Datasheet LTC3541-2 (Analog Devices) - 10

LTC3541-2
OPERATION
The LTC3541-2 contains a high efficiency synchronous
Buck Regulator Control Loop
buck converter, a very low dropout regulator (VLDO), The LTC3541-2 internal buck regulator uses a constant and a linear regulator that can be used to provide up to frequency, current mode, step-down architecture. Both the two output voltages from a single input voltage making main (top, P-channel MOSFET) and synchronous (bottom, the LTC3541-2 ideal for applications with limited board N-channel MOSFET) switches are internal. During normal space. The combination and configuration of these major operation, the internal main switch is turned on at the be- blocks within the LTC3541-2 is determined by way of the ginning of each clock cycle provided the internal feedback control pins ENBUCK and ENVLDO as defined in Table 1. voltage to the buck is less than the reference voltage. The With the ENBUCK pin driven to a logic high and ENVLDO current into the inductor provided to the load increases driven to a logic low, the LTC3541-2 enables the buck until the current limit is reached. Once the current limit is converter to efficiently reduce the voltage provided at reached the main switch turns off and the energy stored the VIN input pin to an output voltage of 1.875V which in the inductor flows through the bottom synchronous is set by an internal feedback resistor network. The buck switch into the load until the next clock cycle. regulator can be configured for Pulse-Skip or Burst Mode The peak inductor current is determined by comparing the operation by driving the MODE pin to a logic high or logic buck feedback signal to an internal 0.8V reference. When low respectively. The buck regulator is capable of provid- the load current increases, the output of the buck and ing a maximum output current of 500mA, which must be hence the buck feedback signal decrease. This decrease taken into consideration when using the buck regulator causes the peak inductor current to increase until the aver- to provide the power for both the VLDO regulator and for age inductor current matches the load current. While the external loads. main switch is off, the synchronous switch is turned on With the ENBUCK pin driven to a logic low and ENVLDO until either the inductor current starts to reverse direction driven to a logic high, the LTC3541-2 enables the linear or the beginning of a new clock cycle. regulator, providing a low noise regulated output voltage When the MODE pin is driven to a logic low, the LTC3541-2 of 1.5V at the LVOUT pin while drawing minimal quiescent buck regulator operates in Burst Mode operation for high current from the VIN input pin. This feature allows output efficiency. In this mode, the main switch operates based voltage LVOUT to be brought into regulation without the upon load demand. In Burst Mode operation the peak presence of the LVIN voltage. inductor current is set to a fixed value, where each burst With the ENBUCK and ENVLDO pins both driven to a event can last from a few clock cycles at light loads to logic high, the LTC3541-2 enables the high efficiency nearly continuous cycling at moderate loads. Between buck converter and VLDO, providing dual output opera- burst events the main switch and any unneeded circuitry tion from a single input voltage. When configured in this are turned off, reducing the quiescent current. In this sleep manner, the LTC3541-2’s auto start-up sequencing feature state, the load is being supplied solely from the output will bring the buck output (1.875V) into regulation in a capacitor. As the output voltage droops, an internal error controlled manner prior to enabling the VLDO regulator amplifier’s output rises until a wake threshold is reached (1.5V) without the need for external pin control. A detailed causing the main switch to again turn on. This process discussion of the transitions between the VLDO regula- repeats at a rate that is dependant upon the load current tor and linear regulator can be found in the VLDO/Linear demand. Regulator Loop section. 35412fc 10 For more information www.linear.com/LTC3541-2 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Operation Applications Information Package Description Revision History Related Parts