IC Cuts Power Switching Voltage Drop

Aug 1, 2002 12:00 PM
By Sam Davis, Editor, Power Electronics Technology

With the continuing need for improvements in cell phones, handheld computers, digital cameras, and UPSs, today's system designers need a controller that's useful in a variety of cost and space-sensitive power control applications. A new product from Linear Technology Corp., the PowerPath^™ LTC4412, is filling this requirement by permitting highly efficient ORing of multiple power sources for extended battery life and low self-heating. This is possible because the IC controls an external P-channel MOSFET to create a near ideal diode function for power switchover or load sharing (Fig. 1).

A battery operating range of 2.5V to 28V supports its use with one to six series-connected Li-ion cells, or most ac-dc wall adapters (3V to 28V). The low quiescent current (11μA typical) is independent of the load current. The gate driver includes an internal voltage clamp for MOSFET protection. The table describes the LTC4412's pin functions.

The STAT pin enables an auxiliary P-channel MOSFET power switch when an auxiliary supply is detected. It also tells a microcontroller that an auxiliary supply is present.

This IC disconnects a load from the battery when connecting an auxiliary power source. Two or more interconnected LTC4412s allow load sharing between multiple batteries or charging of multiple batteries from one charger.

Fig. 2 is a circuit for the automatic switchover of a load between a battery and a wall adapter or other power input, describing a typical application. With the battery applied to VIN, the parasitic drain-source diode of the P-channel MOSFET initially pulls up the load. Then, the LTC4412's GATE pin causes the external MOSFET to turn on, reducing its voltage drop from a parasitic diode drop to as low as 10 mV. The system is now in the low loss forward regulation mode. Applying the wall adapter input causes the Schottky diode to pull up the SENSE pin (connected to the load) above the battery voltage, which turns off the MOSFET. The STAT pin then sinks current, indicating the connection of an auxiliary input. The battery now supplies no load current and all the load current flows through the Schottky diode. Instead of the Schottky, a silicon diode could be used; however, it would result in higher power dissipation and heating due to its higher forward voltage drop.

Selecting the optimum MOSFET is an important design consideration. The maximum allowable drain-source voltage, V_DS, must be high enough to withstand the application's maximum drain-source voltage. Typically, you would use a logic-level MOSFET, but if a low supply voltage limits the gate voltage, consider a sublogic level threshold MOSFET. As a general rule, select a MOSFET with a low enough R_DS(ON) to obtain the desired V_DS while operating at full current load and an achievable V_GS.

Housed in a low-profile (1 mm) ThinSOT package with operating temperature range is -40°C to 85°C, system designers can use this controller in the latest applications with a wall adapter or other auxiliary power source.

Linear Technology Corp., Milpitas, Calif. CIRCLE 349 on Reader Service Card