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LTM8055 Datasheet(PDF) 14 Page - Linear Technology |
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LTM8055 Datasheet(HTML) 14 Page - Linear Technology |
14 / 26 page LTM8055 14 8055fb For more information www.linear.com/LTM8055 Input Current Monitor (IINMON) The IINMON pin produces a voltage equal to approximately 20 times the voltage of IIN – VIN. Since the LTM8055 input current limit engages when IIN – VIN = 50mV, IINMON will be 1V at maximum input current. Output Current Monitor (IOUTMON) The IOUTMON pin produces a voltage proportional to the voltage of VOUT – IOUT. Since the LTM8055 output current limit engages when VOUT – IOUT = 58mV, IOUTMON will be 1.2V at maximum output current. Synchronization TheLTM8055switchingfrequencycanbesynchronizedto an external clock using the SYNC pin. Driving SYNC with a 50% duty cycle waveform is a good choice, otherwise maintainthedutycyclebetweenabout10%and90%.When synchronizing, a valid resistor value (that is, a value that results in a free-running frequency of 100kHz to 800kHz) must be connected from RT to GND. While an RT resistor is required for proper operation, the value of this resistor is independent of the frequency of the externally applied SYNC signal. Be aware, however, that the LTM8055 will switch at the frequency prescribed by the RT value if the SYNC signal terminates, so choose an appropriate resistor value. CLKOUT The CLKOUT signal reflects the internal switching clock of theLTM8055.Itisphaseshiftedbyapproximately180°with respect to the leading edge of the internal clock. If CLKOUT is connected to the SYNC input of another LTM8055, the two devices will switch about 180° out of phase. Input Precaution In applications where the output voltage is deliberately pulled up above the set regulation voltage or the FB pin is abruptlydriventoanewvoltage,theLTM8055mayattempt to regulate the voltage by removing energy from the load for a short period of time after the output is pulled up. Since the LTM8055 is a synchronous switching converter, it delivers this energy to the input. If there is nothing on the LTM8055 input to consume this energy, the input voltage may rise. If the input voltage rises without intervention, it may rise above the absolute maximum rating, damaging the part. Carefully examine the input voltage behavior to see if the application causes it to rise. In many cases, the system load on the LTM8055 input bus will be sufficient to absorb the energy delivered by the μModule regulator. The power required by other devices will consume more than enough to make up for what the LTM8055 delivers. In cases where the LTM8055 is the largest or only power converter, this may not be true and some means may need to be devised to prevent the LTM8055’s input from rising too high. Figure 5a shows a passive crowbar circuit that will dissipate energy during momentaryinputovervoltageconditions.Thebreak-down voltage of the Zener diode is chosen in conjunction with the resistor R to set the circuit’s trip point. The trip point is typically set well above the maximum VIN voltage under normal operating conditions. This circuit does not have a precision threshold, and is subject to both part-to-part and temperature variations, so it is most suitable for ap- plications where the maximum input voltage is much less than the 40VIN absolute maximum. As stated earlier, this type of circuit is best suited for momentary overvoltages. Figure 5a is a crowbar circuit, which attempts to prevent the input voltage from rising above some level by dumping energy to GND through a power device. In some cases, it is possible to simply turn off the LTM8055 when the input voltage exceeds some threshold. An example of this circuit is shown in Figure 5b. When the power source on the output drives VIN above a predetermined threshold, the comparator pulls down on the RUN pin and stops switching in the LTM8055. When this happens, the input capacitance needs to absorb the energy stored within the LTM8055’s internal inductor, resulting in an additional voltage rise. This voltage rise depends upon the input capacitor size and how much current is flowing from the LTM8055 output to input. APPLICATIONS INFORMATION |
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Similar Description - LTM8055 |
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