Datasheet LTC7149 (Analog Devices) - 9

LTC7149 OPERATION The LTC7149 is a current mode monolithic step-down current to be. As a result, when the load is low enough, regulator. The accurate 50μA bias current on the ISET VOUTSNS will rise relative to VISET because the average pin allows the user to program the output voltage in a programmed inductor current is above the load current, unity-gain buffer fashion with just one external resistor, thus driving VITH low. Once the ITH voltage is driven below R – SET, from the ISET pin to VOUT . The output voltage is an internal threshold (~400mV), the switching regulator set such that: will enter its sleep mode and wait for VOUT to drop and V – VITH to rise above the threshold before it starts to switch OUT = –50μA • RSET again. During sleep mode, the quiescent current of the The LTC7149 operates through a wide VIN range, and its part is reduced to 440µA to conserve input power. The frequency can be programmed to a wide range with the RT LTC7149 is designed to operate with single burst pulse resistor. To suit a variety of applications, the MODE/SYNC behavior to minimize output voltage ripple while keeping pin allows the user to trade off output ripple for efficiency. the efficiency high at light loads. Lastly, if at any point the top power MOSFET is on for roughly 8 consecutive
Main Control Loop
cycles, the part will turn on the bottom power MOSFET In normal operation, the internal top power MOSFET is for a brief duration such that the BOOST capacitor can turned on at the beginning of a clock pulse. The inductor be replenished. current is allowed to ramp up to a peak level. Once that level is reached, the top power switch is turned off and
Forced Continuous Mode Operation
the bottom switch is turned on until the next clock cycle. Floating the MODE/SYNC pin defaults the LTC7149 into The peak inductor current is determined by sensing the forced continuous mode operation. In this mode, the part voltage drop across the SW and VIN nodes of the top power switches continuously regardless of load current, and the MOSFET. The voltage on the ITH pin sets the comparator inductor peak current is allowed to decrease to approxi- threshold corresponding to inductor peak current. The mately –1A to allow for significant negative average current. error amplifier, EA, adjusts this ITH voltage by comparing the differential V – OUTSNS voltage to VOUT with the voltage
High Duty Cycle/Dropout Operation
on ISET. In the typical application where the VOUTSNS pin As the input voltage decreases towards the desired output is tied to GND, if the load current into VOUT – increases, it voltage, the duty cycle will increase towards 100%. How- causes an increase in VISET with respect to VOUTSNS. This ever, given the architecture, there are two restrictions that causes the ITH voltage to rise until the current into VOUT – prevent the LTC7149 from operating in full dropout mode. provided by the regulator matches that of the load current. The first restriction is due to how the ISET voltage is
Low Current Operation
programmed. If a resistor is placed between ISET and Burst Mode operation can be selected by connecting the VOUT – to set the output voltage, the 50µA of current out MODE/SYNC pin to GND. In this mode, the LTC7149 will of the ISET pin is only guaranteed to be accurate when automatically transition from continuous mode operation VISET is more than 500mV below VIN. As the input volt- to Burst Mode operation when the load current is low. A age drops below that 500mV threshold, the ISET current reverse current comparator looks at the voltage across SW will decrease, thus limiting the programmed voltage. to GND and turns off the bottom power MOSFET when Typically, VISET will never get within 300mV of VIN. Since that voltage difference approaches zero. This prevents the VISET programs VOUT –, this limitation essentially enforces inductor current from going negative. An internal burst a maximum duty cycle for the switcher. This limitation clamp is set to be approximately 1A, which means that in can be overcome if an accurate external supply is used Burst Mode, the peak inductor current will never go below to drive the ISET pin directly. 1A regardless of what the ITH voltage demands the peak 7149fa For more information www.linear.com/LTC7149 9