Truck, automotive and heavy equipment environments are very demanding on any type of power conversion device. Wide operating voltage range, large transients and temperature offsets present significant challenges for reliable, rugged electronic system design. In addition, some applications require power conversion devices to be installed in the hood, so these devices are required to operate at 150 °C. At the same time, the number of electronic components is increasing and the available space is shrinking, so the ability to pass high and high input surge voltages becomes even more important.
Whether it's load dump, cold start or high temperature under the hood, the car's built-in power supply must be designed to operate reliably under all conditions. Under normal steady state, the 12V battery system varies only from approximately 9V to 18V, while the 24V system varies from approximately 21V to 36V. However, under load dump transient conditions, a voltage of more than 120V may be generated and this voltage may persist for hundreds of milliseconds. When the alternator charges the car battery and the electrical circuit opens, causing the battery to suddenly disconnect from the alternator, a load dump occurs, which is a very common phenomenon. Before the voltage regulator can respond, the entire charging current of the alternator goes directly to the automotive power bus, raising the bus voltage to a potentially dangerous level. Physical disconnection can cause such transients, which can be caused by a faulty battery cable connection or corrosion of the battery terminals.
Other physical factors in vehicle design are also worthy of attention. In particular, there are long power lines in the car that supply power from the distribution box in the engine compartment to various remote corners of the car. Today, there are about a mile long copper wire in a regular car, and in 1948 the copper wire in the car was only 150 feet long. Due to the inductive nature of the long leads, the transient voltage caused by the power line is even higher than the transient generated when the load is dumped. The tailgate electronics safety management performance specification is required to withstand +100V transients. This can be a challenge for IC electronics, such as regulators for LED taillights.
In addition, there are several electronic systems that require continuous power supply, even when the vehicle's motor is not running, such as remote keyless entry, GPS, and vehicle safety systems. For this type of "always on" system, an essential requirement is that the dc current of the DC/DC converter used is very small to maximize battery run time in sleep mode. In such cases, the regulator typically operates in continuous switching mode until the output current drops below a pre-set threshold of approximately 30mA to 50mA. Below this current value, the switching regulator must operate at a lower quiescent current to reduce the current drawn, which in turn reduces battery power consumption, which in turn can extend battery run time.
Critical systems must be protected from damage and must operate seamlessly and uninterrupted when such transients occur. Until now, most vehicles have used a passive protection network consisting of a low-pass LC filter and a transient voltage suppression array to clamp the peak voltage offset of the power bus. However, a recently introduced high input voltage DC/DC step-down controller does not require additional surge suppression devices to operate properly in the event of such high voltage surges and to protect downstream components.
A new IC solutionThe LTC3895 is a non-isolated synchronous buck switching regulator controller that drives the full N-channel MOSFET supply stage. Its 4V to 140V (absolute maximum 150V) input voltage range allows operation with high voltage input supplies or input power supplies with high voltage surges, eliminating the need for external surge suppression devices. The LTC3895 operates continuously at up to 100% duty cycle when the input voltage drops to 4V, making it ideal for automotive and heavy equipment applications.
The device's output voltage can be set from 0.8V to 60V, output current up to 20A, and efficiency up to 96%. The LTC3895 draws only 40μA in sleep mode and the output voltage is stable, making it ideal for always turning the system on. The internal charge pump allows operation at 100% duty cycle with differential pressure, which is a very useful feature for surge suppression applications and battery powered and discharge. The LTC3895's powerful 1? N-channel MOSFET gate driver can be adjusted from 5V to 10V, allowing the use of logic levels or standard-level MOSFETs to maximize efficiency. To prevent large internal power dissipation in high input voltage applications, the LTC3895 provides an NDRV pin that drives the gate of an optional external N-channel MOSFET to power the IC as a low dropout linear regulator. The EXTVCC pin allows the LTC3895 to be powered from the output of the switching regulator or other available power supply, reducing power consumption and increasing efficiency.
The LTC3895 operates over an optional fixed frequency range of 50kHz to 900kHz and can be synchronized to an external clock from 75kHz to 850kHz. At light loads, the user can select forced continuous mode, pulse skip mode, or low ripple Burst Mode® operation. Its current mode architecture provides very easy loop compensation, fast transient response and excellent voltage stability. Current sensing is accomplished by measuring the voltage drop across the output inductor (DCR) for maximum efficiency, or by using an optional sense resistor. A very short 80ns minimum on-time allows for a high step-down ratio at high switching frequencies. In the event of an overload, current foldback limits the amount of heat generated by the MOSFET. Other features include fixed 5V or 3.3V output options, integrated bootstrap diodes, power good output signals, adjustable input overvoltage lockout, and soft start. The LTC3895 is packaged in a TSSOP-38 thermally enhanced package with several pins removed for high voltage separation. The device is available in two operating junction temperature versions, extended and industrial temperature grade versions from –40°C to 125°C, and high temperature automotive grade versions operating from –40°C to 150°C.
The schematic shown in Figure 1 produces a 12V output over the 7V to 140V input range. When the input voltage is below 12V, the output voltage will follow the input voltage because the device will operate at 100% duty cycle when the top MOSFET is continuously turned on. This feature is available because the LTC3895 has a built-in charge pump.
Figure 1: Schematic showing the LTC3895 producing a 12V output over a 7V to 140V input voltage range
VOUT FOLLOWS VIN WHEN VIN < 12V: When VIN < 12V, VOUT follows VIN
Burst mode operationWhen the load current is small, the LTC3895 can enter high efficiency burst mode, constant frequency pulse skip mode or forced continuous conduction mode when it starts up. When configured for burst mode operation and under light load conditions, the converter will generate several pulses in a burst to maintain the charging voltage on the output capacitor, then disconnect the converter and enter sleep mode, then its internal circuitry Most of them are down. The output capacitor provides the load current. When the voltage across the output capacitor drops to a set value, the converter starts up again, providing more current to increase the charging voltage. Most internal circuits are shut down and shut down, which can significantly reduce quiescent current, thus helping to extend battery run time when the "always on" system is in standby mode.
Switch type surge suppressorIn addition to being a high voltage step-down DC/DC controller that can traverse high voltage surges, the LTC3895 can also be designed to operate only as a high efficiency switching surge suppressor. For example, when the input voltage is from a 12V lead-acid battery in a car, the output voltage can be set to 12V. In this configuration, the normal operation is in a "dropout" state and the top MOSFET is continuously turned on. The LTC3895 will then only switch when it is started or in response to an input overvoltage or output short circuit condition. If the switching time exceeds the time set by the OVLO pin, the LTC3895 will shut down to protect itself from overheating. The switching time can be set to a few milliseconds up to a few seconds before stopping.
MOSFET driver and efficiencyThe LTC3895 has a powerful 1.1? built-in N-channel MOSFET gate driver to minimize conversion time and switching losses. The gate drive voltage can be set from 5V to 10V, allowing the use of logic level or standard level N-channel MOSFETs to maximize efficiency. Multiple parallel MOSFETs can be driven in larger current applications due to the large drive current available.
Figure 2 is a typical efficiency curve for the LTC3895 schematic shown in Figure 1 at 24V or 48V input voltage. As shown, the 8.5V output produces a very high 98% efficiency. The efficiency is also over 90% at 3.3V. In addition, this design operates in burst mode, so the efficiency is still over 75% when each output has a 1mA load.
Figure 2: Efficiency curve of the LTC3895 at 24V or 48V input, 12V output
EFFICIENCY: Efficiency
POWER LOSS: Power consumption
LOAD CURRENT: load current
Fast transient responseThe LTC3895 uses a 25MHz fast op amp for output voltage feedback. The amplifier's large bandwidth combined with high switching frequency and low inductance inductors allows for very high crossover frequency gain. This allows the compensation network to be optimized for very fast load transient response. Figure 3 illustrates the transient response of a 2A step load at 12V output, with a deviation from the nominal value of less than 100mV and a recovery time of 200μs.
Figure 3: Transient Response of the LTC3895 at 12V Output, 2A Step Load
100mV/DIV: 100mV per cell
AC COUPLED: AC coupling
in conclusionThe LTC3895 offers a new level of performance for safe and efficient operation in demanding high voltage transient environments such as those found in automotive DC/DC converters. The powerful adjustable gate drive voltage provides the flexibility to drive logic levels or standard level MOSFETs. Its low quiescent current saves battery power in sleep mode, allowing for extended battery run time, a very useful feature in an "always on" system. The 150V absolute maximum input voltage rating, fast transient response, and high temperature version make the LTC3895 an excellent choice for truck, heavy equipment, rail and automotive applications where high voltage transients are a common occurrence.
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