Fast accurate overcurrent limit protection is essential in high voltage power supplies. 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. Alternative designs might use the RDS(ON) of the top or bottom MOSFET to sense the output current. However, this creates a time frame within the switching cycle where the controller is blind with regards to what the output current is and could cause a failure of the converter.
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 powerful 1.1Ohms on-board N-channel MOSFET gate drivers that minimize transition times and switching losses thereby maximizing the efficiency. In addition, it is capable of driving multiple MOSFET’s in parallel for higher current applications.
The LTC3890 efficiency curves in Figure 3 are representative of the Figure 1circuit schematic with a 12 V input voltage. As shown, the 8.5V output produces a very high efficiency at up to 98 percent. The 3.3V is also over 90 percent efficient. In addition, this design is still over 75 percent efficient for each output with a 1mA load, this due to its Burst Mode operation.
Figure 3: LTC3890 efficiency curves for 8.5V & 3.3V outputs from a 12V input
Fast transient response
The LTC3890 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: LTC3890 transient response curve for a 4A load step
The LTC3890 provides features that make it a suitable choice for harsh military power supply applications. It brings a new level of performance in terms of needing to operate safely and efficiently in a harsh high voltage transient environment and wide temperature ranges. Features that include a 60V input capability make it well suited for military vehicles 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.
Finally, the LTC3890 is also easily applied to a wide variety of output voltages with up to a 24V output voltage. Alternatively, its low minimum on-time enables it to be used in high step-down voltage ratio applications. The ability to directly step-down input voltages from 60V without requiring a bulky transformer, or external protection, makes for a simpler, more reliable and compact solution.
About the author:
Bruce Haug is senior product marketing engineer at Linear Technology Corporation
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