FEATURE AUTOMOTIVE ELECTRONICS
HEFTY VEHICLES NEED STRONG POWER CONVERSION
Bruce Haug, senior product marketing engineer, Power by Linear Group, Analog Devices Inc. presents a power conversion solution that is specifically designed for heavy load, industrial, automotive applications
T
ruck, automotive and heavy equipment environments are very
demanding for any type of power conversion devices. Wide operating voltage ranges coupled with large transients and wide temperature excursions combine to make reliable electronic system design challenging. To further complicate the design, some applications install the power converter under the hood, requiring a high temperature operation. At the same time, space requirements are continuing to shrink, making high efficiency conversion critical. Consequently, reliable truck switching
DC/DC regulators need to work over a wide input voltage range. A 60V input rating gives good margin for a nominal 12V system, which is usually clamped in the 36V to 40V range. Double battery applications, commonly found in both trucks and heavy equipment environments require an even higher operating voltage due to their nominal 24V battery voltage. Most of these are clamped to 58V, so a 60V rating is usually sufficient. The onboard automotive and truck overvoltage clamp is required to maintain a maximum transient voltage caused by the inductive kick back voltage from the starter motor, which can cause a much higher transient voltage when left unclamped. There are several electronic systems that require continuous power even when the vehicle’s motor is not running, such as remote keyless entry and security systems. It is essential for these types of “always-on” systems to have a DC/DC converter with low quiescent current in order to maximise the battery run-time when in sleep mode. The LTC3892 from the Power by
Linear Group at Analog Devices, is part of a growing family of 60V input- capable step-down switching regulator controllers that addresses many of the key issues required in automotive and truck applications as outlined herein. Figure 1 shows the LTC3892 operating in an application that converts a 4.5V to 60V input into 3.5V/5A and 8.5V/3A outputs.
12 MARCH 2018 | ELECTRONICS
The LTC3892/-1 are high voltage dual output synchronous step-down DC/DC controllers that draw only 29µA when one output is active and 34µA when both outputs are enabled, and are in regulation. The 4.5V to 60V input supply range is designed to protect against high voltage transients, ensuring continuous operation during automotive cold crank, load dump and to accommodate a broad range of input sources and battery chemistries. Each output can be set from 0.8V to 99% of VIN
at output currents over 20
amps with efficiencies as high as 96%. The LTC3892/-1 operates with a
selectable fixed frequency between 50kHz and 900kHz, and can be synchronised to an external clock from 75kHz to 850kHz. The user can select from continuous operation, pulse- skipping and low ripple Burst Mode operation during light loads. The LTC3892/-1’s 2-phase operation reduces input filtering and capacitance requirements. Its current mode
Figure 2:
LTC3892 efficiency curves for 8.5V and 3.3V outputs from a 12V input
Figure 1:
LTC3892 schematic with 4.5V to 60V input to 8.5V/3A and 3.3V/5A outputs
architecture provides easy loop compensation, fast transient response, fixed frequency operation, excellent line regulation and easy current sharing with paralleled phases for higher current. The LTC3892 is the full-featured version and differs from the LTC3892-1 by providing two power good signals, adjustable current limit and fixed 3.3V or 5V output voltage options. The LTC3892/-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 LTC3892/-1 has powerful 1.1Ω onboard N-channel MOSFET gate drivers that minimise transition times and switching losses. The gate drive voltage can be programmed from 5V to 10V to allow the use of logic- or standard-level N-channel MOSFETs to maximise efficiency. Due to the high drive current available, multiple MOSFETs in parallel can be driven for higher current applications. The LTC3892 efficiency curves in Figure 2 are representative of the Figure 1 schematic with a 12V input voltage. As shown, the 8.5V output produces a very high efficiency at up to 98%. The 3.3V is also over 90% efficient. In addition, this design is still over 75% efficient for each output with a 1mA load, this due to its Burst Mode operation.
Analog Devices Ltd.
www.linear.com www.analog.com T: 01628 477 066
/ ELECTRONICS
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