Datasheet LTM8071 (Analog Devices) - 14

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APPLICATIONS INFORMATION BIAS Pin Considerations
SHARE pins of all the paralleled LTM8071s together. The BIAS pin is used to provide drive power for the inter- To ensure that paralleled modules start up together, the nal power switching stage and operate other internal cir- TR/SS pins may be tied together, as well. If it is inconvenient cuitry. For proper operation, it must be powered by at to tie the TR/SS pins together, make sure that the same least 3.2V. If the output voltage is programmed to 3.2V valued soft-start capacitors are used for each µModule or higher, BIAS may be simply tied to AUX. If V regulator. An example of two LTM8071s configured for OUT is less than 3.2V, BIAS can be tied to V load sharing is given in the Typical Applications section. IN or some other voltage source. If the BIAS pin voltage is too high, the efficiency For closer load sharing, synchronize the LTM8071s to an of the LTM8071 may suffer. The optimum BIAS voltage is external clock source. When load sharing among n units dependent upon many factors, such as load current, input and using a single RFB resistor, the value of the resistor is: voltage, output voltage and switching frequency. In al cases, ensure that the maximum voltage at the BIAS pin 241.5 RFB = is less than 19V. If BIAS power is applied from a remote n(VOUT – 0.97) or noisy voltage source, it may be necessary to apply a decoupling capacitor locally to the pin. A 1µF ceramic where RFB is in kΩ. capacitor works well. The BIAS pin may also be left open
Burst Mode Operation
at the cost of a small degradation in efficiency. If SYNC is less than about 0.4V, the LTM8071 automati- If the LTM8071 is configured to provide a negative output, cally switches to Burst Mode operation which keeps the do not connect BIAS to VOUT or AUX. Instead, tie to BIAS to output capacitor charged to the proper voltage while mini- the LTM8071 GND, which should be the negative output. mizing the input quiescent current. During Burst Mode
Maximum Load
operation, the LTM8071 delivers single cycle bursts of current to the output capacitor followed by sleep periods The maximum practical continuous load that the LTM8071 where most of the internal circuitry is powered off and can drive, while rated at 5A, actually depends upon both energy is delivered to the load by the output capacitor. the internal current limit and the internal temperature. During the sleep time, VIN and BIAS quiescent currents The internal current limit is designed to prevent damage are greatly reduced, so, as the load current decreases to the LTM8071 in the case of overload or short-circuit. towards a no load condition, the percentage of time that The internal temperature of the LTM8071 depends upon the LTM8071 operates in sleep mode increases and the operating conditions such as the ambient temperature, average input current is greatly reduced, resulting in the power delivered, and the heat sinking capability of higher light load efficiency. the system. For example, if the LTM8071 is configured to regulate at 1.2V, it may continuously deliver more than
Minimum Input Voltage
7A from 12VIN if the ambient temperature is controlled The LTM8071 is a step-down converter, so a minimum to less than 55°C; this is more than the 5A continuous amount of headroom is required to keep the output in rating. Please see the Derating, VOUT = 1.2V curve in the regulation. Keep the input above 3.6V to ensure proper Typical Performance Characteristics section. Similarly, if operation. Voltage transients or ripple valleys that cause the output voltage is 15V and the ambient temperature the input to fall below 3.6V may turn off the LTM8071. is 100°C, the LTM8071 will deliver at most 0.25A from 48VIN, which is less than the 5A continuous rating.
Output Voltage Tracking and Soft-Start Load Sharing
The LTM8071 allows the user to program its output volt- age ramp rate by means of the TR/SS pin. An internal 2μA Two or more LTM8071s may be paralleled to pro- pulls up the TR/SS pin to about 2.4V. Putting an external duce higher currents. To do this, tie the VIN, VOUT and capacitor on TR/SS enables soft starting the output to Rev 0 14 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Package Photos Package Description Typical Application Related Parts