Military-Grade LDOs Battle Harsh Realms

Initiatives to incorporate more commercial-o ff-the-shelf (COTS) components into military systems may result in cost reductions, but reduced reliability may be the real price. The -40 to +85C commercial component temperature range, or even the extended industrial -40 to +125C temperature range, may not be adequate for the most demanding applications. Because of the increased demands of military systems operating in hostile environmental conditions, Linear Technology has developed a family of high-reliability MP-grade products, including low-drop-out (LDO) and Module voltage regulators with extended temperature operation to -55C.

With the COTS initiative of 1994, military designers were allowed to integrate commercial and industrial-grade integrated circuits (ICs) into their weapons systems; MIL-STD-883 and JAN-qualified ICs were no longer required. The initiative freed designers of military and avionics (electronics for aircraft) systems to search for new sources of ICs; the benefits of COTS have included reduced system development & maintenance time and costs.

Still, these systems have stringent requirements, and not all COTs solutions may provide the needed performance levels. For example, commercial aircraft and unmanned aerial vehicles (UAVs) operating at altitudes in excess of 30,000 ft. can encounter temperatures at -50C or less, ruling out the use of ICs rated for industrial or commercial temperature ranges. Key applications for ICs in these platforms include outside cabin-mounted environmental sensors (air speed, temperature, humidity, etc.), Flight control computers, infrared image sensors, and cockpit voice recorders. As measures against terrorism increase, there has also been an increase in safer first-strike capability technology as well as predictive and defense counter-measures technology. Applications in this area include integrated defensive electronic countermeasures (IDECM) systems, joint-air-to-ground-missile (JAGM) systems, joint direct attack munitions (JDAM) using Global Positioning System (GPS) targeting, and radar jamming systems. These systems require ICs capable of operating over the full military temperature range due to the wide range of environments in which the systems are deployed.

Many ground-based military applications (some battery-powered) such as soldier man-packs, radar systems, joint tactical radio systems (JTRS), armored vehicles and night vision apparatus also require ICs that operate across hot-to-cold temperature extremes from a high of 125C down to -55C since they can be placed almost anywhere, and further in humid or arid environmental conditions.

Ambient temperature extremes may cause parametric, and, in some cases, catastrophic problems for ICs. High temperature extremes can cause electro-migration, parametric drift, parasitic leakage, hot spots, and high power dissipation. Cold temperatures can lead to board leakage/condensation potentially causing trace shorting, thermal shock if rapid changes from cold to hot occur, low temperature oscillations, instability, overshoot and poor filter performance. IC designs at cold temperatures must carefully compensate for these factors.

The use of linear LDO regulators is commonplace in both non-portable and handheld electronic systems. Improved specifications, reliability, and protection features are enticing system designers to also consider them for harsh environments. For example, lower dropout voltages enable higher efficiency conversion. Running regulators in parallel spreads heat and reduces hot spots on PCBs, while thermally enhanced LDO regulator packaging also helps dissipate heat and improve reliability. Lower input-to-output voltage differentials open up the types of system rails to be powered. Low quiescent currents increase battery run time. Higher input voltage capabilities provide protection against system voltage transients. High reverse input/output and reverse current protection shield the IC and the surrounding electronics while low output voltage noise minimizes system EMI concerns. These features, in addition to their design simplicity, have allowed LDO regulators to occupy 1 to 5 A application spaces that were previously serviced solely by switching regulators.

Battery-powered systems face the threat of damage to an IC and/or the load when an end user inserts a battery with reverse polarity or misconnects the battery, i.e., in a vehicular system. If an IC is exposed to a reverse voltage, large currents flow to ground through parasitic junctions in the IC, potentially destroying the junctions. Single or multiple diodes added to the power circuitry can provide protection, but at the cost of wasted power and reduced supply voltage resulting from diode voltage drops between the battery voltage and the supply rail. An alternative on-chip solution would not only protect the IC and the load but eliminate those trade-offs resulting from adding more components.

Reverse output voltage protection prevents reverse current from flowing through an IC's parasitic diodes under a reverse output voltage condition, or if the load is returned to the negative supply rail, or if the negative supply is turned on before VIN, or if the output is sitting at the negative rail's potential during power up.

Linear regulators can be easily destroyed if they are forced to source excessive current. Therefore, current-limiting (short-circuit) protection circuitry kicks in under short circuit or excessive load conditions in which the output voltage is less than the input voltage, V_OUTIN; the current-limiting circuitry prevents excessive current flow from V_IN to V_OUT. In a short-circuit condition, not only is the pass transistor sourcing excessive current, the voltage across it is at a maximum (since V_OUT is at ground, the voltage across the transistor is equal to V_IN.) Linear regulators typically use one of two types of short-circuit protection on-chip: constant current limit or a more sophisticated form, current limit with foldback. The addition of foldback protection to the current limit decreases the current limit as the input voltage increases to keep the power transistor in its safe operating area (SOA). When V_OUT > V_IN, i.e., if V_IN is shorted or V_OUT is pulled above V_IN, the reverse current (reverse output-to-input) circuitry prevents any flow of reverse current from V_OUT to V_IN.

With thermal shutdown protection, an IC is actually shut o and the die must cool down by the amount of hysteresis built into the thermal shutdown circuitry. Once the part has cooled down sufficiently, it is restarted. If a fault or overload condition exists, the part heats back up to the thermal shutdown temperature and turns itself off. Therefore, the part sits and thermally oscillates at some low frequency and duty cycle depending on the thermal shutdown temperature, the amount of hysteresis, the package, and the associated thermal time constants. Thermal limiting is a slightly less sophisticated technique than thermal shutdown, in which the maximum die temperature is controlled by the protection circuit.

For many years, Linear Technology has provided reliable, high-performance PNP and NPN LDO regulators for the extended industrial junction temperature range of -40 to +125C. Enhanced wafer fabrication technology and improved layout techniques have allowed the extension of this temperature range to -55C, resulting in a new generation of military plastic (MP)-grade devices.

The MP-grade portfolio encompasses output currents from 20 mA (model LT3008) to 5 A (model LT3070), with dropout voltages as low as 85 mV, input voltages as high as 80 V, output voltage noise as low as 20 V root mean square (RMS), sub-5 A quiescent currents, and a full set of protection features (see the table for a sampling of these MP-grade LDO regulators). The voltage regulator ICs incorporate several protection features that make them ideal for use in both battery-powered and non-portable circuits and systems. In addition to the normal protection features associated with monolithic regulators, such as current limiting and thermal limiting/shutdown, these ICs also protect against reverse input voltages, reverse output voltages and reverse output-to-input voltages.

Current limit protection and thermal overload protection protect the devices against overload conditions at its output. e addition of fold-back keeps the power transistor in its SOA. For example, the LT3070 has SOA protection. The safe area protection decreases current limit as the input-to-output voltage increases and keeps the power transistor inside a safe operating region for all values of input-to-output voltage up to the absolute maximum voltage rating. Under conditions of maximum current load (ILOAD) and maximum input-to-out-put-voltage differential (V_IN V_OUT), the LT3070's power dissipation peaks at around 3 W. If the ambient temperature is high enough, the die junction temperature will exceed the +125C maximum operating temperature. If this occurs, the LT3070 relies on two additional thermal safety features. At about +145C, the PWRGD output pulls low providing an early warning of an impending thermal shutdown condition. At typically +165C, the LT3070's thermal shutdown engages and the output is shut down until the IC temperature falls below the thermal hysteresis limit. The SOA protection decreases current limit as the input-to-output voltage increases and keeps the power dissipation at safe levels for all values of input-to-output voltage. The LT3070 provides some output current at all values of input-to-output voltage to the absolute maximum voltage rating (Figs. 1 and 2).

In summary, the MP-grade LDO voltage regulators from Linear Technology are designed to handle the requirements for a wide range of military systems. Some can be used for input voltages as high as 80 V and some may be placed directly in parallel when higher ratings are needed. The family features ICs with quiescent current as low as 3 A, low dropout voltage of typically as low as 90 mV, low output noise, output voltage tolerance as good as 2 percent, fast transient response, and foolproof reverse voltage protection (output, output-to-input, and input protection). In addition to the LDO voltage regulators, Linear Technology manufactures a full line of high-performance, Module voltage regulator devices with design-in simplicity near that of an LDO, in MP and other grades.

STEVE KNOTH Senior Product Marketing Engineer Linear Technology Corp., 1630 McCarthy Blvd., Milpitas, CA 95035-7417; (408) 432-1900, FAX: (408) 434-0507, Internet: www.linear.com.