Datasheet LTC3649 (Analog Devices) - 9

LTC3649
OPERATION Low Current Operation
set the output voltage, the 50µA of current out of the ISET Burst Mode operation can be selected by connecting the pin is only guaranteed to be accurate when VISET is more MODE/SYNC pin to GND. In this mode, the LTC3649 will than 500mV below VIN. As the input voltage drops below automatically transition from continuous mode operation that 500mV threshold, the ISET current will decrease, thus to Burst Mode operation when the load current is low. A limiting the programmed voltage. Typically, VISET will never reverse current comparator looks at the voltage across SW get within 300mV of VIN. Since VISET programs VOUT, this to GND and turns off the bottom power MOSFET when limitation essentially enforces a maximum duty cycle for that voltage difference approaches zero. This prevents the the switcher. This limitation can be overcome if an accu- rate external supply is used to drive the ISET pin directly. inductor current from going negative. An internal burst clamp is set to be approximately 1A, which means that in The second limitation against full dropout operation Burst Mode operation, the peak inductor current will never is the requirement for the BOOST to SW capacitor to go below 1A regardless of what the ITH voltage demands refresh. When the top power MOSFET is on for multiple the peak current to be. As a result, when the load is low clock cycles during dropout operation, the BOOST to SW enough, VOUT will rise relative to VISET because the average capacitor slowly gets depleted by the internal circuitry of programmed inductor current is above the load current, the chip. When the bottom switch does not turn on for at thus driving V least 80ns for 8 periods, it is forced to turn on in order to ITH low. Once the ITH voltage is driven below an internal threshold (~400mV), the switching regulator guarantee sufficient voltage on the bootstrap capacitor. will enter its sleep mode and wait for V During a refresh, the bottom switch will only turn on for OUT to drop and V roughly 30% of the period to limit inductor ripple, thus ITH to rise above the threshold before it starts to switch again. During sleep mode, the quiescent current of the limiting output voltage ripple. part is reduced to less than 400µA to conserve input power. The LTC3649 is designed to operate with single
Output Current Monitoring and Regulation
burst pulse behavior to minimize output voltage ripple The LTC3649 has the ability to accurately sense the aver- while keeping the efficiency high at light loads. Lastly, if age inductor current without the use of an external sense at any point the top power MOSFET is on for roughly 8 resistor. The IMON pin output current is 1/40000th scale consecutive clock cycles, the part will turn on the bottom of the inductor current. Placing a resistor from IMON to power MOSFET for a brief duration such that the BOOST GND allows the voltage on that node to be equal to: capacitor can be replenished. V
Forced Continuous Mode Operation
IMON = RIMON • IL 40000 Floating the MODE/SYNC pin defaults the LTC3649 into Since the IMON current mirrors the inductor current, it forced continuous mode operation. In this mode, the is necessary to place a capacitor from IMON to GND to part switches continuously regardless of load current, filter the voltage on the node. The choice of this capacitor and the inductor peak current is allowed to decrease to is discussed below. approximately –1A to allow for negative average current. In addition to simply sensing the inductor current, the
High Duty Cycle/Dropout Operation
LTC3649 can also be programmed to regulate the aver- age output current limit. The regulator will limit the peak As the input voltage decreases towards the desired output inductor current if it senses that the voltage on IMON voltage, the duty cycle will increase towards 100%. How- has exceeded 2V. As a result, the programmed average ever, given the architecture, there are two restrictions that inductor current depends on the size of R prevent the LTC3649 from operating in full dropout mode. IMON such that: The first restriction is due to how the ISET voltage is pro- ILAVG = 2V • 40000 grammed. If a resistor is placed between ISET and GND to RIMON 3649fb For more information www.linear.com/LTC3649 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts