Datasheet LTM8028 (Analog Devices) - 9

LTM8028
operaTion
Current generation FPGA and ASIC processors place the LTM8028 only loses 4W, while the traditional linear stringent demands on the power supplies that power the regulator theoretically dissipates over 110W. core, I/O and transceiver channels. Power supplies that The LTM8028 switching buck converter utilizes fixed- power these processors have demanding output voltage frequency, forced continuous current mode control to specifications, especially at low voltages, where they regulate its output voltage. This means that the switching require tight tolerances, small transient response excur- regulator will stay in fixed frequency operation even as the sions, low noise and high bandwidth to achieve the lowest LTM8028 output current falls to zero. The LTM8028 has bit-error rates. This can be accomplished with some high an analog control pin, I performance linear regulators, but this can be inefficient MAX, to set the maximum allow- able current output of the LTM8028. The analog control for high current and step-down ratios. range of IMAX is from 0V to 1.5V. The RUN pin functions The LTM8028 is a 5A high efficiency, UltraFast transient as a precision shutdown pin. When the voltage at the RUN response linear regulator. It integrates a buck regulator with pin is lower than 1.55V, switching is terminated. Below a high performance linear regulator, providing a precisely this threshold, the RUN pin sinks 5.5µA. This current can regulated output voltage digitally programmable from 0.8V be used with a resistor between RUN and VIN to set the to 1.8V. As shown in the Block Diagram, the LTM8028 hysteresis. During start-up, the SS pin is held low until the contains a current mode controller, power switches, part is enabled, after which the capacitor at the soft-start power inductor, linear regulator, and a modest amount pin is charged with an 11μA current source. The switching of capacitance. To achieve high efficiency, the integrated frequency is determined by a resistor at the RT pin. The buck regulator is automatically controlled (Input-Output LTM8028 may also be synchronized to an external clock Control on the Block Diagram) to produce the optimal through the use of the SYNC pin. voltage headroom to balance efficiency, tight regulation The output linear regulator supplies up to 5A of output and transient response at the linear regulator output. current with a typical dropout voltage of 85mV. Its high Figure 1 is a composite graph of the LTM8028’s power bandwidth provides UltraFast transient response using low loss compared to the theoretical power loss of a traditional ESR ceramic output capacitors, saving bulk capacitance, linear regulator. Note that the power loss (left hand Y axis) PCB area and cost. The output voltage for the LTM8028 is plotted on the log scale. For 1.2VOUT at 5A and 24VIN is digitally selectable in 50mV increments over a 0.8V to 1.8V range, and analog margining function allows the user 1000 60 to adjust system output voltage over a continuous ±10% TRADITIONAL LINEAR 55 range. It also features a remote sense pin for accurate TEMPERATURE RISE (°C) REGULATOR POWER LOSS regulation at the load, and a PGOOD circuit that indicates 100 50 whether the output is in or out of regulation or if an internal fault has occurred. 45 TEMPERATURE RISE The LTM8028 is equipped with a thermal shutdown to POWER LOSS (W) 10 40 POWER LOSS protect the device during momentary overload conditions. 35 It is set above the 125°C absolute maximum internal tem- perature rating to avoid interfering with normal specified 0 30 0 10 20 30 40 operation, so internal device temperatures will exceed INPUT VOLTAGE (V) the absolute maximum rating when the overtemperature 8028 F01 protection is active. So, continuous or repeated activation
Figure 1. This Graph Shows the Full Load Power Loss and
of the thermal shutdown may impair device reliability.
Temperature Rise of the LTM8028 over a Range of Input
During thermal shutdown, all switching is terminated and
Voltages. Compare These Numbers to a Traditional Linear Regulator Powering the Same Load an Operating Condition.
the SS pin is driven low.
Note the Log Scale for Power Loss.
8028fb For more information www.linear.com/LTM8028 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Package Photo Revision History Typical Application Related Parts