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LTC1475 Datasheet(PDF) 6 Page - Linear Technology |
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LTC1475 Datasheet(HTML) 6 Page - Linear Technology |
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6 / 20 page  6 LTC1474/LTC1475 OPERATIO (Refer to Functional Diagram) The LTC1474/LTC1475 are step-down converters with internal power switches that use Burst Mode operation to keep the output capacitor charged to the proper output voltage while minimizing the quiescent current. Burst Mode operation functions by using short “burst” cycles to ramp the inductor current through the internal power switch and external Schottky diode, followed by a sleep cycle where the power switch is off and the load current is supplied by the output capacitor. During sleep mode, the LTC1474/LTC1475 draw only 9 µA typical supply current. At light loads, the burst cycles are a small percentage of the total cycle time; thus the average supply current is very low, greatly enhancing efficiency. Burst Mode Operation At the beginning of the burst cycle, the switch is turned on and the inductor current ramps up. At this time, the internal current comparator is also turned on to monitor the switch current by measuring the voltage across the internal and optional external current sense resistors. When this volt- age reaches 100mV, the current comparator trips and pulses the 1-shot timer to start a 4.75 µs off-time during which the switch is turned off and the inductor current ramps down. At the end of the off-time, if the output voltage is less than the voltage comparator threshold, the switch is turned back on and another cycle commences. To minimize supply current, the current comparator is turned on only during the switch-on period when it is needed to monitor switch current. Likewise, the 1-shot timer will only be on during the 4.75 µs off-time period. The average inductor current during a burst cycle will normally be greater than the load current, and thus the output voltage will slowly increase until the internal volt- age comparator trips. At this time, the LTC1474/LTC1475 go into sleep mode, during which the power switch is off and only the minimum required circuitry is left on: the voltage comparator, reference and low battery compara- tor. During sleep mode, with the output capacitor supply- ing the load current, the VFB voltage will slowly decrease until it reaches the lower threshold of the voltage com- parator (about 5mV below the upper threshold). The voltage comparator then trips again, signaling the LTC1474/ LTC1475 to turn on the circuitry necessary to begin a new burst cycle. Peak Inductor Current Programming The current comparator provides a means for program- ming the maximum inductor/switch current for each switch cycle. The 1X sense MOSFET, a portion of the main power MOSFET, is used to divert a sample (about 5%) of the switch current through the internal 5 Ω sense resistor. The current comparator monitors the voltage drop across the series combination of the internal and external sense resistors and trips when the voltage exceeds 100mV. If the external sense resistor is not used (Pins 6 and 7 shorted), the current threshold defaults to the 400mA maximum due to the internal sense resistor. Off-Time The off-time duration is 4.75 µs when the feedback voltage is close to the reference; however, as the feedback voltage drops, the off-time lengthens and reaches a maximum value of about 65 µswhenthisvoltageiszero.Thisensures that the inductor current has enough time to decay when the reverse voltage across the inductor is low such as during short circuit. Shutdown Mode Both LTC1474 and LTC1475 provide a shutdown mode that turns off the power switch and all circuitry except for the low battery comparator and 1.23V reference, further reducing DC supply current to 6 µA typical. The LTC1474’s run/shutdown mode is controlled by a voltage level at the RUN pin (ground = shutdown, open/high = run). The LTC1475’s run/shutdown mode, on the other hand, is controlled by an internal S/R flip-flop to provide pushbutton on/off control. The flip-flop is set (run mode) by a momen- tary ground at the ON pin and reset (shutdown mode) by a momentary ground at the LBI/OFF pin. Low Battery Comparator The low battery comparator compares the voltage on the LBI pin to the internal reference and has an open drain N-channel MOSFET at its output. If LBI is above the reference, the output FET is off and the LBO output is high impedance. If LBI is below the reference, the output FET is on and sinks current. The comparator is still active in shutdown. |
Similar Part No. - LTC1475 |
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Similar Description - LTC1475 |
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