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Harsh military environments demand high performance power conversion

Harsh military environments demand high performance power conversion

Technology News |
By eeNews Europe




Background

Military vehicles are the platform for increasingly complex electronic systems for command and control, communications, weapons management, surveillance and countermeasures. Size, weight and power represent a major challenge for designers trying to combine a myriad of systems with soldiers and their supplies into a small and cluttered space. In particular heat management and power distribution are critical and new high performance power conversion technology helps designers to develop smaller, more thermally efficient and reliable systems.

A further complication for designers of military vehicle electronics is the need to comply with MIL-STD-1275D or similar national standards which specify the steady state and transient voltage requirements of 28V power systems, which have a typical operating input voltage range of 16V to 40V. The MIL-STD-1275D specifies voltage surges to 60V and 100V, requiring operation at these higher voltage levels or needing an extra clamp circuit to maintain safe operating input voltage. To further complicate design considerations, the number of rails within an electronic system is also increasing. For example, a typical navigation system can have six or more different supplies including 8.5V, 5V, 3.3V, 2.5V, 1.8V and 1.5V. At the same time, as the number of components increase, space requirements continue to shrink. Therefore, high efficiency conversion to minimize power dissipation becomes more critical due to the space limitations and high temperature conditions.

Many military vehicle systems require continuous power to the onboard electronics even when the motor is not running. It is essential for these types of “Always-on” systems to have a DC/DC converter with low quiescent current in order to maximize the battery run-time when in sleep mode. In such circumstances, the regulator runs in normal continuous switching mode until the output current drops below a predetermined threshold of 30-50mA or so. Below this level, the switching regulator must go into Burst Mode® operation to lower the quiescent current into tens of micro amps, thereby lowering the power drawn from the battery in order to extend its run-time.

With 60V input DC/DC converters in short supply, designers have resorted to a transformer-based topology or external high side drivers to operate from up to 60V. Others have used an intermediate bus converter requiring an additional power stage. Both of the alternatives increase the design complexity and, in most cases, reduce the overall efficiency. However, the LTC3890 from Linear Technology is the newest member in a growing family of 60V input capable step-down switching regulator controllers that specifically addresses many of the key issues found in automotive, military vehicle and truck applications as outlined above. Figure 1 shows a schematic of the LTC3890 operating in an application that converts a 9V to 60V input into 3.3V/5A and 8.5V/3A outputs.


Figure 1: LTC3890 with 9V to 60V input to 8.5V/3A & 3.3V/5A outputs
Click on image to enlarge

Introducing the LTC3890
The LTC3890/-1 is a high voltage dual output synchronous step-down DC/DC controller that draws only 50uA when one output is active and 60uA when both outputs are enabled. With both outputs shut down, the LTC3890/-1 draws only 14uA. The 4V to 60V input supply range is designed to protect against high voltage transients, continue operation during automotive heavy equipment and truck cold cranking along with covering a broad range of input sources and battery chemistries. Each output can be set from 0.8V to 24V at output currents up to 25 amps, with efficiencies as high as 98 percent making it well suited for 12V, 24V or 28V automotive, truck, heavy equipment and industrial control applications.

The LTC3890/-1 operates with a selectable fixed frequency between 50kHz and 900kHz, and can be synchronized to an external clock from 75kHz to 850kHz with its phase-locked loop (PLL). The user can select from continuous operation, pulse skipping and low ripple Burst Mode operation during light loads. The LTC3890’s 2-phase operation reduces input filtering and capacitance requirements. Its current mode architecture provides easy loop compensation, fast transient response and excellent line regulation. Output current sensing is accomplished by measuring the voltage drop across the output inductor (DCR) for the highest efficiency or by using an optional sense resistor. Current foldback limits MOSFET heat dissipation during overload conditions. These features, combined with a minimum on-time of just 95ns, make this controller an ideal choice for high step-down ratio applications.

The device is available in two versions; the LTC3890 is the fully featured part with functions including a clock out, clock phase modulation, two separate power good outputs and adjustable current limit. The LTC3890-1 does not have these extra features and is available in a 28-pin SSOP package. The LTC3890 is available is a 32-lead 5mm x 5mm QFN package. Both packaged versions are available in H-grade and MP-grades which have an operating junction temperature from -40°C to 150°C and -55°C to 150°C, respectively.

Burst Mode operation, pulse skipping or forced continuous mode
The LTC3890/-1 can be enabled to enter high efficiency Burst Mode operation, constant frequency pulse skipping, or forced continuous conduction mode at low load currents. When configured for Burst Mode operation and during a light load condition, the converter will burst out a few pulses to maintain the charge voltage on the output capacitor. It then turns off the converter and goes into sleep mode with most of its internal circuits shut down. The output capacitor supplies the load current and when the voltage across the output capacitor drops to a programmed level, the converter starts back up delivering more current to replenish the charge voltage. The action of shutting down and turning off most of its internal circuits significantly reduces quiescent current, thereby helping to extend the battery run-time in an “always-on” system when the system is not running. Figure 2 shows the conceptual timing diagram of how this works.



Figure 2: Burst Mode operation voltage diagram for the LTC3890
Click on image to enlarge

The Burst Mode output ripple is load independent so only the length of the sleep intervals will change. In sleep mode, much of the internal circuitry is turned off except for the critical circuitry needed to respond quickly, further reducing its quiescent current. When the output voltage drops low enough, the sleep signal goes low and the controller resumes normal Burst Mode operation by turning on the top external MOSFET. Alternatively, there are instances when the user will want to operate in forced continuous or constant frequency pulse skipping mode at light load currents. Both of these modes are easily configurable but will have a higher quiescent current and a lower peak to peak output ripple.

In addition, when the controller is enabled for Burst Mode operation, the inductor current is not allowed to reverse. The reverse current comparator, IR, turns off the bottom external MOSFET just before the inductor current reaches zero, preventing it from going negative. Thus, the controller also operates in discontinuous mode when configured for Burst Mode operation.

Furthermore, in forced continuous operation or when clocked by an external clock source, the inductor current is allowed to reverse at light loads or under large transient conditions. Continuous operation has the advantage of lower output voltage ripple and results a higher quiescent current.

Overcurrent protection
Fast accurate overcurrent limit protection is essential in high voltage power supplies. Because of the high voltage across the inductor when the output is shorted, the inductor can saturate quickly causing excessive currents to flow. The LTC3890/-1 has the option of either using a sense resistor in series with the output or using the voltage drop across the output inductor to sense the output current. Either way, the output current is monitored continuously and provides the highest level of protection. Alternative designs might use the RDS(ON) of the top or bottom MOSFET to sense the output current. However, this creates a time frame within the switching cycle where the controller is blind with regards to what the output current is and could cause a failure of the converter.

Strong gate drivers
Switching losses are proportional to the square of the input voltage and these losses can dominate in high input voltage applications with an inadequate gate driver. The LTC3890/-1 has powerful 1.1Ohms on-board N-channel MOSFET gate drivers that minimize transition times and switching losses thereby maximizing the efficiency. In addition, it is capable of driving multiple MOSFET’s in parallel for higher current applications.

Efficiency
The LTC3890 efficiency curves in Figure 3 are representative of the Figure 1circuit schematic with a 12 V input voltage. As shown, the 8.5V output produces a very high efficiency at up to 98 percent. The 3.3V is also over 90 percent efficient. In addition, this design is still over 75 percent efficient for each output with a 1mA load, this due to its Burst Mode operation.


Figure 3: LTC3890 efficiency curves for 8.5V & 3.3V outputs from a 12V input

Fast transient response
The LTC3890 uses a fast 25MHz bandwidth operation amplifier for voltage feedback. The high bandwidth of the amplifier, along with high switching frequencies and low value inductors, allow for a very high gain crossover frequency. This allows the compensation network to be optimized for a very fast load transient response. Figure 4 illustrates the transient response of a 4A step load on a 3.3V output with a less than 100mV deviation from nominal.

Figure 4: LTC3890 transient response curve for a 4A load step


Conclusion

The LTC3890 provides features that make it a suitable choice for harsh military power supply applications. It brings a new level of performance in terms of needing to operate safely and efficiently in a harsh high voltage transient environment and wide temperature ranges. Features that include a 60V input capability make it well suited for military vehicles and heavy equipment applications. Its low quiescent current preserves battery energy during sleep mode allowing for increased battery run-time, a very useful feature in “always-on” bus systems.

Finally, the LTC3890 is also easily applied to a wide variety of output voltages with up to a 24V output voltage. Alternatively, its low minimum on-time enables it to be used in high step-down voltage ratio applications. The ability to directly step-down input voltages from 60V without requiring a bulky transformer, or external protection, makes for a simpler, more reliable and compact solution.

About the author:
Bruce Haug is senior product marketing engineer at Linear Technology Corporation

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