Hot Swap and Circuit Protection
245
Victor Fleury
Protect sensitive circuits from overvoltage and reverse supply connections
Undervoltage, overvoltage and reverse supply protection
The LTC4365 is a unique solution that elegantly and robustly protects sensitive circuits from unpredictably high or nega- tive supply voltages. The LTC4365 blocks positive voltages as high as 60V and negative voltages as low as −40V. Only volt- ages in the safe operating supply range are passed along to the load. The only external active component required is a dual N-channel MOSFET connected between the unpredictable supply and the sensitive load.
Figure 245.1 shows a complete application. A resistive divider sets the overvoltage (OV) and undervoltage (UV) trip points for connecting/disconnecting the load from VIN. If the input supply wanders outside this voltage window, the LTC4365 quickly disconnects the load from the supply.
The dual N-channel MOSFET blocks both positive and negative voltages at VIN. The LTC4365 provides 8.4V of enhancement to the gate of the external MOSFET during
Introduction
What would happen if someone connected 24V to your 12V circuits? If the power and ground lines were inadvertently reversed, would the circuits survive? Does your application reside in a harsh environment, where the input supply can ring very high or even below ground? Even if these events are unlikely, it only takes one to destroy a circuit board.
To block negative supply voltages, system designers tradi- tionally place a power diode or P-channel MOSFET in series with the supply. However, diodes take up valuable board space and dissipate a significant amount of power at high load currents. The P-channel MOSFET dissipates less power than the series diode, but the MOSFET and the circuitry required to drive it increases costs. Both of these solutions sacrifice low supply operation, especially the series diode. Also, neither protects against voltages that are too high—protection that requires more circuitry, including a high voltage window com- parator and charge pump.
Figure 245.1 • Complete 12V Automotive Undervoltage, Overvoltage and Reverse Supply Protection Circuit Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00245-3
normal operation. The valid operating range of the LTC4365 is as low as 2.5V and as high as 34V—the OV to UV window can be anywhere in this range. No protective clamps at VIN are needed for most applications, further simplifying board design.
Accurate and fast overvoltage and undervoltage protection
Two accurate (±1.5%) comparators in the LTC4365 monitor for overvoltage (OV) and undervoltage (UV) conditions at VIN. If the input supply rises above the OV or below the UV thresholds, respectively, the gate of the external MOSFET is quickly turned off. The external resistive divider allows a user to select an input supply range that is compatible with the load at VOUT. Furthermore, the UV and OV inputs have very low leakage currents (typically <1nA at 100°C), allowing for large values in the external resistive divider.
Figure 245.2 shows how the circuit of Figure 245.1 reacts as VIN slowly ramps from −30V to 30V. The UV and OV thresholds are set to 3.5V and 18V, respectively. VOUT tracks VIN when the supply is inside the 3.5V to 18V window. Out- side of this window, the LTC4365 turns off the N-channel MOSFET, disconnecting VOUT from VIN, even when VIN is negative.
Novel reverse supply protection
The LTC4365 employs a novel negative supply protection circuit. When the LTC4365 senses a negative voltage at VIN, it quickly connects the GATE pin to VIN. There is no diode drop between the GATE and VIN voltages. With the gate of the external N-channel MOSFET at the most negative poten- tial (VIN), there is minimal leakage from VOUT to the negative voltage at VIN.
Figure 245.3 shows what happens when VIN is hot-plugged to −20V. VIN, VOUT and GATE start out at ground just before the connection is made. Due to the parasitic inductance of the
VIN and GATE connections, the voltage at VIN and GATE pins ring significantly below −20V. The external MOSFET must have a breakdown voltage that survives this overshoot.
The speed of the LTC4365 reverse protection circuits is evident by how closely the GATE pin follows VIN during the negative transients. The two waveforms are almost indistin- guishable on the scale shown. Note that no additional external circuits are needed to provide reverse protection.
There’s more! AC blocking, reverse VIN Hot Swap control when VOUT is powered
After either an OV or UV fault has occurred (or when VIN
goes negative), the input supply must return to the valid oper- ating voltage window for at least 36ms in order to turn the external MOSFET back on. This effectively blocks 50Hz and 60Hz unrectified AC.
LTC4365 also protects against negative VIN connections even when VOUT is driven by a separate supply. As long as the breakdown voltage of the external MOSFET is not exceeded (60V), the 20V supply at VOUT is not affected by the reverse polarity connection at VIN.
Conclusion
The LTC4365 controller protects sensitive circuits from over- voltage, undervoltage and reverse supply connections using back-to-back MOSFETs and no diodes. The supply voltage is passed to the output only if it is qualified by the user-adjust- able UV and OV trip thresholds. Any voltage outside this window is blocked, up to 60V and down to −40V.
The LTC4365’s novel architecture results in a rugged, small solution size with minimal external components, and it is available in tiny 8-pin 3mm × 2mm DFN and TSOT-23 packages. The LTC4365 has a wide 2.5V to 34V operating range and consumes only 10μA during shutdown.
246
Tim Regan
Simple energy-tripped circuit breaker with automatic delayed retry
Higher currents permitted for shorter time intervals
The circuit of Figure 246.1 has three distinct parts—circuit breaking, current sensing and timing.
The circuit breaking function can be any type of electroni- cally controlled relay or solid state switch, properly sized for voltage and current ratings of the load being protected.
Load current sensing is achieved via an LT6108-2 current sense amplifier with built-in comparator. The LT6108-2 converts the voltage drop across a small valued sense resistor to a ground-referenced output voltage that is directly proportional to the load current. The trip threshold is created by scaling the output voltage via resistor divider and feeding the result to the integrated comparator with a preci- sion 400mV voltage reference. The comparator changes state when the load current exceeds the threshold.
To prevent short duration transients from causing nuisance trips, an LTC6994-2 TimerBlox delay timer is added between
Introduction
A circuit breaker protects sensitive load circuits from excessive current flow by opening the power supply when the current reaches a predetermined level. The simplest circuit breaker is a fuse, but blown fuses require physical replacement. An electronic circuit breaker provides the same measure of circuit protection as a fuse without the single-use problem. Nevertheless, an electronic circuit breaker with a fixed trip current threshold, while effective for protection, can become a nuisance if tripped by short duration current transients—even if the circuit breaker self-resets.
One way to minimize nuisance breaks is to employ a slow- blow technique, which allows relatively high levels of current for short intervals of time without tripping the breaker. Ide- ally, the breaker’s trip threshold would be a function of total transient energy, instead of just current. This article describes an electronic circuit breaker, combining current sensing with timing to create an energy-tripped breaker, which protects sensitive circuits while minimizing nuisance trips.
Figure 246.1 • Energy-Tripped Circuit Breaker Trips After a Time Interval that Varies as a Function of Sensed Load Current
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00246-5
the comparator output and the circuit breaker. Once tripped, the comparator falling edge starts a variable time delay inter- val, which, if allowed to complete, signals the circuit breaker to open. Nothing happens if the transient duration is shorter than the delay.
A current-controlled delay interval
The LTC6994-2 delays from an edge appearing at its IN pin by a time ranging from 1μs to 33s. The delay time is con- trolled by the current sourced by the SET pin, which pro- grams an internal oscillator frequency, while the bias voltage on the DIV pin selects a frequency divide ratio.
The LT1783 op amp circuit takes the output voltage from the current sense amplifier and adjusts the SET pin current, thereby making the delay time a function of the load current (see Figure 246.2). As shown, the current sense comparator trip threshold is 500mA. A current of 500mA creates a fall- ing edge and starts a time delay of 350ms. Should the load current drop below 500mA before the delay time expires, the timer output remains high and the circuit breaker does not trip.
Higher load currents correspond to higher current sense amplifier output voltages, which in turn reduce the delay time interval (Figure 246.2). For instance, a 5A load current trips the circuit breaker in only 60ms. Depending on the average load current in excess of the 500mA threshold, the delay interval or trip time will fall somewhere between 30ms and 400ms.
Once tripped, the load current drops to zero. This resets the current sense comparator high. This rising edge is also delayed by the LTC6994-2. The minimum current sense out-
put voltage stretches this delay to a maximum time of ∼1.3s.
After this delay the circuit breaker closes and reapplies power to the load. This automatic retry function requires no addi- tional components.
The response of the circuit to a 5A load current spike and automatic retry is shown in Figure 246.3. If the load current remains too high, the trip/retry cycle repeats continually.
A current surge is fairly common when the circuit breaker is first closed and can trip the comparator. If the duration is less than the timer delay, the breaker remains closed, thus avoiding an endless loop of self-induced nuisance trips.
Extending the retry time interval
The LTC6994-2 delay timer has eight divider settings for a wide range of timing intervals. Adding the single optional resistor shown in Figure 246.1 shifts the delay block to a new setting, increasing the retry time interval if desired. This can give any fault condition more time to subside. The circuit breaker response time interval is not affected.
For the values shown, when the circuit breaker trips and the current drops to zero, the comparator high level biases the DIV pin to a higher voltage level, resulting in a longer retry delay time of 10 seconds.
Conclusion
The circuit shown here can be easily modified to different timing requirements with a few resistor value changes. Other Figure 246.3 • An Example Trip and Retry Sequence. At Time Point A, the 5A Load Current Spike Trips the Comparator and 60ms Later the Breaker Is Opened. At Time B, After a Delay Time of 1.3 sec, the Timer Closes the Breaker. The Resulting Short Duration Spike of Start-Up Current Is Not Large Enough or Long Enough in Duration to Trip the Breaker Again
247
Vladimir Ostrerov
Hot Swap controller, MOSFET and sense resistor are integrated in a 5mm × 3mm DFN for accurate current limit and load current
monitoring in tight spaces
5mm ×3mm DFN package (or 20-lead TSSOP). This 2A integrated Hot Swap controller fits easily onto boards operating in the voltage range from 2.9V to 26.5V. A dedicated 12V version, LTC4217-12, is also available, which contains preset 12V specific thresholds. Figure 247.1 shows how little space is required for a complete Hot Swap circuit.
LTC4217 features
Figure 247.2 shows a simplified block diagram of the LTC4217.
The controller provides inrush current control and a 5% accu- rate 2A current limit with foldback. For soft-start, an internal current source charges the gate of the N- channel MOSFET with 300V/s slew rate. Lower soft-start output voltage slew rates can be set by adding an external gate capacitor.
Introduction
In general, a Hot Swap controller provides two important functions to boards that can be plugged and unplugged from a live backplane:
• It limits the potentially destructive inrush current when a board is plugged in.
• It acts as a circuit breaker, with the maximum current and maximum time at that current factored into the breaker function.
Two of the components required to implement these functions, the power MOSFET and sense resistor, tend to dominate the board real estate taken by the Hot Swap circuit. The LTC4217 saves space by combining these two components with a Hot Swap controller in a 16-pin
Figure 247.1 • Tiny Integrated Controller Package
Results in a Small Footprint Figure 247.2 • Simplified Block Diagram of the LTC4217
Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00247-7
Integrated MOSFET and sense resistor
The LTC4217 integrates a 25mΩ MOSFET and 7.5mΩ cur- rent sense resistor. The default value of the active current limit is 2A, which can be adjusted to a lower value by add- ing an external resistor. Using an external analog switch for the connection of this resistor allows a start-up current to be larger than the maximum load current in steady state operation.
Adjustable current limit
The voltage at the ISET pin determines the active current limit, which is 2A by default. This pin is driven by a 0.618mV voltage reference through a 20k resistor. An external resis- tor placed between the ISET pin and ground forms a resis- tive divider with the internal 20k resistor. The divider acts to lower the voltage at the ISET pin and therefore lower the cur- rent limit threshold.
The ISET pin voltage increases linearly with temperature, with a slope of 3.2V/°C when no external resistor is pre- sent. This compensates for the temperature coefficient of the sense resistor—important for applications that must maintain
monitoring accuracy over a wide temperature range. This also provides a convenient means to monitor the MOSFET tem- perature. If the die temperature exceeds 145°C, the MOS- FET is turned off. It is turned on again when the temperature drops below 125°C.
Voltage and current monitoring
The LTC4217 protects the load from overvoltage and under- voltage conditions with a 2% accurate comparator threshold.
The LTC4217 also features an adjustable current limit timer, a current monitor output and a fault output.
The adjustable current limit timer sets the time duration for current limit before the MOSFET is turned off. The cur- rent monitor produces a voltage signal scaled to the load cur- rent. The fault output is an open drain that pulls low when an overcurrent fault has occurred and the circuit breaker trips.
Typical application
The LTC4217 application circuit shown in Figure 247.3 operates with a 100ms auto-retry time and a 2ms overcurrent condition when the load current reaches 2A. It also produces a voltage signal for an ADC to monitor load current.
Figure 247.3 • Typical Application with Current Monitored by an ADC
248
Vladimir Ostrerov
Hot Swap solution meets AMC and MicroTCA standards
also features an adjustable analog current limit with a circuit breaker timeout for the 12V supply.
The LTC4223 monitors 12V load current by sensing voltage across an external resistor and outputs a ground- referenced voltage (at the 12IMON pin) proportional to the load current. It also provides separate power-good outputs for the two supplies and a single, common fault output.
Additional features include card detection and independent control of the two supplies. The LTC4223-1 latches off after a circuit breaker fault timeout expires while the LTC4223-2 provides automatic retry after a fault. A fault on the 12V supply shuts down only the 12V path, leaving the 3.3V auxiliary power available for system management functions. A fault on the 3.3V AUX supply shuts down both supplies.
Introduction
The LTC4223 is a dual Hot Swap controller that meets the power requirements of the Micro Telecommunication Com- puting Architecture (MicroTCA) specification recently rati- fied by the PCI Industrial Computer Manufacturers Group (PICMG).
The LTC4223 includes an internal pass FET for the 3.3V auxiliary supply and a driver for an external N-channel pass FET for the 12V payload supply. Inrush current for both sup- plies is controlled: the auxiliary supply has a fixed 240mA active current limit and the 12V ramp rate is controlled by an external capacitor. A timed circuit breaker and fast current limit protect both supplies against severe overcurrent faults. It
Figure 248.1 • Advanced Mezzanine Card Application Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00248-9
Advanced Mezzanine Card application
Figure 248.1 shows a typical MicroTCA application. The cur- rent limit on the 12V rail is 7.6A, determined by the 6mΩ sense resistor. Auxiliary rail current limit is internally set to 240mA. (Section 4.2.1 of the MicroTCA specification details the requirements for payload and auxiliary voltage, current,
and protection.) Figure 248.2 shows the power-up transients when a card is inserted. Figures 248.3 and 248.4 show the two modes of overcurrent protection on the 12V supply. In Figure 248.3, the load current is increased above the analog current limit (ACL) threshold. The LTC4223 responds by reducing the current to the ACL threshold, allowing the card to ride out short overcurrent faults. If the fault persists, the timer expires and power is turned off. In the event of a severe overcurrent fault, load current is reduced to the ACL limit in 8μs as shown in Figure 248.4. Once again, if the fault persists and the timer expires, power is turned off entirely.
Conclusion
The LTC4223 aims to simplify Hot Swap control for Advanced Mezzanine Cards in ATCA and MicroTCA systems. It succeeds by meeting all MicroTCA requirements for controlling both payload and auxiliary power with only a 5mm × 4mm DFN package and a few minimal external components. Individual card monitoring and control functions are further simplified by LTC4223’s status and control lines.
Figure 248.2 • Normal Power-Up Waveform
Figure 248.3 • Overcurrent Fault on 12V Output Figure 248.4 • Short-Circuit Fault on 12V Output
249
Mark Thoren
An easy way to add auxiliary control functions to Hot Swap cards
Additional control
There are many functions on a card that are considered part of the “power gateway,” apart from the actual function of the board (telecommunications, data acquisition, etc.). These include sequencing power supplies, providing supply status information, monitoring pushbuttons, etc. The LTC4215-1 GPIO pins are well suited to these functions. Tying the ON pin high turns on the pass FET after a 100ms power-on delay.
Grounding the ON pin enables software control of the FET.
The state of the GPIO pins can be set before enabling the FET, ensuring a known state when downstream power is ena- bled. GPIO1 defaults high on power-up, and can sink 5mA.
GPIO2 defaults high and can sink 3mA. GPIO3 defaults low and can sink 100μA.
For instance, Figure 249.1 shows an application that moni- tors a “request to remove card” pushbutton and lights an
Introduction
A Hot Swap controller is essential to any system in which boards are inserted into a live backplane. The controller must gently ramp up the supply voltage and current into the card’s bypass capacitors, thus minimizing disturbances on the back- plane and to other cards. Likewise, it must disconnect a faulty card from the backplane if it draws too much current. The controller also monitors undervoltage and overvoltage condi- tions on the backplane supply, ensuring reliable operation of the card’s circuitry. The LTC4215-1 takes the obvious next step and integrates three general purpose I/O (GPIO) lines and an accurate ADC into the Hot Swap controller to pro- vide quantitative information on board voltage and current.
Upgrading to the LTC4215-1 is analogous to replacing a car’s venerable “Check Engine” light with a modern dashboard information display.
Figure 249.1 • The LTC4215-1 in a Typical Card Resident Application Analog Circuit Design: Design Note Collection. http://dx.doi.org/10.1016/B978-0-12-800001-4.00249-0