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LTC1430AC Datasheet(PDF) 8 Page - Linear Technology |
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LTC1430AC Datasheet(HTML) 8 Page - Linear Technology |
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8 / 24 page  8 LTC1430A APPLICATIONS INFORMATION OVERVIEW The LTC1430A is a voltage feedback PWM switching regulator controller (see Block Diagram) designed for use in high power, low voltage step-down (buck) converters. It includes an onboard PWM generator, a precision refer- ence trimmed to ±0.5%, two high power MOSFET gate drivers and all necessary feedback and control circuitry to form a complete switching regulator circuit. The PWM loop nominally runs at 200kHz. The 16-lead versions of the LTC1430A include a current limit sensing circuit that uses the upper external power MOSFET as a current sensing element, eliminating the need for an external sense resistor. Also included in the 16-lead version is an internal soft start feature that requires only a single external capacitor to operate. In addition, 16-lead parts feature an adjustable oscillator which can run at frequencies from 50kHz to 500kHz, allowing added flexibility in external component selection. The 8-lead version does not include current limit, internal soft start or frequency adjustability. THEORY OF OPERATION Primary Feedback Loop The LTC1430A senses the output voltage of the circuit at the output capacitor with the SENSE + and SENSE – pins and feeds this voltage back to the internal transconduc- tance amplifier FB. FB compares the resistor-divided out- put voltage to the internal 1.265V reference and outputs an error signal to the PWM comparator. This is then com- pared to a fixed frequency sawtooth waveform generated by the internal oscillator to generate a pulse width modu- lated signal. This PWM signal is fed back to the external MOSFETs through G1 and G2, closing the loop. Loop compensation is achieved with an external compensation network at COMP, the output node of the FB transconduc- tance amplifier. MIN, MAX Feedback Loops Two additional comparators in the feedback loop provide high speed fault correction in situations where the FB amplifier may not respond quickly enough. MIN compares the feedback signal to a voltage 40mV (3%) below the internal reference. At this point, the MIN comparator overrides the FB amplifier and forces the loop to full duty cycle, set by the internal oscillator at about 93.5%. Simi- larly, the MAX comparator monitors the output voltage at 3% above the internal reference and forces the output to 0% duty cycle when tripped. These two comparators prevent extreme output perturbations with fast output transients, while allowing the main feedback loop to be optimally compensated for stability. Current Limit Loop The 16-lead LTC1430A devices include yet another feed- back loop to control operation in current limit. The current limit loop is disabled in the 8-lead device. The ILIM ampli- fier monitors the voltage drop across external MOSFET Q1 with the IFB pin during the portion of the cycle when G1 is high. It compares this voltage to the voltage at the IMAX pin. As the peak current rises, the drop across Q1 due to its RDS(ON) increases. When IFB drops below IMAX, indicating that Q1’s drain current has exceeded the maximum level, ILIM starts to pull current out of the external soft start capacitor, cutting the duty cycle and controlling the output current level. At the same time, the ILIM comparator generates a signal to disable the MIN comparator to prevent it from conflicting with the current limit circuit. If the internal feedback node drops below about 0.8V, indi- cating a severe output overload, the circuitry will force the internal oscillator to slow down by a factor of as much as 100. If desired, the turn on time of the current limit loop can be controlled by adjusting the size of the soft start capacitor, allowing the LTC1430A to withstand short overcurrent conditions without limiting. By using the RDS(ON) of Q1 to measure the output current, the current limit circuit eliminates the sense resistor that would otherwise be required and minimizes the number of components in the external high current path. Because power MOSFET RDS(ON) is not tightly controlled and varies with temperature, the LTC1430A current limit is not de- signed to be accurate; it is meant to prevent damage to the power supply circuitry during fault conditions. The actual current level where the limiting circuit begins to take effect may vary from unit to unit, depending on the power MOSFETs used. See Soft Start and Current Limit for more details on current limit operation. |
Similar Part No. - LTC1430AC |
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Similar Description - LTC1430AC |
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