Datasheet LTC3541 (Analog Devices) - 10

LTC3541
OPERATION
need for external pin control. A detailed discussion of the When the MODE pin is driven to a logic high the LTC3541 transitions between the VLDO and linear regulator can be operates in Pulse-Skip mode for low output voltage rip- found in the VLDO/Linear Regulator Loop section. ple. In this mode, the LTC3541 continues to switch at a constant frequency down to very low currents, where it
Buck Regulator Control Loop
will begin skipping pulses used to control the main (top) The LTC3541 internal buck regulator uses a constant fre- switch to maintain the proper average inductor current. quency, current mode, step-down architecture. Both the If the input supply voltage is decreased to a value ap- main (top, P-channel MOSFET) and synchronous (bottom, proaching the output voltage, the duty cycle of the buck N-channel MOSFET) switches are internal. During normal is increased toward maximum on-time and 100% duty operation, the internal main switch is turned on at the be- cycle. The output voltage will then be determined by the ginning of each clock cycle provided the internal feedback input voltage minus the voltage drop across the main voltage to the buck is less than the reference voltage. The switch and the inductor. current into the inductor provided to the load increases until the current limit is reached. Once the current limit is
VLDO/Linear Regulator Loop
reached the main switch turns off and the energy stored in the inductor flows through the bottom synchronous In the LTC3541, the VLDO and linear regulator loops consist switch into the load until the next clock cycle. of an amplifier and N-channel MOSFET output stages that, when connected with the proper external components, will The peak inductor current is determined by comparing the servo the output to maintain a regulator output voltage, buck feedback signal to an internal 0.8V reference. When LVOUT. The internal reference voltage provided to the the load current increases, the output of the buck and hence amplifier is 0.4V allowing for a wide range of output volt- the buck feedback signal decrease. This decrease causes ages. Loop configurations enabling the VLDO or the linear the peak inductor current to increase until the average regulator are stable with an output capacitance as low as inductor current matches the load current. While the main 2.2μF and as high as 100μF. Both the VLDO and the linear switch is off, the synchronous switch is turned on until regulators are capable of operating with an input voltage, either the inductor current starts to reverse direction or VIN, as low as 2.7V, but are subject to the constraint that the beginning of a new clock cycle. VIN must be greater than LVOUT + 1.4V. When the MODE pin is driven to a logic low, the LTC3541 The VLDO is designed to provide up to 300mA of output buck regulator operates in Burst Mode operation for high current at a very low LVIN to LVOUT voltage. This allows efficiency. In this mode, the main switch operates based a clean, secondary, analog supply voltage to be provided upon load demand. In Burst Mode operation the peak with a minimum drop in efficiency. The VLDO is provided inductor current is set to a fixed value, where each burst with thermal protection that is designed to disable the event can last from a few clock cycles at light loads to VLDO function when the output, pass transistor’s junction nearly continuous cycling at moderate loads. Between temperature reaches approximately 160°C. In addition to burst events the main switch and any unneeded circuitry thermal protection, short-circuit detection is provided to are turned off, reducing the quiescent current. In this sleep disable the VLDO function when a short-circuit condition is state, the load is being supplied solely from the output sensed. This circuit is designed such that an output current capacitor. As the output voltage droops, an internal error of approximately 1A can be provided before this circuit amplifier’s output rises until a wake threshold is reached will trigger. As detailed in the Electrical Characteristics, the causing the main switch to again turn on. This process VLDO regulator will be out of regulation when this event repeats at a rate that is dependent upon the load current occurs. Both the thermal and short-circuit faults when demand. detected are treated as catastrophic fault conditions. The 3541fb 10 For more information www.linear.com/LTC3541 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 BUCK Regulator VLDO/Linear regulator Applications Information Efficiency Considerations Thermal Considerations Applications Information Pc Board Layout Checklist Design Example Typical Applications Package Description Revision History Related Parts