Robust Power Conversion, a Must Have for Truck, Automotive & Heavy Equipment

Robust Power Conversion, a Must Have for Truck, Automotive & Heavy Equipment

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By eeNews Europe

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, GPS 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 maximize the battery run time when in sleep mode. Under such circumstances, the regulator runs in normal continuous switching mode until the output current drops below a predetermined threshold of around 30-50mA. Below this level, the switching regulator must go into lower quiescent current operation in order to reduce the current draw to tens of micro amps, thereby lowering the power drawn from the battery which in turn extends the battery run time.

With 60V input capable DC/DC converters that have high step-down ratios in short supply, designers have resorted to a transformer-based topology or external high side drivers to operate up to 60V. Others have used an intermediate bus converter, requiring an additional power stage. Both of these alternatives increase the design complexity and, in most cases, reduce the overall efficiency. However, the LTC3892 from Linear Technology, is the latest part in 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.

Figure 1. LTC3892 Schematic with 4.5V to 60V Input to 8.5V/3A & 3.3V/5A Outputs. For higher resolution click here.

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%, making it well suited for 12V or 24V automotive, truck and heavy equipment applications.

The LTC3892/-1 operates with a selectable fixed frequency between 50kHz and 900kHz, and can be synchronized 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 architecture provides easy loop compensation, fast transient response, fixed frequency operation, excellent line regulation and easy current sharing with paralleled phases for higher current. 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 for high accuracy.

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 is available in a 5mm X 5mm QFN-32 and the LTC3892-1 is available in a TSSOP-28 package. Four temperature grades are available, with operation from –40 to 125°C for the extended and industrial grades, a high temp automotive range of –40°C to 150°C and a military grade guaranteed from –55°C to 150°C.

Burst Mode Operation

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 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 LTC3892/-1

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.

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 but results in a higher quiescent current.


Each channel has an overvoltage comparator that guards against transient overshoots as well as other more serious conditions that may overvoltage the output. When the VFB1, 2 pin rises by more than 10% above its regulation point of 0.800V, the top MOSFET is turned off and the bottom MOSFET is turned on until the overvoltage condition is cleared. Fast accurate over current limit protection is essential in a high voltage power supply. When the output voltage falls to less than 70% of its nominal level, foldback current limiting is activated, progressively lowering the peak current limit in proportion to the severity of the overcurrent or short-circuit condition.

MOSFET Drivers & Efficiency

The LTC3892/-1 has powerful 1.1Ω onboard N-channel MOSFET gate drivers that minimize 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 maximize efficiency. Due to the high drive current available, multiple MOSFET’s in parallel can be driven for higher current applications.

The LTC3892 efficiency curves in Figure 3 are representative of the Figure 1 schematic with a 12 V 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.

Figure 3. LTC3892 Efficiency Curves for 8.5V & 3.3V Outputs from a 12V Input

Fast Transient Response

The LTC3892 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. LTC3892 Transient Response Curve for the 3.3VOUT/12VIN shown in Figure 1


The LTC3892/-1 brings a new level of performance in terms of needing to operate safely and efficiently in a demanding high voltage transient environment. The powerful adjustable gate drive voltage provides the flexibility of driving logic- or standard-level MOSFETS. 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. The 60 voltage maximum input voltage, fast transient and high temp grades make the
LTC3892/-1 an excellent choice for truck, heavy equipment and automotive applications. As a result, system designers now have an optimum choice for their DC/DC conversion needs.

About the author:
Bruce Haug is Senior Product Marketing Engineer at Linear Technology Corporation.

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