Power Control & Ideal Diode Design
189
Meilissa Lum
Ideal diodes protect against power supply wiring errors
such mishaps. It is important that active ideal diodes give similar protection.
Types of misconnections
Figure 189.2 shows the correct power supply connections.
RTNA and RTNB are close in potential by virtue of the com- mon connections to safety ground represented by RGND.
Figure 189.3 shows a reversed input connection with RTNA and NEGA swapped. The associated ideal diodes are reverse biased, making the wiring error transparent to the load with BATTERY B providing power.
Figure 189.4 shows another misconnection with RTNB and NEGA swapped, so one power supply is connected across the RTN inputs of the LTC4355 and the other supply across the
−48V inputs of the LTC4354. In this case, the reverse input protection network of three diodes shown in Figure 189.1 prevents damage to the LTC4355. The load operates from BATTERY B, but only after the current has passed through the ground wiring.
Introduction
High availability systems often employ dual feed power distribution to achieve redundancy and enhance system reli- ability. ORing diodes join the feeds together at the point of load, most often using Schottky diodes for low loss.
MOSFET-based ideal diodes can be used to replace Schottky diodes for a significant reduction in power dissipation, sim- plifying the thermal layout and improving system efficiency.
Figure 189.1 shows the LTC4355 and LTC4354 combining the inputs and returns in a −48V, 5A dual feed application.
This solution reduces the power dissipation from 6W using Schottky diodes to just 1.1W with MOSFETs.
With two supply sources and four supply connections there are plenty of ways to incorrectly connect the wires.
Although the likelihood of a wiring error is small, the cost is high if downstream cards are not designed to tolerate such errors. Wiring errors could include reverse polarity or cross- feed connections. Knowing this, circuit designers are accus- tomed to using discrete diode solutions to protect against
Figure 189.1 • −48V Ideal Diode-OR Figure 189.2 • Correct Power Supply Connections Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00189-7
Figure 189.5 shows BATTERY B installed incorrectly. The reversed battery has no effect on the load because the diode connected to NEGB is reverse biased. The voltage across the LTC4354 can exceed 100V and an external clamp may be added to protect its DRAIN pin.
Figures 189.2–189.5 have the correct safety ground con- nections to RTNA and RTNB. Damage can occur if there is a large potential difference between RTNA and RTNB.
Figure 189.6 shows the safety ground, RGND, mistakenly con- nected to NEGB instead of RTNB. This connects the power supplies in series and the voltage seen across the load nears 100V which can cause damage, a situation no different than encountered with a discrete diode solution. A TransZorb
placed across the output protects the load, until a fuse on the input opens to isolate the high voltage from the load.
Conclusion
In dual feed applications, the supply connections can be erro- neously wired, potentially causing damage to the load. An ideal diode solution using the LTC4355 and LTC4354 pro- vides protection similar to Schottkys, but with much lower power dissipation. The end result is a compact layout and improved efficiency.
Figure 189.3 • Reversed RTNA and NEGA Connection
Figure 189.4 • RTNB and NEGA Swapped
Figure 189.5 • Reversed BATTERY B
Figure 189.6 • GND Misconnected to NEGB
190
David Laude
Ideal diode controller eliminates energy wasting diodes in power OR-ing applications
• Protection of MOSFET from excessive gate-to-source volt- age with VGS limiter
• Low quiescent current of 11μA with a 3.6V supply, inde- pendent 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
Applications include anything that takes power from two or more inputs:
• 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
• Multibattery charging from a single charger
• Logic controlled power switches
Automatic power switching between two power sources
Figure 190.2 illustrates an application circuit for the auto- matic 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 neg- ligible battery power. The STAT pin is an open circuit while the battery provides power. When a wall adapter or other
Introduction
Many modern electronic devices need a means to automati- cally switch between power sources when prompted by the insertion or removal of any source. The LTC4412 simplifies PowerPath 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 190.1). The tiny forward volt- age drop reduces power losses and self-heating, resulting in extended battery life. Features include:
• Voltage drop across the controlled external MOSFET of only 20mV (typical)
• Low component count helps keep overall system cost low
• 6-pin ThinSOT package permits a compact design solution
• Wide supply operating range of 2.5V to 28V (36V absolute maximum)
Figure 190.1 • LTC4412 Ideal Diode Controller vs Schottky Diode Characteristics
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00190-3
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 190.3 shows a dual battery load sharing application with automatic switchover of power between the batter- ies 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 pack- age, a compact design as well. Its versatility is useful in a vari- ety of applications (see the LTC4412 data sheet for additional applications).
Figure 190.2 • Automatic Power Switching between a Battery
and a Wall Adapter Figure 190.3 • Dual Battery Load Sharing with Automatic
Switchover of Power from Batteries to Wall Adapter
191
James Herr Mitchell Lee
Replace ORing diodes with MOSFETs to reduce heat and save space
This is easily dissipated in the circuit board with no additional heat sinking.
The LTC4354 implements two ideal diodes, simultaneously controlling two external N-channel MOSFETs with the source pins tied together, as shown in Figure 191.2. This common source node is connected to the VSS pin, the negative supply of the device. Its positive supply is derived from −48V_RTN through an external current limiting resistor. The LTC4354 includes an internal shunt to regulate the VCC pin at 11V.
Introduction
High availability telecom systems employ redundant power supplies or battery feeds to enhance system reliability. Dis- crete diodes are commonly used to combine these power sources at the point of load. The disadvantage of this approach is the significant forward voltage drop and resulting power dissipation, even with Schottky diodes. This drop also reduces the available supply voltage, which is sometimes criti- cal at the low end of the input operating range. A circuit with
“ideal” diode behavior overcomes the dissipation and voltage loss problems by eliminating the forward drop.
The LTC4354 negative voltage diode-OR controller real- izes near-ideal diode behavior. External N-channel MOSFETs, actively driven by the LTC4354, act as pass transistors to replace the diodes. The device maintains a small 30mV for- ward voltage drop across the MOSFETs at light load; under heavy load the voltage drop becomes a function of RDS(ON). For example, an 18mΩ MOSFET and 5A load current pro- duce a drop of 90mV, representing a more than fivefold improvement in drop and power dissipation over a Schottky diode, which exhibits a 500mV drop under the same operating conditions. Lower power dissipation conserves board space and saves the cost of heat sinks. At the same time, 410mV of input operating range is added—a critical factor when the system is running on hold-up capacitors with only a few volts of headroom.
Ideal − 48V ORing diode
Figure 191.1 shows a comparison of power dissipation for a diode and a MOSFET driven by the LTC4354. At 10A, the voltage drop across a 100V Schottky diode (MBR10100) is around 620mV; a heat sink is required to handle resulting 6.2W of power dissipation. Using an LTC4354 driving a 100V N-channel MOSFET (IRFR3710Z), the dissipation is only 1.8W due to the low 18mΩ (max) RDS(ON) of the MOSFET.
Figure 191.1 • Comparison of Power Dissipation for a Diode and a MOSFET Driven by the LTC4354
Figure 191.2 • The LTC4354 Implementing Two Ideal Diodes, Controlling Two N-Channel MOSFETs with the Source Pins Tied Together
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00191-5
At power-up, the initial load current flows through the body diode of the MOSFET and returns to the supply with the lower (most negative) voltage. The associated gate pin immediately ramps up and turns on the MOSFET. The LTC4354 tries to regulate the voltage drop across the source and drain termi- nals to 30mV. If the load current causes more than 30mV of drop, the gate is driven higher to further enhance the MOSFET.
Eventually the MOSFET is driven fully on and the voltage drop increases as dictated by RDS(ON)ã ILOAD.
If the power supply voltages are nearly equal, this regu- lation technique ensures that the load current is smoothly shared between them without oscillation. The current level flowing through each MOSFET depends on the RDS(ON) of the MOSFETs, the output impedance of each supply and dis- tribution resistance.
In the case of supply failure, such as an input supply short to −48V_RTN, a potentially large reverse current could flow from the −48V_RTN through the MOSFET that is on.
The LTC4354 detects this condition as soon as it appears and turns off the MOSFET in less than 1μs. This fast turn- off prevents reverse current from reaching a damaging level, exhibiting a behavior not unlike a discrete diode with a recov- ery time measured in hundreds of nanoseconds.
Fault output detects damaged MOSFETs and fuses
The LTC4354 monitors each MOSFET and reports any exces- sive forward voltage that is indicative of an overcurrent fault.
When the pass transistor is fully on but the voltage drop across it exceeds the 260mV fault threshold, the open-drain FAULT pin goes high. This allows an LED or optocoupler to turn on and flag the system controller. It is important to rec- ognize excessive voltage drop in the MOSFETs because extra heat is being dissipated. If the condition persists the system controller can take action and shut down the load.
Positive low voltage ideal diodes
LTC4354 is also suited for positive, low voltage applications, as shown in Figure 191.3. With this circuit the outputs of multiple high current switching converters can be combined, without concern about back feeding or supply failure short- ing out the common bus. One diode “channel” comprises the LTC4354 and six parallel MOSFETs, supplying 100A to a 1.2V load. The circuit is easily adapted to any supply voltage
between 0V and 5V, provided there is a path for up to 4mA VEE current to ground at either the input or the output. Most high current switching converters can easily sink 4mA and no preload is necessary. No circuit changes are necessary for dif- ferent operating voltages.
Conclusion
The trend in today’s telecom infrastructure is toward higher current and smaller module space. Traditional diode ORing is increasingly cumbersome. The LTC4354 provides an improved solution by controlling low RDS(ON) N-channel MOSFETs to reduce power dissipation and save board space and heat sinks, on both sides of the isolation barrier. Further- more, the LTC4354 monitors and reports fault conditions, information not provided by a traditional diode-OR circuit.
Figure 191.3 • Positive Low Voltage Diode-OR Combines Multiple Switching Converters
192
Andy Bishop
Dual monolithic ideal diode manages multiple power inputs
3mm × 3mm, 10-pin DFN package. Current-limit and ther- mal shutdown features further enhance system reliability.
Triple supply power management
Figure 192.1 shows a schematic with the LTC4413 config- ured to automatically switchover from a battery to either a USB supply or to a wall adapter. This circuit provides unin- terrupted power to the load—while isolating all three power sources—allowing the user to remove the battery without affecting load voltage when either of the two other supplies is present. This circuit exploits the LTC4413 to provide low loss uninterrupted power while automatically prioritizing which source should be connected to the load.
Referring to Figure 192.1, if a wall adapter is applied in the absence of a USB supply, the body diode in MP1 forward biases, pulling the output voltage above the battery voltage and turning off the ideal diode connected between INA and OUTA. This causes the STAT voltage to fall, turning on MP1.
Introduction
The LTC4413 dual monolithic ideal diode helps reduce the size and improve the performance and reliability of handheld battery-operated devices. The LTC4413 is a single-chip solu- tion that can automatically select between and isolate up to three power sources; such as a wall adapter, an auxiliary sup- ply and a battery. It provides a low loss automatic PowerPath management solution for demanding applications that may require short-circuit protection, thermal management and sys- tem-level power management and control.
The LTC4413 contains two isolated low voltage (2.5V to 5.5V) monolithic ideal diodes. Each ideal diode chan- nel provides a low forward voltage drop (typically as low as 40mV when conducting low current) and a low RDS(ON)
(below 100mΩ) when conducting high current—features that are important to extend battery life and reduce heat in portable applications. Furthermore, each channel is capable of providing up to 2.6A of continuous current from a small
Figure 192.1 • Automatic Switchover from a Battery to a USB Supply or Wall Adapter Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00192-7
The load then draws current from the wall adapter and the battery is disconnected from the load. If the USB is present when the wall adapter is removed, the output voltage falls until the USB voltage exceeds the output voltage causing the STAT voltage to rise, disabling the external PFET; the USB then provides power to the load. In the absence of a USB sup- ply when the wall adapter is removed, the load voltage droops until the battery voltage exceeds the load voltage, likewise causing the STAT voltage to rise and disabling MP1; the bat- tery then provides the load power.
When a USB supply is applied, the voltage divider at ENBA disables the power path from battery to OUT. The USB then provides load current, unless a wall adapter is pre- sent as described above.
Automatic switchover between a battery and a wall adapter with a battery charger
Figure 192.2 illustrates an application where the LTC4413 performs the function of automatically switching a load over from a battery to a wall adapter, while controlling an LTC4059A battery charger. When no wall adapter is present,
the LTC4413 connects the load to the Li-Ion battery. In this configuration, the STAT voltage is high, thereby disabling the battery charger. If a wall adapter is connected, the load volt- age rises as the ideal diode from INB to OUTB conducts. As soon as the load voltage exceeds the battery voltage, the bat- tery is disconnected from the load and the STAT voltage falls, turning on the LTC4059A battery charger and beginning a charge cycle.
When the wall adapter is removed, the load voltage col- lapses until it is below the battery voltage. The battery reverts to supplying power to the load and the STAT pin falls disa- bling the battery charger.
Conclusion
The LTC4413 performs automatic PowerPath management functions in high performance battery-powered applications.
The applications described herein have the added benefit that while an alternate supply powers the load, the battery may be replaced without disturbing load voltage. This feature demon- strates the benefit of the LTC4413 as compared with alter- native solutions that do not allow the battery to be replaced without impacting load power.
Figure 192.2 • Automatic Switchover from a Battery to a Wall Adapter with a Battery Charger
193
Doug La Porte
PCMCIA socket voltage switching
drives require 5V at 600mA to 800mA for a short duration during spin-up. Current draw drops to 300mA to 420mA dur- ing read and write operations. A low switch resistance on the 3.3V switch is critical to assure that the specified 3.0V mini- mum is maintained. The VPP supply must source 12V at up to 120mA and 3.3V or 5V at lesser currents. The VPP supply is intended solely for flash memory programming. The 120mA current requirement allows writing to flash devices and simul- taneously erasing two other parts as required by many flash drives.
The host PCMCIA socket designer also has several other practical aspects of the design to consider. The exposed socket pins are vulnerable to being shorted by foreign objects such as paper clips. In addition the users will attempt to install damaged cards. In short, once in the hands of the con- sumer, the designer and manufacturer have little control over use and abuse. To ensure a robust system and a satisfied cus- tomer, switch protection features such as current limiting
Introduction
Most portable systems have built-in PCMCIA sockets as the sole means of expansion. The requirements of the PCMCIA specification have led to some confusion among system designers. This Design Note will attempt to lessen the confu- sion and highlight other practical system issues.
Host power delivery to the PC card socket flows through two paths: the main VCC supply pins and the VPP program- ming pins. Both supplies are switchable to different voltages to accommodate a wide range of card types. The VCC main card supply must be capable of delivering up to 1A at either 3.3V or 5V. The 1A rating is an absolute maximum derived from the contact rating of 500mA per pin for both VCC pins and assumes that both pins are in good condition and cur- rent is shared equally. One of the most stringent actual cur- rent requirements is during hard drive spin-up. Present hard
Why your portable system needs SafeSlot protection
Figure 193.1 • Typical LTC1472 Application with the LT1301 3.3V Boost Regulator Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00193-9