Design Ideas:October 27, 1994
A boost-topology switching regulator is the simplest solution for converting a two- to three-cell input to a 5V output. Unfortunately, boost regulators have several inherent disadvantages, including a lack of short-circuit protection and shutdown capability. In some battery-operated products, external chargers or adapters can raise the battery voltage to a potential higher than the 5V output. Because of this increase, a boost converter can't maintain regulation because the high input voltage feeds through the diode to the output.
The circuit in Fig 1 overcomes these problems. ICis a conventional boost converter that preserves simplicity and high efficiency in the boost mode. Qadds the features of short- circuit limiting, true shutdown, and regulation under high-input-voltage conditions.
When the input voltage is lower than 4V and the regulator is enabled, the circuit drives Q1's emitter above its base and saturates the transistor. As a result, the voltages on Cand Care roughly the same, and the circuit operates as a conventional boost regulator. If the input voltage increases above 4V, the internal error amplifier acts to keep the output at 5V and boosts the voltage on Cto a level about 1V above the input. This voltage controls Qto provide the desired output, and the transistor operates as a linear pass element. The output does not experience any abrupt changes during the switchover between step-up and -down modes because the same error amplifier monitors the output in both modes.
The circuit's efficiency peaks at approximately 88% with an input of 4.25V. At 4.25V, Qbegins to operate in a linear mode with an attendant roll-off of efficiency for higher input voltages. When operating in the boost mode, efficiency is between 82 and 88% for input voltages as low as 2V.
Pulling IC1's shutdown pin high turns off the circuit. ICceases switching, and Qautomatically turns off and fully disconnects the output. Qalso provides overload protection. When the output shorts, ICoperates in a cycle-by-cycle current limit. The short-circuit current, which typically reaches 200 mA, depends on IC1's maximum switch current and Q1's gain. Qcan withstand overload for several seconds before heating up. For sustained faults, you must also consider the thermal effects on Q1. (DI # 1605)