advertisement Ideal Diode Controller Eliminates Energy Wasting Diodes in Power OR-ing Applications – Design Note 1003 David Laude Introduction Many modern electronic devices need a means to automatically switch between power sources when prompted by the insertion or removal of any source. The LTC®4412 simplifies PowerPathTM management and control by providing a low loss, near ideal diode controller function. Any circuit that could otherwise use a diode OR to switch between power sources can benefit from the LTC4412. The forward voltage drop of an LTC4412 ideal diode is far less than that of a conventional diode, and the reverse current leakage can be smaller for the ideal diode as well (see Figure 1). The tiny forward voltage drop reduces power losses and self-heating, resulting in extended battery life. Features include: • Wide supply operating range of 2.5V to 28V (36V absolute maximum) • Protection of MOSFET from excessive gate-to-source voltage with VGS limiter • Low quiescent current of 11µA with a 3.6V supply, independent of the load current • A status pin that can be used to enable an auxiliary MOSFET power switch or to indicate to a microcontroller that an auxiliary supply, such as a wall adapter, is present • A control input pin for external control, such as from a microcontroller • Voltage drop across the controlled external MOSFET of only 20mV (typical) Applications include anything that takes power from two or more inputs: • Low component count helps keep overall system cost low • Cellular phones TM • 6-pin ThinSOT package permits a compact design solution 1 • Uninterruptible power supplies for alarm and emergency systems LTC4412 CURRENT (A) • MP3 players and electronic video and still cameras • Wire-ORed multipowered equipment • Systems with standby capabilities • Systems that use load sharing between two or more batteries SCHOTTKY DIODE • Multibattery charging from a single charger 0 0.02 0.5 FORWARD VOLTAGE (V) DN1003 F01 Figure 1. LTC4412 Ideal Diode Controller vs Schottky Diode Characteristics 12/03/1003 • PDAs • USB peripherals CONSTANT RON (VGS LIMITED) CONSTANT VOLTAGE (VGS VARIABLE) • Portable computers • Logic controlled power switches , LTC and LT are registered trademarks of Linear Technology Corporation. PowerPath and ThinSOT are trademarks of Linear Technology Corporation. Automatic Power Switching Between Two Power Sources Figure 2 illustrates an application circuit for the automatic switchover of load between two power sources, in this example a wall adapter and a battery. While the wall adapter is absent, the LTC4412 controls the gate of Q1 to regulate the voltage drop across the MOSFET to 20mV, thus wasting negligible battery power. The STAT pin is an open circuit while the battery provides power. When a wall adapter or other supply connected to the auxiliary input is applied, the SENSE pin voltage rises. As the SENSE pin voltage rises above VIN – 20mV, the LTC4412 pulls the GATE voltage up to turn off the P-channel MOSFET. When the voltage on SENSE exceeds VIN + 20mV, the STAT pin sinks 10µA of current to indicate that an AC wall adapter is present. The system is now in the reverse turn-off mode, where power to the load is delivered through the external diode and no current is drawn from the battery. The external diode is used to protect the battery against some auxiliary input faults such as a short to ground. Note that the external MOSFET is wired so that the drain to source diode will reverse bias and not deliver current to the battery when a wall adapter input is applied. Load Sharing Figure 3 shows a dual battery load sharing application with automatic switchover of power between the batteries and a wall adapter. In this example, the battery with the higher voltage supplies all of the power until it has discharged to the voltage of the other battery. Once both batteries have the same voltage, they share the load with the battery with the higher capacity providing proportionally higher current to the load. In this way, the batteries discharge at a relatively equal rate, maximizing battery run time. When a wall adapter input is applied, both MOSFETs turn off and no load current is drawn from the batteries. The LTC4412’s STAT pins provide information as to which input is supplying the load current. The ganging of the LTC4412s can be applied to as many power inputs as are needed. Conclusion The LTC4412 provides a simple means to implement a low loss ideal diode controller that extends battery life and reduces self heating. The low external parts count results in low implementation cost and with its ThinSOT 6-pin package, a compact design as well. Its versatility is useful in a variety of applications (see the LTC4412 data sheet for additional applications). 1N5819 WALL ADAPTER INPUT Q1 * TO LOAD BAT1 COUT LTC4412 6 VIN SENSE 5 2 GND GATE 4 3 CTL STAT 1 VCC 470k STATUS IS HIGH WHEN BAT1 IS SUPPLYING LOAD CURRENT 1N5819 WALL ADAPTER INPUT * Q2 * TO LOAD BATTERY CELL(S) COUT Q1 LTC4412 6 1 VIN SENSE 5 2 GND GATE 4 3 CTL STAT BAT2 VCC 470k DN1003 F02 LTC4412 6 VIN SENSE 5 2 GND GATE 4 3 CTL STAT 1 STATUS OUTPUT LOW WHEN WALL ADAPTER IS SUPPLYING LOAD CURRENT *PARASITIC DRAIN-SOURCE DIODE OF MOSFET Q1: FAIRCHILD SEMICONDUCTOR FDN306P (408) 822-2126 Figure 2. Automatic Power Switching Between a Battery and a Wall Adapter WHEN BOTH STATUS LINES ARE HIGH, THEN BOTH BATTERIES ARE SUPPLYING LOAD CURRENTS. WHEN BOTH STATUS LINES ARE LOW, THEN WALL ADAPTER IS PRESENT AND SUPPLYING FULL LOAD CURRENT VCC 470k DN1003 F03 *PARASITIC DRAIN-SOURCE DIODE OF MOSFET STATUS IS HIGH WHEN BAT2 IS SUPPLYING LOAD CURRENT Q1, Q2: SILICONIX Si4953DY (800) 554-5565 Figure 3. Dual Battery Load Sharing with Automatic Switchover of Power from Batteries to Wall Adapter Data Sheet Download http://www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900, Ext. 2593 dn1003f LT 1203 • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2003