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LTC3520EUF-PBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC3520EUF-PBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LTC3520 11 3520f OPERATION The LTC3520 combines a synchronous buck DC/DC converter and a four-switch buck-boost DC/DC converter in a single 4mm x 4mm QFN 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 converter provides a high efficiency lower voltage output and supports 100% duty cycle operation to extend battery life. In Burst Mode operation, the total quiescent current for both converters is reduced to 55µA (typical). Both converters operate synchronously from a common internal oscillator whose frequency is programmed via an external resistor. In ad- dition, the LTC3520 contains an uncommitted gain block which can be configured as a comparator for low battery detection or as a power-good indicator. Alternatively, the gain block can be utilized in conjunction with an external PNP to create an LDO, thereby allowing the LTC3520 to generate a third low noise output voltage. BUCK CONVERTER OPERATION PWM Mode Operation WhenthePWM2pinisheldhigh,theLTC3520buckconverter uses a constant-frequency, current mode control architec- ture. 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 current waveform with superimposed slope compensation 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 converter operates with discontinuous inductor cur- rent at light loads which improves efficiency. At extremely light loads, the minimum on-time of the main switch will be reached and the buck converter will begin turning off for multiple cycles in order to maintain regulation. Burst Mode Operation Burst Mode operation is enabled by either connecting PWM2 to ground through a resistor, RBURST, or by shorting PWM2 to ground. The buck converter will automatically transition between PWM mode at high load current and Burst Mode operation at light currents. Typical curves for the Burst Mode entry threshold are provided in the Typical Performance Characteristics section of this datasheet. Under dropout and near dropout conditions, Burst Mode operation will not be entered. The value of RBURST controls the load current at which Burst Mode operation will be entered. Larger resistor values will cause Burst Mode operation to be entered at lighter load currents. However, if the value of RBURST is too large, then the buck converter will not enter Burst Mode operation at any current, especially when operating with VIN close to the buck output voltage. Conversely, if RBURST is too small, the ripple in Burst Mode operation may become objectionable, especially at high input volt- ages. For most applications, choosing RBURST = 301k represents a reasonable compromise. The output voltage ripple in Burst Mode operation is de- pendent upon the value of RBURST, the input voltage, the output voltage, the inductor value and the output capaci- tor. The Burst Mode operation output voltage ripple can be reduced by increasing the size of the output capacitor, increasing the value of the inductor or increasing the value of RBURST. Low 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 power P-channel MOSFET switch to remain on for more than one cycle until 100% duty cycle operation is reached and the power switch remains on continuously. In this dropout state, the output voltage 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 waveform at high duty cycle operation. This is ac- complished internally on the LTC3520 through the addition of a compensating ramp to the current sense signal. In some current mode ICs, current limiting is performed by clamping the error amplifier voltage to a fixed maximum. |
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