) from the USB supply line at a current of 300 to 400 mA. Although the converter is not actually a true current source, it is good enough (even with a shorted cell) to effectively charge the battery. The advantage of using a DC/DC converter is that the current drawn from the USB port is only 150 mA when the NiMH cell is being charged at current of 400 mA. This leaves 350 mA for directly powering the connected device, in addition to the charging current. The second IC, U2, is a DC/DC boost converter (MAX1722) that transforms the voltage from the single rechargeable battery back to 3.3 V. When the battery is being charged, this converter is disabled and the output voltage is 4.7 V (derived from the USB voltage via diode Dl). The drawback of the charging circuit in Figure 3 is the absence of overcharge protection; the user must manually disconnect the circuit from the USB port when the battery is fully charged.





Protection







The USB specifications do not make any clearly defined statements regarding protection of USB ports. Some PC manufacturers provide reliable electronic current limiting for each port, while others only provide simple fuses on the circuit board. It is thus possible lo ’blow up’ a USB port if you try to draw too much current from it. National Semiconductor has developed a special IC that protects host ports against excessive output currents. The block diagram of this IC (LM3525) is shown in Figure 4. As soon as the output voltage drops more than 100 mV below the input supply voltage, the output current is limited to 1 A by the built-in power FET. Even with simple charging circuits such as the one shown in Figure 1, it’s a good idea to include an LM3525 between the USB connector and the charging circuit, in order to prevent damage to the PC or notebook computer.

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