Harsh automotive environments demand high performance power conversion
28 April 2011
Automotive and heavy equipment environments are very harsh for any type of electronics. Bruce Haug looks at a new power conversion device capable of holding its own in such environments
Wide operating voltage requirements coupled with large voltage transients and wide temperature excursions combine to make life tough on electronic systems.
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 rails including 8.5 V, 5 V, 3.3 V, 2.5 V, 1.8 V and 1.5 V.
At the same time, space requirements continue to shrink. Therefore, high efficiency conversion to minimise power dissipation becomes more critical due to the space limitations and high temperature conditions.
Most cars and trucks have an over-voltage clamp circuit that maintains a maximum voltage that is caused by the inductive kick back voltage from the starter motor.
A 12 V system is usually clamped in the 36 V to 40 V range. Double battery 24 V applications found in trucks and heavy equipment are normally clamped to 58 V, so a 60 V rated input voltage switching regulator is usually sufficient for both applications.
There are many automotive and truck systems that require continuous power even when the vehicle’s motor is not running, such as remote keyless entry and alarm 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.
In such circumstances, the regulator runs in normal continuous switching mode until the output current drops below a predetermined threshold of about 30 mA.
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 the battery run-time.
With 60 V 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 60 V.
Others have used an intermediate bus converter requiring an additional power stage. Both of these alternatives increase the design complexity and, in most cases, reduces the overall efficiency.
However, the LTC3890 from Linear Technology is the latest part in a growing family of 60 V input capable step-down switching regulator controllers that addresses many of the key issues required in automotive and truck applications as outlined above.
Figure 1 shows a schematic of the LTC3890 operating in an application that converts a 9 V to 60 V input into 3.5 V/5 A and 8.5 V/3 A outputs.
The LTC3890/-1 is a high voltage dual output synchronous step-down DC/DC controller that draws only 50 uA when one output is active and 60 uA when both outputs are enabled.
With both outputs shut down, the LTC3890/-1 draws only 14 uA.
The 4 V to 60 V 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 0.8 V to 24 V at output currents up to 20 amps, with efficiencies as high as 97% making it well suited for 12 V or 24 V automotive, truck, heavy equipment and industrial control applications.
The LTC3890/-1 operates with a selectable fixed frequency between 50 kHz and 900 kHz, and can be synchronised to an external clock from 75 kHz to 850 kHz with its phased-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.
These features, combined with a minimum on-time of just 95 ns, make this controller a suitable choice for high step-down ratio applications.
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 in a 32-lead 5 mm x 5 mm package.
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, 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.
Overcurrent protection
Fast accurate over current limit protection is essential in a high voltage power supply.
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.
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 1.1 Ohm on-board N-channel MOSFET gate drivers that minimise transition times and switching losses thereby maximising the efficiency. In addition, it is capable of driving multiple MOSFETs in parallel for higher current applications.
Efficiency
The LTC3890 efficiency curves in figure 2 are representative of the figure 1 schematic with a 12 V input voltage.
As shown, the 8.5 V output produces a high efficiency at up to 98 %. The 3.3 V is also over 90% efficient. In addition, this design is still over 75% efficient for each output with a 1 mA load, due to its Burst Mode operation.
Fast Transient Response
The LTC3890 uses a 25 MHz 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 optimised for a very fast load transient response. Figure 3 illustrates the transient response of a 4 A step load on a 3.3 V output with a less than 100 mV deviation from nominal.
Conclusion
The LTC3890/-1 is aimed at bringing safe and efficient operation in a harsh high voltage transient environment. Features that include a 60 V input capability make it well suited for automotive double battery, truck 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.
The ability to directly step-down input voltages from 60 V without requiring a bulky transformer, or external protection, makes for a cost effective and compact solution.
The author is Senior Product Marketing Engineer for Linear Technology Corporation
Contact Details and Archive...
Related Articles...
Most Viewed Articles...