Datasheet LTC3522 (Analog Devices) - 9

LTC3522
OPERATION
The LTC3522 combines a synchronous buck DC/DC Characteristics section of this data sheet. Under dropout converter and a 4-switch buck-boost DC/DC converter and near dropout conditions, Burst Mode operation will in a single 3mm × 3mm QFN package. The buck-boost not be entered. converter utilizes a proprietary switching algorithm which allows its output voltage to be regulated above, below or
Dropout Operation
equal to the input voltage. The buck converter provides a As the input voltage decreases to a value approaching the high effi ciency lower voltage output and supports 100% output regulation voltage, the duty cycle increases toward duty cycle operation to extend battery life. In Burst Mode the maximum on-time. Further reduction of the supply operation, the combined quiescent current for both con- voltage will force the main switch to remain on for more verters is reduced to 25μA. Both converters operate from than one cycle until 100% duty cycle operation is reached the same internal 1MHz oscillator. where the main switch remains on continuously. In this dropout state, the output voltage will be determined by
BUCK CONVERTER OPERATION
the input voltage less the resistive voltage drop across the main switch and series resistance of the inductor.
PWM Mode Operation Slope Compensation
When the PWM pin is held high, the LTC3522 buck con- verter uses a constant frequency, current mode control Current mode control requires the use of slope compensa- architecture. Both the main (P-channel MOSFET) and tion to prevent sub-harmonic oscillations in the inductor synchronous rectifi er (N-channel MOSFET) switches are current waveform at high duty cycle operation. This is ac- internal. At the start of each oscillator cycle, the P-chan- complished internally on the LTC3522 through the addition nel switch is turned on and remains on until the current of a compensating ramp to the current sense signal. In waveform with superimposed slope compensation ramp some current mode ICs, current limiting is performed by exceeds the error amplifi er output. At this point, the syn- clamping the error amplifi er voltage to a fi xed maximum. chronous rectifi er is turned on and remains on until the This leads to a reduced output current capability at low inductor current falls to zero or a new switching cycle is step-down ratios. In contrast, the LTC3522 performs cur- initiated. As a result, the buck converter operates with rent limiting prior to addition of the slope compensation discontinuous inductor current at light loads which im- ramp and therefore achieves a peak inductor current limit proves effi ciency. At extremely light loads, the minimum that is independent of duty cycle. on-time of the main switch will be reached and the buck converter will begin turning off for multiple cycles in order
Short-Circuit Protection
to maintain regulation. When the output is shorted to ground, the error amplifi er will saturate high and the P-channel MOSFET switch will
Burst Mode Operation
turn on at the start of each cycle and remain on until the When the PWM pin is forced low, the buck converter will current limit trips. During this minimum on-time, the in- automatically transition between Burst Mode operation ductor current will increase rapidly and will decrease very at suffi ciently light loads (below approximately 10mA) slowly during the remainder of the period due to the very and PWM mode at heavier loads. Burst Mode entry is small reverse voltage produced by a hard output short. determined by the peak inductor current and therefore To eliminate the possibility of inductor current runaway the load current at which Burst Mode operation will be in this situation, the buck converter switching frequency entered depends on the input voltage, the output voltage is reduced to approximately 250kHz when the voltage on and the inductor value. Typical curves for Burst Mode FB2 falls below 0.3V. entry threshold are provided in the Typical Performance 3522fa 9