LINEAR TECHNOLOGY VOLUME XII NUMBER 4 DECEMBER 2002 COVER ARTICLE Ideal Diode Controller Eliminates Energy Wasting Diodes in Power OR-ing Applications ...................... 1 David Laude Issue Highlights ............................ 2 LTC® in the News ........................... 2 DESIGN FEATURES High Performance 400MHz Quadrature IF Demodulator Runs from 1.8V Supply .......................... 5 Min Zou and Vladimir Dvorkin New Power for Ethernet—Powered Devices (Part 2 of a 3-Part Series) .. 9 Dave Dwelley 2500V/µs Slew Rate Op Amps Process Large Signals with Low Distortion at High Frequencies ........................ 11 Kris Lokere and Glen Brisebois SOT-23 Digitally Controlled Amp Puts Programmable Gain Anywhere .... 16 Max W. Hauser Programmable Quad Supervisors Offer Unparalleled Flexibility for MultiVoltage Monitoring Applications ................................................... 19 Bob Jurgilewicz Tiny 1.25MHz Monolithic Boost Regulator Has 1.5A Switch and Wide Input Voltage Range ........... 25 Keith Szolusha Secondary Side Synchronous Post Regulator Provides Precision Regulation and High Efficiency for Multiple Output Isolated Power Supplies ...................................... 29 Charlie Y. Zhao, Wei Chen and Chiawei Liao DESIGN INFORMATION A 14-Bit ADC that is Both Fast and Low Noise .................................... 33 Richard Reay and Dave Thomas DESIGN IDEAS .............................................. 34–36 (complete list on page 34) New Device Cameos ..................... 37 Design Tools ................................ 39 Sales Offices ............................... 40 Ideal Diode Controller Eliminates Energy Wasting Diodes in Power OR-ing Applications by David Laude Introduction Many modern electronic devices need a means to automatically and smoothly switch between power sources when prompted by the insertion or removal of any source. The LTC4412 permits low loss OR-ing of multiple power sources for extended battery life and low self-heating. The LTC4412 controls external P-channel MOSFET power switches to create a near ideal diode function for power switchover applications or load sharing PowerPath™ management applications. When conducting, the voltage drop across the MOSFET is typically only 20mV. It also provides power monitoring circuitry and external control for integration with other parts of the power management system. The low component count results in a low overall system cost and, with its ThinSOT™ 6-pin package, a compact design solution. It’s versatile enough to be used in a variety of diode OR-ing applications. For battery powered applications that also can be powered from a wall adapter or other “auxiliary” power source, the load is automatically disconnected from the battery when the auxiliary source is connected, so that no current is drawn from the battery. When the auxiliary source is disconnected, operation reverts back to the 1 CONSTANT RON LTC4412 CURRENT (A) IN THIS ISSUE… CONSTANT VOLTAGE SCHOTTKY DIODE 0 0.02 0.5 FORWARD VOLTAGE (V) 4412 F01b Figure 1. LTC4412 Ideal diode controller vs Schottky diode characteristics battery. An AC adapter present signal is available. The LTC4412 also has built in reverse supply protection. Multiple LTC4412s can be ganged together to provide load sharing between multiple batteries, or to allow multiple batteries to be charged from a single battery charger. The precisely controlled ideal diode behavior of the LTC4412 is crucial to maintain current balance when multiple batteries are discharged or charged while connected to a single load or source, The advantages of the LTC4412 ideal diode PowerPath controller are shown in Figure 1. The forward voltage drop of the ideal diode is far less than that of a conventional diode and the reverse current leakage can be continued on page 3 , LTC, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology Corporation. Adaptive Power, C-Load, DirectSense, FilterCAD, Hot Swap, LinearView, Micropower SwitcherCAD, Multimode Dimming, No Latency ∆Σ, No Latency Delta-Sigma, No RSENSE, Operational Filter, PowerPath, PowerSOT, SoftSpan, SwitcherCAD, ThinSOT and UltraFast are trademarks of Linear Technology Corporation. Other product names may be trademarks of the companies that manufacture the products. DESIGN FEATURES LT1970, continued from page 1 smaller for the ideal diode as well. The tiny forward voltage drop lowers power losses and self-heating, resulting in extended battery life. The very low reverse leakage, when compared to Schottky diodes, is also beneficial in some applications. The wide supply operating range of 2.5V to 28V supports operation with one to six Li-Ion batteries in series. The types of power sources that can be used include all those that are within the supply operating range. The low quiescent current of 11µA with a 3.6V supply is independent of the load current. The LTC4412 also features a status pin that can be used to enable an auxiliary MOSFET power switch for additional power savings when an auxiliary input is utilized. It may also be used to indicate to a microcontroller that an auxiliary supply, such as a wall adapter, is present. A control input pin is provided to extend applications to those that can benefit from external control, such as from a microcontroller. Applications include anything that must take power from several inputs, including cellular phones, portable computers, PDAs, MP3 players and electronic video and still cameras, USB peripherals, wire-ORed multipowered equipment, uninterruptible power supplies for alarm and emergency systems, systems with standby capabilities, systems that use load sharing between two or more batteries, multi-battery chargers, and logic controlled power switches. 1N5819 WALL ADAPTER INPUT BATTERY CELL(S) * TO LOAD LTC4412 6 VIN SENSE 5 2 GND GATE 4 3 CTL STAT 1 Linear Technology Magazine • December 2002 Q1 VCC 470k 4412 F01 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 is absent. At about 0.6V the drainsource diode of the P-channel MOSFET transistor begins to forward bias and pulls the output up (time B1). Once the primary input reaches a voltage sufficient to drive the MOSFET gate and power the LTC4412, the forward regulation mode is achieved (B2). Now the output voltage is regulated to typically 20mV (10mV min) “The forward voltage drop of the ideal diode is far less than that of a conventional diode…” below the input. If the load current varies, the GATE pin voltage is controlled to maintain 20mV unless the load current exceeds the P-channel MOSFET’s ability to deliver the current with a 20mV VDS. If the RON is not low enough to maintain forward regulation then the gate voltage reaches ground or clamps 7V below the higher of the voltages on the VIN or SENSE B1 B2 pins. Once clamped the MOSFET behaves as a constant low value resistor, and the forward voltage increases slightly. During this forward regulation mode of operation the STAT pin is an open circuit and the 470k resistor pulls the voltage up to the VCC supply, which can be as high as 28V. When a wall adapter or other supply connected to the auxiliary input is applied the SENSE pin voltage rises. The battery voltage also rises slightly from being unloaded. As the SENSE voltage pulls above VIN – 20mV the LTC4412 pulls the GATE voltage up to turn off the P-channel MOSFET (A1). 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. Power to the load is being delivered through the external diode and no current is drawn from the battery. Should the primary input now be removed there is no effect (A2). When the primary input is applied and the B3 A1 A2 5 How It Works Figure 2 shows a circuit that automatically switches the power supply between a battery and a wall adapter (or other types of power inputs). The supply inputs are slowly ramped, as seen in Figure 3, to illustrate operation of the circuit. For the sake of this discussion the load is purely resistive, and the terms primary and auxiliary are arbitrary and interchangeable. First the battery primary input, which powers the VIN pin, is ramped up from 0V while the auxiliary input COUT PRIMARY INPUT (V) 0 0 7.5 AUXILIARY INPUT (V) 0 7 4.98 6.9 OUTPUT TO LOAD (V) 0 3 GATE (V) STATUS (V) 0 VCC 0 0.2 Figure 3. Operation waveforms 3 DESIGN FEATURES Q2 * WALL ADAPTER INPUT Q1 * TO LOAD BATTERY CELL(S) COUT LTC4412 6 VIN SENSE 5 2 GND GATE 4 3 CTL STAT 1 470k 4412 F02 *PARASITIC DRAIN-SOURCE DIODE OF MOSFET STATUS OUTPUT DROPS WHEN A WALL ADAPTER IS PRESENT AND SUPPLYING THE LOAD CURRENT Q1, Q2: SILICONIX Si4953DY (800) 554-5565 Figure 4. Lowest loss automatic power switching between a battery and a wall adapter auxiliary input is removed the circuit reverts to battery operation (B3). The external diode is used to protect against auxiliary input faults. A silicon diode could be used instead of the Schottky, but will result in higher power dissipation and heating due to the higher forward voltage drop. The Lowest Loss Automatic PowerPath Control 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 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 Q2 * BAT2 LTC4412 6 VIN SENSE 5 2 GND GATE 4 3 CTL STAT 1 VCC 470k 4412 F05 *PARASITIC DRAIN-SOURCE DIODE OF MOSFET STATUS IS HIGH WHEN BAT2 IS SUPPLYING LOAD CURRENT Q1, Q2: SILICONIX Si4953DY (800) 554-5565 Figure 5. Dual battery load sharing with automatic switchover of power from batteries to wall adapter Q1 * BATTERY CHARGER INPUT TO LOAD OR PowerPath BAT1 CONTROLLER LTC4412 6 1 VIN SENSE 5 2 GND GATE 4 3 CTL STAT VCC 470k STATUS IS HIGH WHEN BAT1 IS CHARGING Q2 * LTC4412 6 VIN SENSE 5 2 GND GATE 4 3 CTL STAT 1 TO LOAD OR PowerPath BAT2 CONTROLLER VCC 470k STATUS IS HIGH WHEN BAT2 IS CHARGING *PARASITIC DRAIN-SOURCE DIODE OF MOSFET 4412 F06 Q1, Q2: SILICONIX Si4953DY (800) 554-5565 Figure 6. Automatic dual battery charging from single charging source 4 wall adapter voltage must be high enough to overcome the diodes forward voltage drop. Note that the external MOSFET is wired so that the drain to source diode will reverse bias when a wall adapter input is applied. If the control input (CTL pin) is asserted high the GATE voltage is forced to the higher of the voltages on the VIN pin or the SENSE pin turning the MOSFET off. Also the STAT pin will sink 10µA of current if connected. This feature is useful for forced switching of the load between two power sources. The drain to source diode inherent in MOSFETs require that series back to back MOSFETs, with gates tied together, be used to fully isolate a power source. Figure 4 illustrates an application circuit for automatic switchover of load between a battery and a wall adapter that features lower power loss in the auxiliary path than the circuit of Figure 2. Operation is similar to Figure 2 except that a P-channel MOSFET (Q2) replaces the diode. The STAT pin is used to turn on Q2 once the SENSE pin voltage exceeds the battery voltage by 20mV. When the wall adapter input is applied the drainsource diode of Q2 turns on first to pull up the SENSE pin and load voltages, followed by Q2’s gate turning on. Once Q2 has turned on, the voltage drop across it can be very low depending on the MOSFET’s characteristics. Load Sharing Figure 5 illustrates an application circuit for dual battery load sharing with automatic switchover of power between batteries and a wall adapter. Whichever battery can supply the higher voltage provides the load current until it is discharged to the voltage of the other battery. Then, the load is shared between the two batteries with the higher capacity battery providing proportionally higher current to the load. When a wall adapter input is applied both MOSFETs turn off and continued on page 15 Linear Technology Magazine • December 2002 DESIGN FEATURES fore obtained in the lowest gain configurations. The 2500V/µs slew rate specified on the data sheet is measured in a noninverting unity gain configuration. The 1800V/µs production tested slew rate is measured in an (inverting) gain of –1, which is equivalent to a noninverting gain of 2. The internal current generated across the input resistor can be much higher than the quiescent supply current (up to 80mA). In normal transient closed loop operation this does not present a problem, since after a few nanoseconds the feedback brings the differential input signal back to zero. However, sustained (i.e. open loop) differential input voltages may result in excessive power dissipation and therefore this amplifier should not be used as a comparator. The output stage buffers the high impedance node from the load by providing current gain. Emitter followers Q17–Q20 provide a current gain equal to BetaNPN × BetaPNP, but the effective current gain is greatly enhanced by the dynamic base current compensation provided by Q24–Q26 and Q21–Q23. Q24 measures a fraction of the output current that flows through Q19, and mirror Q25–Q26 injects the appropriate current back into the base of Q19. This signal-dependent boost improves the linearity of the amplifier by reducing the amount of differential input signal required for a given output current. An additional advantage is that the output devices can be smaller, which requires less quiescent current for a given amplifier speed. LTC4412, continued from page 4 High Side Power Switch voltage, therefore denying power to the load. The MOSFET is connected with its source connected to the power source. This prevents the drain-source diode from supplying voltage to the load when the MOSFET is off. no load current is drawn from the batteries. The STAT pins provide information as to which input is supplying the load current. This concept can be applied to as many power inputs as are needed. Multiple Battery Charging Figure 6 shows an application circuit for automatically charging two batteries from a single charger. Whichever battery has the lower voltage receives the charging current until both battery voltages are equal then both will be charged. When both are charged simultaneously the higher capacity battery receives proportionally higher current from the charger. For Li-Ion batteries both batteries achieve the float voltage of the battery charger minus the forward regulation voltage of 20mV. This concept can apply to more than two batteries. The STAT pins provide information as to which batteries are being charged. Figure 7 illustrates an application circuit for a logic controlled high side power switch using the control input pin. When the CTL pin is a logical low the LTC4412 turns on the MOSFET. Because the SENSE pin is grounded the LTC4412’s internal controller functions as an open-loop comparator and applies maximum gate drive voltage to the MOSFET. When the CTL pin is a logical high the LTC4412 turns off the MOSFET by pulling its gate voltage up to the supply input Q1 * SUPPLY INPUT LOGIC INPUT TO LOAD LTC4412 6 1 VIN SENSE 5 2 GND GATE 4 3 CTL STAT COUT Conclusion The ultrafast slew rate and high bandwidth allow the LT1818 and LT1819 op amps to process large signals at high frequencies with low distortion. Combined with the low noise and moderate supply current, these amplifiers are a good choice for receivers, filters, or drivers of cables and ADCs in high-speed communication or data acquisition systems. Conclusion The LTC4412 provides a simple and efficient way to implement a low loss ideal diode controller that extends battery life and significantly reduces self-heating. The low external parts count translates directly to low overall system cost and its ThinSOT 6-pin package makes for compact design solutions. It’s versatile enough to be used in a variety of diode OR-ing applications covering a wide range of supply voltages. 4412 F07 *PARASITIC DRAIN-SOURCE DIODE OF MOSFET Q1: FAIRCHILD SEMICONDUCTOR FDN306P (408) 822-2126 Figure 7. Logic controlled high side power switch for the latest information on LTC products, visit www.linear.com Linear Technology Magazine • December 2002 15