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LTC3521 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3521 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 22 page  LTC3521 10 3521fb For more information www.linear.com/LTC3521 operaTion The LTC3521 combines dual synchronous buck DC/DC converters and a 4-switch buck-boost DC/DC converter in a 4mm × 4mm QFN package and a 20-pin thermally enhanced TSSOP package. The buck-boost converter utilizes a proprietary switching algorithm which allows its output voltage to be regulated above, below or equal to the input voltage. The buck converters provide a high ef- ficiencylowervoltageoutputandsupport100%dutycycle operation to extend battery life. In Burst Mode operation, the total quiescent current for the LTC3521 is reduced to 30μA. All three converters are synchronized to the same internal 1.1MHz oscillator. BUCK CONVERTER OPERATION PWM Mode Operation When the PWM pin is held high, the LTC3521 buck con- verters use a constant-frequency, current mode control architecture. Both the main (P-channel MOSFET) and synchronous rectifier (N-channel MOSFET) switches are internal. At the start of each oscillator cycle, the P-channel switch is turned on and remains on until the currentwaveformwithsuperimposedslopecompensation ramp exceeds the error amplifier output. At this point, the synchronous rectifier is turned on and remains on until the inductor current falls to zero or a new switching cycle is initiated. As a result, the buck converters operate with discontinuous inductor current at light loads, which improvesefficiency.Atextremelylightloads,theminimum on-time of the main switch will be reached and the buck converters will begin turning off for multiple cycles in order to maintain regulation. Burst Mode Operation When the PWM pin is forced low, the buck converters will automatically transition between Burst Mode operation at sufficiently light loads (below approximately 15mA) and PWM mode at heavier loads. Burst Mode entry is determined by the peak inductor current. Therefore, the load current at which Burst Mode operation will be entered depends on the input voltage, the output voltage and the inductor value. Typical curves for Burst Mode entry threshold are provided in the Typical Performance Characteristics section of this data sheet. In dropout and neardropoutconditions,BurstModeoperationisdisabled. Dropout Operation As the input voltage decreases to a value approaching the output regulation voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage will force the main switch to remain on for more than one cycle until 100% duty cycle operation is reached where the main switch remains on continuously. In this dropout state, the output will be determined by the input voltage less the resistive voltage drop across the main switch and series resistance of the inductor. Slope Compensation Current mode control requires the use of slope compensa- tion to prevent subharmonic oscillations in the inductor current at high duty cycle operation. This is accomplished internally on the LTC3521 through the addition of a com- pensatingramptothecurrentsensesignal.Insomecurrent mode ICs, current limiting is performed by clamping the error amplifier voltage to a fixed maximum. This leads to a reduced output current capability at low step-down ratios. In contrast, the LTC3521 performs current limiting prior to addition of the slope compensation ramp and therefore achieves a peak inductor current limit that is independent of duty cycle. Short-Circuit Protection When the output is shorted to ground, the error amplifier will saturate high and the P-channel MOSFET switch will turn on at the start of each cycle and remain on until the current limit trips. During this minimum on-time, the in- ductor current will increase rapidly and will decrease very slowly during the remainder of the period due to the very small reverse voltage produced by a hard output short. To eliminate the possibility of inductor current runaway in this situation, the buck converter switching frequency is reducedto250kHzwhenthevoltageonthebuckFBpinfalls below 0.25V. The buck soft-start circuit is reset when the buck FB pin falls below 0.25V to provide a smooth restart once the short-circuit condition at the output voltage is no longer present. Additionally, the PMOS current limit is decreased from 1050mA to 700mA when the voltage on the buck FB pin falls below 0.25V. |
Similar Part No. - LTC3521_15 |
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Similar Description - LTC3521_15 |
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