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LTC3774 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3774 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 38 page  LTC3774 10 3774fb For more information www.linear.com/LTC3774 operaTion Main Control Loop The LTC3774 uses an LTC proprietary current sensing, current mode step-down architecture. During normal operation, the top MOSFET is turned on every cycle when the oscillator sets the RS latch, and turned off when the main current comparator, ICMP, resets the RS latch. The peak inductor current at which ICMP resets the RS latch is controlled by the voltage on the ITH pin, which is the outputoftheerroramplifier,EA.Theremotesenseamplifier (diffamp)producesasignalequaltothedifferentialvoltage sensed across the output capacitor divided down by the feedback divider and re-references it to the local IC ground reference.Theerroramplifierreceivesthisfeedbacksignal and compares it to the internal 0.6V reference. When the load current increases, it causes a slight decrease in the VOSNS+ pin voltage relative to the 0.6V reference, which in turn causes the ITH voltage to increase until the inductor’s average current equals the new load current. After the top MOSFET has turned off, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by the reverse current comparator, IREV, or the beginning of the next cycle. The main control loop is shut down by pulling the RUN pin low. Releasing RUN allows an internal 1.0µA current source to pull up the RUN pin. When the RUN pin reaches 1.22V, the main control loop is enabled and the IC is powered up. When the RUN pin is low, all functions are kept in a controlled state. Sensing Signal of Very Low DCR The LTC3774 employs a unique architecture to enhance the signal-to-noise ratio that enables it to operate with a small sense signal of a very low value inductor DCR, 1mΩ or less, to improve power efficiency, and reduce jitter due to the switching noise which could corrupt the signal. The LTC3774 can sense a DCR value as low as 0.2mΩ with careful PCB layout.The LTC3774 comprises two positive sense pins, SNSD+ and SNSA+, to acquire signals and processes them internally to provide the response as with a DCR sense signal that has a 14dB signal-to-noise ratio improvement. In the meantime, the current limit threshold is still a function of the inductor peak current and its DCR value, and can be accurately set from 10mV to 30mV in 5mV steps with the ILIM pin. The filter time constant, R1C1, of the SNSD+ should match the L/DCR of the output inductor, while the filter at SNSA+ should have a bandwidth of five times larger than SNSD+, R2•C2 equals R1•C1/5. Internal Soft-Start By default, the start-up of the output voltage is normally controlled by an internal soft-start ramp. The internal soft- start ramp represents a noninverting input to the error amplifier. The VOSNS+ pin is regulated to the lower of the error amplifier’s three noninverting inputs (the internal soft-start ramp, the TK/SS pin or the internal 600mV ref- erence). As the ramp voltage rises from 0V to 0.6V over approximately 600µs, the output voltage rises smoothly from its prebiased value to its final set value. Certain applications can result in the start-up of the con- verter into a non-zero load voltage, where residual charge is stored on the output capacitor at the onset of converter switching. In order to prevent the output from discharging under these conditions, the bottom MOSFET is disabled until soft-start is greater than VOSNS+. Shutdown and Start-Up (RUN and TK/SS Pins) The LTC3774 can be shut down using the RUN pin. Pulling the RUN pin below 1.14V shuts down the main control loop for the controller and most internal circuits, including the INTVCC regulator. Releasing the RUN pin allows an internal 1.0µA current to pull up the pin and enable the controller. Alternatively, the RUN pin may be externally pulled up or driven directly by logic. Be careful not to exceed the absolute maximum rating of 6V on this pin. The start-up of the controller’s output voltage, VOUT, is controlled by the voltage on the TK/SS pin. When the voltage on the TK/SS pin is less than the 0.6V internal reference, the LTC3774 regulates the VOSNS+ voltage to the TK/SS pin voltage instead of the 0.6V reference. This allows the TK/SS pin to be used to program a soft-start by connecting an external capacitor from the TK/SS pin to GND. An internal 1.25µA pull-up current charges this capacitor, creating a voltage ramp on the TK/SS pin. As the TK/SS voltage rises linearly from 0V to 0.6V (and beyond), the output voltage, VOUT, rises smoothly from zero to its final value. Alternatively, the TK/SS pin can be |
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