Harsh automotive environments require high performance power conversion

　　Automotive and heavy equipment environments are very harsh for any type of electronics. Wide operating voltage requirements combined with large voltage transients and wide temperature variations combine to create a very tough operating environment for electronic systems. To further complicate considerations, the number of voltage rails in electronic systems is also increasing. For example, a typical navigation system may have 6 or more different power supplies , including 8.5V, 5V, 3.3V, 2.5V, 1.8V, and 1.5V. At the same time, as the number of components increases, the available space continues to shrink. Therefore, high efficiency conversion to minimize power consumption due to space constraints and high temperature conditions becomes more important.

As a result, a good switching regulator 　　for cars and trucks needs to be specified to operate over a wide input voltage range of 4V to 60V. A 60V rating provides good margin for 12V systems that are typically clamped in the 36V to 40V range. Furthermore, dual battery applications found in trucks and heavy equipment require even higher operating voltages due to their 24V nominal battery voltages. Most of these applications are clamped to a maximum operating voltage of 58V, so a 60V rating is usually sufficient. Overvoltage clamping is required on cars and trucks to limit the maximum transient voltage caused by the inductive flyback voltage of the engine starter.

　　There are many car and truck systems that require continuous power even when the vehicle engine is not running, such as remote keyless entry systems and alarm systems. For these "always-on" systems, it is very important to have a DC/DC converter with low quiescent current to maximize battery run time when in sleep mode. In these environments, the regulator operates in the usual continuous switching mode until the output current drops below a preset threshold of around 30mA to 50mA. Below this value, the switching regulator must enter Burst Mode (Burst Mode?) operation to reduce the quiescent current to the tens of uA range, thereby reducing the power drawn from the battery to extend battery run time.

　　Due to the shortage of 60V input DC/DC converters, some designers have turned to transformer -based topologies or external high-side drivers to operate at voltages up to 60V. Others have resorted to intermediate bus converters that require additional power stages. Both of these alternative solutions increase design complexity and, in most cases, reduce overall efficiency. However, Linear Technology offers the LTC3890. This is the latest device in a growing family of 60V input step-down switching regulator controllers that address many of the key issues encountered in the above-mentioned automotive and truck applications. Figure 1 shows the operating schematic of the LTC3890 in an application that converts a 9V to 60V input into 3.5V/5A and 8.5V/3A outputs.

　　Figure 1: The LTC3890 converts a 9V to 60V input to 8.5V/3A and 3.3V/5A outputs

　　The LTC3890/-1 is a high voltage dual output synchronous step-down DC/DC controller that consumes only 50uA when one output is running and 60uA when both outputs are enabled. When both outputs are off, the LTC3890/-1 consumes only 14uA. Its 4V to 60V input supply range is used to protect the device from high voltage transients and to operate continuously during cold cranking of cars, heavy equipment and trucks and covering a variety of input power supplies and battery chemistries. At output currents up to 20A, each output can be set from 0.8V to 24V, and the efficiency is up to 98%, making the device ideal for 12V or 24V cars, trucks, heavy equipment and industrial control applications.

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

　　The device is available in two versions: The LTC3890 is a full-featured device with features including clock output, clock phase modulation, two separate power good outputs, and adjustable current limit. The LTC3890-1 does not have these additional features and is available in a 28-pin SSOP package. The LTC3890 is available in a 32-lead 5mm x 5mm QFN package.

　　Burst Mode Operation, Pulse Skipping or Forced Continuous Mode

　　The LTC3890/-1 can start up in 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 at light loads, the converter generates several bursts of pulses to keep the charge voltage on the output capacitor constant. The device then shuts down the converter and enters a sleep mode where most of the internal circuitry is off. The output capacitor supplies the load current, and when the voltage across the output capacitor drops to the set value, the converter starts to supply more current to replenish the charge voltage. Shutting down most of the internal circuitry significantly reduces the quiescent current.

　　In addition, when the controller starts up in Burst Mode operation, the inductor current is not allowed to reverse. Just before the inductor current reaches zero, the reverse current comparator IR turns off the bottom external MOSFET to prevent it from going negative. Therefore, when configured in Burst Mode, the controller also operates in Discontinuous Mode.

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

　　Overcurrent protection

　　In high voltage power supplies, fast and accurate current limiting protection is important. Because the voltage across the inductor is high when the output is shorted, the inductor may saturate quickly, causing excessive current to flow. The LTC3890/-1 offers the following options: using a sense resistor in series with the output to sense the output current; or using the voltage drop across the output inductor to sense the output current. In either case, the output current is continuously monitored and provides the highest level of protection. Some alternative designs may use the RDS(ON) of the top or bottom MOSFET to sense the output current. However, this results in a period of time during the switching cycle when the controller "cannot see" how much the output current is, and may cause converter failure.

　　Powerful gate driver

　　Switching losses are proportional to the square of the input voltage, and these losses can be most prominent in high input voltage applications where the gate driver is not good enough. The LTC3890/-1 has a powerful 1.1Ω built-in N-channel MOSFET gate driver that minimizes transition time and switching losses, thereby maximizing efficiency. In addition, it can drive multiple MOSFETs in parallel for higher current applications.

　　efficiency

　　The LTC3890 efficiency curves shown in Figure 2 are examples of the Figure 1 schematic with a 12V input voltage. As shown, the 8.5V output produces very high efficiencies of up to 98%. The efficiency is also over 90% at 3.3V. Furthermore, this design still has an efficiency of over 75% with a 1mA load on each output due to Burst Mode operation.

　　Figure 2: LTC3890 12V input to 8.5V and 3.3V output efficiency curves

　　Efficiency vs Load Current: Efficiency and load current

　　Fast transient response

　　The LTC3890 implements voltage feedback using an amplifier operating with a fast 25MHz bandwidth. The high bandwidth amplifier combined with a high switching frequency and low value inductor allows a very high gain crossover frequency. This allows the compensation network to be optimized for very fast load transient response. Figure 3 illustrates the transient response of a 4A step load on a 3.3V output with less than 100mV deviation from nominal.

　　Figure 3: Transient response of the LTC3890 to a 4A load step

　　Load Step: Load step

　　Forced Continuous Mode: Forced Continuous Mode

　　2A/DIV: 2A per grid

　　in conclusion

　　The LTC3890 offers features that make it ideal for high input voltage power supplies. It provides a higher level of performance when it needs to operate safely and efficiently in harsh high voltage transient environments. Features including 60V input capability make it ideal for automotive dual battery, truck and heavy equipment applications. Its low quiescent current saves battery energy in sleep mode, allowing longer battery run time, which is a very useful feature in "always on" bus systems.

　　In addition, with an output voltage up to 24V, the LTC3890 can also be easily used to generate multiple output voltages. Alternatively, its small minimum on-time enables the LTC3890 to be used in high step-down ratio applications. Directly stepping down the input voltage from 60V without the need for bulky transformers or external protection capabilities makes it possible to form an economical and compact solution.