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Battery Charger Open Loop Shunt Regulator

IP.com Disclosure Number: IPCOM000010451D
Publication Date: 2002-Dec-03
Document File: 4 page(s) / 48K

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Abstract

Key Terms: Battery Voltage, Shunt Regulator and Series Transistor Many portable applications use and many more are planned to use low voltage devices. The batteries used in low voltage devices range in voltage between 3 and 5 volts. Wall adapters are often used to charge these batteries, and the control of the battery charge is via a control circuit that depends on the battery charge. Where the battery voltage is too low, the control circuit must rely on power from the wall adapter. The voltage needed by the battery must be above a given threshold (e.g., 3.2 volts) to provide enough power to the control circuit in order to control the battery charge. A regulator is required when the battery voltage falls below the given threshold. With the supply voltage to the regulator possibly varying as much as 6 to 22 volts, and the value of the external loading capacitor possibly varying considerably, the design of a regulator for these battery charging applications is a substantial challenge. Much of the technology today does not provide the voltage regulation needed. A series regulator cannot be used and the shunt regulator has severe thermal limitations, because the current draw is prohibitive (e.g., 95 milliamps) on the available shunt regulator. The method shown here does not use the shunt regulator to drive the load directly, but instead uses a series transistor M1 (Figure 1) to provide the required load current. In addition, this method uses a band gap multiplier to provide accurate voltage regulation.

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Battery Charger Open Loop Shunt Regulator

Key Terms: Battery Voltage, Shunt Regulator and Series Transistor

Many portable applications use and many more are planned to use low voltage devices.� The batteries used in low voltage devices range in voltage between 3 and 5 volts.� Wall adapters are often used to charge these batteries, and the control of the battery charge is via a control circuit that depends on the battery charge.� Where the battery voltage is too low, the control circuit must rely on power from the wall adapter.� The voltage needed by the battery must be above a given threshold (e.g., 3.2 volts) to provide enough power to the control circuit in order to control the battery charge.� A regulator is required when the battery voltage falls below the given threshold.� With the supply voltage to the regulator possibly varying as much as 6 to 22 volts, and the value of the external loading capacitor possibly varying considerably, the design of a regulator for these battery charging applications is a substantial challenge.�

Much of the technology today does not provide the voltage regulation needed.� A series regulator cannot be used and the shunt regulator has severe thermal limitations, because the current draw is prohibitive (e.g., 95 milliamps) on the available shunt regulator.� The method shown here does not use the shunt regulator to drive the load directly, but instead uses a series transistor M1 (Figure 1) to provide the required load current.� In addition, this method uses a band gap multiplier to provide accurate voltage regulation.�

Figure 1

The source current, called IB, provides an external current source to the circuit.� This source current is derived by the difference of the base-emitter junction of two bipolar transistors (not shown) divided by the value of a resistor matched to resistor R3.� This current is proportional to the absolute temperature (i.e., IB= n x kT/qR).

By using two current mirrors (i.e., M5/M4 and M2/M3), the currents are made equal (i.e., IB=I3, and I4=I5).� The difference between I3 and I4 is c...