PDF LTC3520 Data sheet ( Hoja de datos )

Número de pieza	LTC3520
Descripción	Synchronous 1A Buck-Boost and 600mA Buck Converters
Fabricantes	Linear Technology
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No Preview Available ! FEATURES ■ Dual High Efﬁciency DC/DC Converters: Buck-Boost (VOUT: 2.2V to 5.25V, IOUT = 1A at VOUT = 3.3V, VIN ≥ 3V) Buck (VOUT: 0.8V to VIN, IOUT = 600mA) ■ 2.2V to 5.5V Input Voltage Range ■ Pin-Selectable Burst Mode® Operation www■.DaUtnacShoememt4Uit.cteodmGain Block for LDO Controller, Battery Good Indication or Sequencing ■ Programmable 100kHz to 2MHz Switching Frequency ■ 55µA Total Quiescent Current for Both Converters in Burst Mode Operation ■ Thermal and Overcurrent Protection ■ <1µA Quiescent Current in Shutdown ■ 24-Lead 4mm × 4mm QFN Package APPLICATIONS ■ Portable Media Players ■ Digital Cameras ■ Handheld PCs, PDAs ■ GPS Receivers , LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents, including 5481178, 6166527, 6304066, 6404251, 6580258. LTC3520 Synchronous 1A Buck-Boost and 600mA Buck Converters DESCRIPTION The LTC®3520 combines 1A buck-boost and 600mA synchronous buck DC/DC converters in a tiny 4mm × 4mm package. A programmable switching frequency allows the efﬁciency to be optimized while minimizing the solution footprint. Both converters feature soft-start and current limit protection. The uncommitted gain block can be conﬁgured as an LDO or utilized as a battery-good comparator. The buck converter is current mode controlled with internal synchronous rectiﬁcation to improve efﬁciency. Pin-select- able Burst Mode operation can be enabled to improve light load efﬁciency, or the buck converter can be operated in low noise PWM mode for sensitive applications. The buck-boost converter provides continuous conduc- tion operation to maximize efﬁciency and minimize noise. At light loads, use of Burst Mode operation will improve efﬁciency. The LTC3520 provides a <1µA shutdown mode and over- temperature shutdown on both converters. The LTC3520 is available in a low proﬁle (0.75mm) 24-lead 4mm × 4mm QFN package. TYPICAL APPLICATION 3.3V at 500mA, 1.8V at 600mA and 1.5V at 200mA Converter VIN 2.2V TO 5.5V VOUT2 1.8V 600mA 10µF 22µF 255k 200k BURST 4.7µH PVIN1 PVIN2 PVIN3 SVIN SW1A SW1B SW2 VOUT1 4.7µH 470pF 27pF FB2 0.01µF SS2 54.9k RT LTC3520 VC1 15k FB1 0.01µF SS1 VOUT2 PWM PWM1 PWM2 AOUT SD3 OFF ON SD2 SD1 AIN PGND1 SGND PGND2 56pF 10k 1M 309k 100k 33pF 110k 47µF VOUT1 3.3V 500mA 1A FOR VIN ≥ 3V VOUT 1.5V 200mA 4.7µF 3520 TA01 Efﬁciency vs VIN 100 BUCK-BOOST 95 IOUT = 150mA BUCK 90 IOUT = 250mA 85 80 75 70 2.2 2.7 3.2 3.7 4.2 4.7 5.2 VIN (V) 3520f 1 1 page TYPICAL PERFORMANCE CHARACTERISTICS LTC3520 Buck Efﬁciency Lithium-Ion to 1.8V 100 Burst Mode 90 OPERATION 80 70 60 50 800 PWM MODE 700 600 VIN = 4.2V 500 VIN = 3V 400 40 30 www.DataSheet4U.com20 L = SUMIDA 10 CDRH3D16NP-4R7N RBURST = 249k 0 1 10 Burst Mode OPERATION POWER LOSS 100 LOAD CURRENT (mA) 300 200 100 0 1000 3520 G03 LDO Load Transient Response LDO VOUT 100mV/DIV LOAD CURRENT 100mA/DIV (20mA TO 210mA STEP) VIN = 3.6V VOUT = 1.5V 50µs/DIV 3520 G05 COUT = 4.7µF LDO INPUT VOLTAGE = 1.8V Switching Frequency vs RT 10000 1000 100 10 100 RT (k) Buck-Boost Load Transient Response VOUT VIN = 5V 200mV/DIV VOUT VIN = 2.2V 500mV/DIV LOAD CURRENT 500mA/DIV COUT = 47µF L = 4.7µH VIN = 3.3V 200µs/DIV 1000 3520 G04 3520 G06 Buck Load Transient Response (PWM Mode) BUCK VOUT 100mV/DIV LOAD CURRENT 500mA/DIV (50mA TO 500mA STEP) VIN = 3.6V VOUT = 1.8V COUT = 10µF 100µs/DIV 3520 G07 Buck Load Transient Response (Burst Mode Operation) BUCK VOUT 100mV/DIV LOAD CURRENT 200mA/DIV (5mA TO 300mA STEP) VIN = 3.6V VOUT = 1.8V 100µs/DIV COUT = 22µF RBURST = 249k 3520 G08 3520f 5 5 Page LTC3520 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 wwws.yDnactahSrhoeneotu4Us.lcyofmrom 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 When the PWM2 pin is held high, the LTC3520 buck converter 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. 3520f 11 11 Page

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