LTC3522 Synchronous 400mA Buck-Boost and 200mA Buck Converters FEATURES DESCRIPTION ■ The LTC®3522 combines a 400mA buck-boost DC/DC converter with a 200mA synchronous buck DC/DC converter in a tiny 3mm × 3mm package. The 1MHz switching frequency minimizes the solution footprint while maintaining high efficiency. Both converters feature internal soft-start and compensation, simplifying the design process. ■ ■ ■ ■ ■ ■ ■ ■ Dual High Efficiency DC/DC Converters: Buck-Boost (VOUT: 2.2V to 5.25V, IOUT: 400mA for VIN > 3V, VOUT = 3.3V) Buck (VOUT: 0.6V to VIN, IOUT: 200mA) 2.4V to 5.5V Input Voltage Range Pin Selectable Burst Mode® Operation 25μA Total Quiescent Current for Both Converters in Burst Mode Operation Independent Power Good Indicator Outputs Integrated Soft-Start Thermal and Overcurrent Protection <1μA Quiescent Current in Shutdown Small 0.75mm × 3mm × 3mm QFN Package APPLICATIONS ■ ■ ■ ■ ■ The buck converter is current mode controlled and utilizes an internal synchronous rectifier for high efficiency. The buck converter supports 100% duty cycle operation to extend battery life. If the PWM pin is held low, the buck converter automatically transitions from Burst Mode operation to PWM mode. With the PWM pin held high, the buck converter remains in low noise, 1MHz PWM mode. The buck-boost converter provides continuous conduction operation to maximize efficiency and minimize noise. At light loads, the buck-boost converter can be placed in Burst Mode operation to improve efficiency and reduce no-load standby current. Flash-Based MP3 Players Medical Instruments Digital Cameras PDAs, Handheld PCs Personal Navigation Devices The LTC3522 provides a 1μA shutdown mode, overtemperature shutdown and current limit protection on both converters. The LTC3522 is available in a 16-pin low profile 3mm × 3mm QFN package. , 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 6404251 and 6166527. TYPICAL APPLICATION Efficiency vs VIN VOUT2 1.8V 200mA + Li-Ion 4.7μF L1 8.2μH SW1A SW2 6.8μF 137k 12pF 68.1k ON FB2 OFF SW1B LTC3522 VOUT1 1M SHDN2 SHDN1 FB1 432k PGOOD2 PWM BURST L2 4.7μH PVIN1 PVIN2 PWM PGOOD1 PGND1 GND PGND2 3522 TA01a L1: COILCRAFT MSS6132-8.2μH L2: COILCRAFT MSS6132-4.7μH VOUT1 3.3V 300mA (400mA 4.7μF VIN > 3V) EFFICIENCY (%) VIN 2.4V TO 4.2V 100 98 96 94 92 90 88 86 84 82 80 78 76 74 72 70 2.4 BUCK-BOOST IOUT = 100mA VOUT = 3.3V BUCK IOUT = 100mA VOUT = 1.8V 4.4 3.4 5.4 VIN (V) 3522 TA01b 3522f 1 LTC3522 ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATION (Note 1) PVIN1, PVIN2 Voltage .................................... –0.3V to 6V SW1A, SW1B, SW2 Voltage DC............................................................ –0.3V to 6V Pulsed < 100ns ........................................... –1V to 7V Voltage, All Other Pins ................................. –0.3V to 6V Operating Temperature Range (Note 2) ... –40°C to 85°C Maximum Junction Temperature (Note 5) ............ 125°C Storage Temperature Range................... –65°C to 125°C PGND1 SW2 PVIN2 SHDN2 TOP VIEW 16 15 14 13 FB2 1 12 VOUT1 PWM 2 11 SW1A 17 GND 3 10 SW1B PGOOD2 4 6 7 8 FB1 PGOOD1 SHDN1 PVIN1 9 5 PGND2 UD PACKAGE 16-LEAD (3mm × 3mm) PLASTIC QFN TJMAX = 125°C, θJA = 68°C/W EXPOSED PAD (PIN 17) IS GND AND MUST BE SOLDERED TO PCB GROUND ORDER PART NUMBER UD PART MARKING LTC3522EUD LCRQ Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. PVIN1 = PVIN2 = 3.6V, VOUT1 = 3.3V unless otherwise noted. PARAMETER CONDITIONS MIN ● Input Voltage Quiescent Current—Shutdown VSHDN1 = VSHDN2 = 0V Burst Mode Quiescent Current VFB1 = 1.1V, VFB2 = 0.66V, VPWM = 0V 2.4 ● MAX 5.5 0.01 1 25 ● Oscillator Frequency TYP SHDN1, SHDN2, PWM Input High Voltage 0.8 1.07 UNITS V μA μA 1.33 1.4 MHz V SHDN1, SHDN2, PWM Input Low Voltage 0.4 V Power Good Outputs Low Voltage IPGOOD1 = IPGOOD2 = 1mA 0.02 0.1 V Power Good Outputs Leakage VPGOOD1 = VPGOOD2 = 5.5V 0.1 10 μA Buck Converter PMOS Switch Resistance 0.41 Ω NMOS Switch Resistance 0.34 Ω NMOS Switch Leakage VSW2 = 5V, PVIN1 = PVIN2 = 5V 0.1 5 μA PMOS Switch Leakage VSW2 = 0V, PVIN1 = PVIN2 = 5V 0.1 10 μA Feedback Voltage (Note 4) 0.594 0.606 V 1 50 nA ● 0.582 Feedback Input Current Peak Current Limit (Note 3) 300 Maximum Duty Cycle VFB2 = 0.54V ● Minimum Duty Cycle VFB2 = 0.66V ● 400 mA 100 % 0 % 3522f 2 LTC3522 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. PVIN1 = PVIN2 = 3.6V, VOUT1 = 3.3V unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS PGOOD Threshold VFB2 Falling –11.3 –7.7 –4.1 % Power Good Hysteresis 2.5 % Buck-Boost Converter ● Output Voltage 2.2 5.25 V PMOS Switch Resistance 0.29 Ω NMOS Switch Resistance 0.22 Ω NMOS Switch Leakage VSW1A = VSW1B = 5V, PVIN1 = PVIN2 = 5V 0.1 5 μA PMOS Switch Leakage VSW1A = VSW1B = 0V, PVIN1 = PVIN2 = 5V 0.1 10 μA Feedback Voltage (Note 4) 0.97 1 1.03 V 1 50 nA 0.65 0.85 A 230 340 mA 250 mA 70 80 % –12 –10 ● Feedback Input Current Average Current Limit (Note 3) Burst Mode Current Limit Reverse Current Limit (Note 3) Maximum Duty Cycle VFB1 = 0.9V ● Minimum Duty Cycle VFB1 = 1.1V ● PGOOD Threshold VFB1 Falling Power Good Hysteresis Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC3522 is guaranteed to meet performance specifications from 0°C to 85°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. Note 3: Current measurements are performed when the LTC3522 is not switching. The current limit values in operation will be somewhat higher due to the propagation delay of the comparators. 0 2.5 % –8 % % Note 4: The LTC3522 is tested in a proprietary non-switching test mode that connects each FB pin to the output of the respective error amplifier. Note 5: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125°C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability. 3522f 3 LTC3522 TYPICAL PERFORMANCE CHARACTERISTICS Buck-Boost Efficiency, Li-Ion to 3.3V Buck Efficiency, Li-Ion to 2.5V 160 100 Burst Mode OPERATION 90 80 40 60 Burst Mode POWER LOSS 30 VIN = 4.2V VIN = 2.7V 20 EFFICIENCY (%) PWM Mode 0 1 PWM Mode 60 40 100 80 50 Burst Mode POWER LOSS 40 VIN = 3.7V VIN = 4.2V L: COILCRAFT MSS6132-8.2μH 30 20 20 L: COILCRAFT MSS6132-4.7μH 0 1000 100 10 LOAD CURRENT (mA) 10 120 70 10 60 40 20 0 1000 0 1 100 10 LOAD CURRENT (mA) 3522 G02 3522 G01 L = 4.7μH 80 50 Burst Mode POWER LOSS 60 VIN = 4.2V VIN = 2.7V 30 20 40 L: COILCRAFT MSS6132-8.2μH 10 1 30 100 10 LOAD CURRENT (mA) 300 VOUT = 1.8V 25 20 VOUT = 2.5V 15 0 2.4 2.9 3.4 4.4 3.9 VIN (V) 600 500 CHANGE FROM 25°C (%) RDS(ON) (mΩ) 4.9 NMOS 300 200 100 3522 G06 20 40 60 80 TEMPERATURE (°C) 100 120 Switching Frequency vs VIN 10 8 8 4 2 0 –2 –4 –10 –50 –30 –10 10 30 50 70 TEMPERATURE (°C) 6 4 2 0 –2 –4 –6 –8 –8 100 120 0 3522 G05 10 –6 20 40 60 80 TEMPERATURE (°C) 0 –40 –20 5.4 6 400 0 150 Switching Frequency vs Temperature Buck Switch RDS(0N) PMOS NMOS (SWITCHES B AND C) 3522 G04 3522 G03 0 –40 –20 200 50 5 0 1000 250 100 10 20 0 VOUT = 1.2V 35 CHANGE FROM VIN = 3.6V (%) 40 POWER LOSS (mW) 100 60 LOAD CURRENT (mA) 120 PMOS (SWITCHES A AND D) 350 40 PWM Mode 400 RDS(ON) (mΩ) 45 140 80 EFFICIENCY (%) 50 160 Burst Mode 90 OPERATION 70 Buck-Boost Switch RDS(0N) Buck Burst Mode Threshold Buck Efficiency, Li-Ion to 1.8V 100 POWER LOSS (mW) 100 60 140 80 120 70 160 Burst Mode 90 OPERATION POWER LOSS (mW) EFFICIENCY (%) 100 140 80 50 (TA = 25°C unless otherwise noted) –10 90 110 3522 G07 2.5 2.9 3.4 4.4 3.9 VIN (V) 4.9 5.4 3522 G08 3522f 4 LTC3522 TYPICAL PERFORMANCE CHARACTERISTICS 0.4 0.3 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 20 40 60 80 100 120 140 TEMPERATURE (°C) Buck-Boost Maximum Load Current, Burst Mode Operation 0.5 90 0.4 80 MAXIMUM LOAD CURRENT (mA) 0.5 –0.5 –40 –20 0 Buck Feedback Voltage vs Temperature CHANGE IN FEEDBACK VOLTAGE FROM 20°C (%) CHANGE IN FEEDBACK VOLTAGE FROM 20°C (%) Buck-Boost Feedback Voltage vs Temperature 0.3 0.2 0.1 0 –0.1 –0.2 –0.3 –0.5 –40 –20 0 60 VOUT = 5V 50 40 30 20 20 40 60 80 100 120 140 TEMPERATURE (°C) 0 2.4 50 L = 4.7μH QUIESCENT CURRENT (μA) VOUT = 3.3V 400 300 VOUT = 5V 200 100 2.9 3.4 3.9 4.4 VIN (V) 4.9 5.4 3522 G11 Buck-Boost Burst to PWM Transition No Load Quiescent Current vs VIN BOTH CONVERTERS ENABLED 45 LOAD CURRENT (mA) VOUT = 3V 70 3522 G09 Buck-Boost Maximum Load Current, PWM Mode 500 L = 4.7μH 10 –0.4 3522 G09 600 (TA = 25°C unless otherwise noted) INDUCTOR CURRENT 200mA/DIV 40 35 30 VOUT 100mV/DIV 25 20 VIN = 3.6V VOUT = 3.3V L = 4.7μH COUT = 4.7μF 15 10 5 50μs/DIV 3522 G14 0 0 2.4 2.9 3.4 3.9 4.4 VIN (V) 4.9 5.4 2.4 2.9 3.4 4.4 3.9 VIN (V) 4.9 5.4 3522 G13 3522 G12 Buck Load Step, PWM Mode, 5mA to 200mA Buck-Boost Load Step, 0mA to 300mA Buck Load Step, Burst Mode Operation, 5mA to 200mA VOUT 100mV/DIV VOUT 100mV/DIV VOUT 100mV/DIV INDUCTOR CURRENT 200mA/DIV INDUCTOR CURRENT 100mA/DIV INDUCTOR CURRENT 100mA/DIV VIN = 3.6V VOUT = 3V L = 4.7μH COUT = 4.7μF 100μs/DIV 3522 G15 VIN = 3.6V VOUT = 1.8V L = 4.7μH COUT = 4.7μF 100μs/DIV 3522 G16 VIN = 3.6V VOUT = 1.8V L = 4.7μH COUT = 4.7μF 100μs/DIV 3522 G17 3522f 5 LTC3522 PIN FUNCTIONS FB2 (Pin 1): Feedback Voltage for the Buck Converter Derived from a Resistor Divider on the Buck Output Voltage. The buck output voltage is given by the following equation where R1 is a resistor between FB2 and ground and R2 is a resistor between FB2 and the buck output voltage: ⎛ R2⎞ VOUT = 0.594V ⎜1+ ⎟ ⎝ R1⎠ PWM (Pin 2): Logic Input Used to Choose Between Burst and PWM Mode Operation for Both Converters. This pin cannot be left floating. PWM = Low: Burst Mode operation is enabled on both converters. The buck converter will operate in Burst Mode operation at light current but will automatically transition to PWM operation at higher currents. The buck converter can supply its maximum output current (200mA) in this mode. The buck-boost converter will operate in variable frequency mode and can only supply a reduced load current (typically 50mA). PWM = High: Both converters are forced into low noise 1MHz PWM mode operation. The buck converter will remain at constant frequency operation until its minimum on-time is reached. The buck-boost converter will remain in PWM mode at all load currents. GND (Pin 3): Small-Signal Ground Used as a Ground Reference for the Internal Circuitry of the LTC3522. PGOOD2 (Pin 4): This pin is an open-drain output which will only pull low if the buck converter is enabled and one or more of the following conditions occurs: the buck output voltage is out of regulation, the part is in overtemperature shutdown or the part is in undervoltage lockout. FB1 (Pin 5): Feedback Voltage for the Buck-Boost Converter Derived from a Resistor Divider on the Buck-Boost Output Voltage. The buck-boost output voltage is given by the following equation where R1 is a resistor between FB1 and ground and R2 is a resistor between FB1 and the buck-boost output voltage: ⎛ R2⎞ VOUT = 1V ⎜1+ ⎟ ⎝ R1⎠ PGOOD1 (Pin 6): This pin is an open-drain output which will only pull low if the buck-boost converter is enabled and one or more of the following conditions occurs: the buck-boost output voltage is out of regulation, the part is in overtemperature shutdown, the part is in undervoltage lockout or the buck-boost converter is in current limit. See the Operation section of this data sheet for details on the functionality of this pin in PWM mode. SHDN1 (Pin 7): Buck-Boost Active-Low Shutdown Pin. Forcing this pin above 1.4V enables the buck-boost converter. Forcing this pin below 0.4V disables the buck-boost converter. This pin cannot be left floating. PVIN1 (Pin 8): High Current Power Supply Connection Used to Supply Switch A of the Buck-Boost Converter. This pin should be bypassed by a 4.7μF or larger ceramic capacitor. The bypass capacitor should be placed as close to the pin as possible and should have a short return path to ground. Pins PVIN1 and PVIN2 must be connected together in the application circuit. PGND2 (Pin 9): High Current Ground Connection for the Buck-Boost Switch C. The PCB trace connecting this pin to ground should be made as short and wide as possible. 3522f 6 LTC3522 PIN FUNCTIONS SW1B (Pin 10): Buck-Boost Switch Node That Must be Connected to One Side of the Buck-Boost Inductor. SW1A (Pin 11): Buck-Boost Switch Node That Must be Connected to One Side of the Buck-Boost Inductor. VOUT1 (Pin 12): Buck-Boost Output Voltage Node. This pin should be connected to a low ESR output capacitor. The capacitor should be placed as close to the IC as possible and should have a short return to ground. PGND1 (Pin 13): High Current Ground Connection for Buck-Boost Switch B and the Buck Converter Synchronous Rectifier. The PCB trace connecting this pin to ground should be made as short and wide as possible. SW2 (Pin 14): Buck Converter Switch Node That Must be Connected to the Buck Inductor. PVIN2 (Pin 15): High Current Power Supply Connection Used to Supply the Buck Converter Power Switch. In addition this pin is the supply pin for the internal circuitry of the LTC3522. This pin should be bypassed by a 4.7μF or larger ceramic capacitor. The bypass capacitor should be placed as close to the pin as possible and should have a short return path to ground. Pins PVIN1 and PVIN2 must be connected together in the application circuit. SHDN2 (Pin 16): Buck Active-Low Shutdown Pin. Forcing this pin above 1.4V enables the buck converter. Forcing this pin below 0.4V disables the buck converter. This pin cannot be left floating. Exposed Pad (Pin 17): The Exposed Pad must be electrically connected to ground. Pins PGND1, PGND2, GND, and the Exposed Pad must be connected together in the application circuit. 3522f 7 LTC3522 BLOCK DIAGRAM 10 12 VOUT1 SW1B D A B C FILTER + – 0.85A 0.250A + – 0A + – PGND1 PGND2 IZERO 0.9V REVERSE ILIMIT SW1A FB1 1.00V PVIN2* PWM OSCILLATOR INTERNAL VCC 5 SOFT-START RAMP SHDN1 15 6 + – VOUT1 BUCK-BOOST PWM LOGIC GATE DRIVES PGOOD1 FORWARD ILIMIT 11 PVIN1* + + – 8 7 2 UVLO 14 SW2 GATE DRIVES BUCK PWM LOGIC SHDN2 16 PGND1 0A + – ZERO CROSSING 0.4A + – + 1 FB2 0.594V –gm + + 1.00V 0.594V ILIMIT SLOPE COMPENSATION 0.9V 0.548V BANDGAP REFERENCE AND OT SHUTDOWN + – PGOOD2 SOFT-START RAMP 0.548V GND PGND1 PGND2 3 13 9 + – 4 3522 BD *PVIN1 AND PVIN2 MUST BE CONNECTED TOGETHER IN THE APPLICATION. 3522f 8 LTC3522 OPERATION The LTC3522 combines a synchronous buck DC/DC converter and a 4-switch buck-boost DC/DC converter in a single 3mm × 3mm 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 combined quiescent current for both converters is reduced to 25μA. Both converters operate from the same internal 1MHz oscillator. BUCK CONVERTER OPERATION Characteristics section of this data sheet. Under dropout and near dropout conditions, Burst Mode operation will not be entered. 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 voltage will be determined by the input voltage less the resistive voltage drop across the main switch and series resistance of the inductor. PWM Mode Operation When the PWM pin is held high, the LTC3522 buck converter uses 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 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 current 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 When the PWM pin is forced low, the buck converter will automatically transition between Burst Mode operation at sufficiently light loads (below approximately 10mA) and PWM mode at heavier loads. Burst Mode entry is determined by the peak inductor current and 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 Slope Compensation Current mode control requires the use of slope compensation to prevent sub-harmonic oscillations in the inductor current waveform at high duty cycle operation. This is accomplished internally on the LTC3522 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. This leads to a reduced output current capability at low step-down ratios. In contrast, the LTC3522 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 inductor 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 reduced to approximately 250kHz when the voltage on FB2 falls below 0.3V. 3522f 9 LTC3522 OPERATION Soft-Start The buck converter has an internal voltage mode soft-start circuit with a nominal duration of 600μs. The converter remains in regulation during soft-start and will therefore respond to output load transients which occur during this time. In addition, the output voltage rise time has minimal dependency on the size of the output capacitor or load current. Error Amplifier and Compensation The LT3522 buck converter utilizes an internal transconductance error amplifier. Compensation of the feedback loop is performed internally to reduce the size of the application circuit and simplify the design process. The compensation network has been designed to allow use of a wide range of output capacitors while simultaneously ensuring rapid response to load transients. PGOOD2 Comparator The PGOOD2 pin is an open-drain output which indicates the status of the buck converter. If the buck output voltage falls 7.7% below the regulation voltage, the PGOOD2 open-drain output will pull low. The output voltage must rise 2.5% above the falling threshold before the pull-down will turn off. In addition, there is a 60μs typical deglitching delay in the flag in order to prevent false trips due to voltage transients on load steps. The PGOOD2 output will also pull low during overtemperature shutdown and undervoltage lockout to indicate these fault conditions. The PGOOD2 output is only active if the buck converter is enabled. BUCK-BOOST CONVERTER OPERATION PWM Mode Operation When the PWM pin is held high, the LTC3522 buck-boost converter operates in a constant frequency PWM mode with voltage mode control. A proprietary switching algorithm allows the converter to switch between buck, buck-boost and boost modes without discontinuity in inductor current or loop characteristics. The switch topology for the buck-boost converter is shown in Figure 1. L PVIN1 A SW1A B LTC3522 PGND1 SW1B D VOUT1 C PGND2 3522 F01 Figure 1. Buck-Boost Switch Topology When the input voltage is significantly greater than the output voltage, the buck-boost converter operates in buck mode. Switch D turns on continuously and switch C remains off. Switches A and B are pulse width modulated to produce the required duty cycle to support the output regulation voltage. As the input voltage decreases, switch A remains on for a larger portion of the switching cycle. When the duty cycle reaches approximately 85%, the switch pair AC begins turning on for a small fraction of the switching period. As the input voltage decreases further, the AC switch pair remains on for longer durations and the duration of the BD phase decreases proportionally. As the input voltage drops below the output voltage, the AC phase will eventually increase to the point that there is no longer any BD phase. At this point, switch A remains on continuously while switch pair CD is pulse width modulated to obtain the desired output voltage. At this point, the converter is operating solely in boost mode. This switching algorithm provides a seamless transition between operating modes and eliminates discontinuities in average inductor current, inductor current ripple, and loop transfer function throughout all three operational modes. These advantages result in increased efficiency and stability in comparison to the traditional 4-switch buck-boost converter. Error Amplifier and Compensation The buck-boost converter utilizes a voltage mode error amplifier with an internal compensation network as shown in Figure 2. Notice that resistor R2 of the external resistor divider network plays an integral role in determining the frequency 3522f 10 LTC3522 OPERATION VOUT1 LTC3522 + – 1V VOUT R2 FB1 R1 GND 3522 F02 Figure 2. Buck-Boost Error Amplifier and Compensation response of the compensation network. The ratio of R2 to R1 must be set to program the desired output voltage but this still allows the value of R2 to be adjusted to optimize the transient response of the converter. Increasing the value of R2 generally leads to greater stability at the expense of reduced transient response speed. Increasing the value of R2 can yield substantial transient response improvement in cases where the phase margin has been reduced due to the use of a small value output capacitor or a large inductance (particularly with large boost step-up ratios). Conversely, decreasing the value of R2 increases the loop bandwidth which can improve the speed of the converter’s transient response. This can be useful in improving the transient response if a large valued output capacitor is utilized. In this case, the increased bandwidth created by decreasing R2 is used to counteract the reduced converter bandwidth caused by the large output capacitor. Current Limit Operation The buck-boost converter has two current limit circuits. The primary current limit is an average current limit circuit which injects an amount of current into the feedback node which is proportional to the extent that the switch A current exceeds the current limit value. Due to the high gain of this loop, the injected current forces the error amplifier output to decrease until the average current through switch A decreases approximately to the current limit value. The average current limit utilizes the error amplifier in an active state and thereby provides a smooth recovery with little overshoot once the current limit fault condition is removed. Since the current limit is based on the average current through switch A, the peak inductor current in current limit will have a dependency on the duty cycle (i.e., on the input and output voltages in the overcurrent condition). The speed of the average current limit circuit is limited by the dynamics of the error amplifier. On a hard output short, it would be possible for the inductor current to increase substantially beyond current limit before the average current limit circuit would react. For this reason, there is a second current limit circuit which turns off switch A if the current ever exceeds approximately 165% of the average current limit value. This provides additional protection in the case of an instantaneous hard output short. Reverse Current Limit The reverse current comparator on switch D monitors the inductor current entering VOUT1. When this current exceeds 250mA (typical) switch D will be turned off for the remainder of the switching cycle. Burst Mode Operation With the PWM pin held low, the buck-boost converter operates utilizing a variable frequency switching algorithm designed to improve efficiency at light load and reduce the standby current at zero load. In Burst Mode operation, the inductor is charged with fixed peak amplitude current pulses. These current pulses are repeated as often as necessary to maintain the output regulation voltage. The typical output current which can be supplied in Burst Mode operaton is dependent upon the input and output voltage as given by the following formula: IOUT(MAX ),BURST = 0.11 • VIN (A) VIN + VOUT In Burst Mode operation, the error amplifier is not used but is instead placed in a low current standby mode to reduce supply current and improve light load efficiency. Soft-Start The buck-boost converter has an internal voltage mode soft-start circuit with a nominal duration of 600μs. The converter remains in regulation during soft-start and will therefore respond to output load transients that occur during this time. In addition, the output voltage rise time 3522f 11 LTC3522 OPERATION has minimal dependency on the size of the output capacitor or load. During soft-start, the buck-boost converter is forced into PWM operation regardless of the state of the PWM pin. PGOOD1 Comparator The PGOOD1 pin is an open-drain output which indicates the status of the buck-boost converter. In Burst Mode operation (PWM = Low), the PGOOD1 open-drain output will pull low when the output voltage falls 10% below the regulation voltage. There is approximately 2.5% hysteresis in this threshold when the output voltage is returning good. In addition, there is a 60μs typical deglitching delay to prevent false trips due to short duration voltage transients in response to load steps. In PWM mode, operation of the PGOOD1 comparator is complicated by the fact that the feedback pin voltage is driven to the reference voltage independent of the output voltage through the action of the voltage mode error amplifier. Since the soft-start is voltage mode, the feedback voltage will track the output voltage correctly during soft-start, and the PGOOD1 output will correctly indicate the point at which the buck-boost attains regulation at the end of soft-start. Therefore, the PGOOD1 output can be utilized for sequencing purposes. Once in regulation, the feedback voltage will no longer track the output voltage and the PGOOD1 pin will not directly respond to a loss of regulation in the output. However, the only means by which a loss of regulation can occur is if the current limit has been reached thereby preventing the buck-boost converter from delivering the required output current. In such cases, the occurrence of current limit will cause the PGOOD1 flag to fall indicating a fault state. There can be cases, however, when the buck-boost converter is continuously in current limit, causing the PGOOD1 output to pull low, but the output voltage still remains slightly above the PGOOD1 comparator trip point. The PGOOD1 output also pulls low during overtemperature shutdown and undervoltage lockout. The PGOOD1 output is only active if the buck-boost converter is enabled. COMMON FUNCTIONS Thermal Shutdown If the die temperature exceeds 150°C (typical) both converters will be disabled. All power devices will be turned off and all switch nodes will be high impedance. The soft-start circuits for both converters are reset during thermal shutdown to provide a smooth recovery once the overtemperature condition is eliminated. Both converters will restart (if enabled) when the die temperature drops to approximately 140°C. Undervoltage Lockout If the supply voltage decreases below 2.3V (typical) then both converters will be disabled and all power devices will be turned off. The soft-start circuits for both converters are reset during undervoltage lockout to provide a smooth restart once the input voltage rises above the undervoltage lockout threshold. 3522f 12 LTC3522 APPLICATIONS INFORMATION The basic LTC3522 application circuit is shown as the typical application on the front page of this data sheet. The external component selection is determined by the desired output voltages, output currents and ripple voltage requirements of each particular application. However, basic guidelines and considerations for the design process are provided in this section. Buck Inductor Selection The choice of buck inductor value influences both the efficiency and the magnitude of the output voltage ripple. Larger inductance values will reduce inductor current ripple and will therefore lead to lower output voltage ripple. For a fixed DC resistance, a larger value inductor will yield higher efficiency by lowering the peak current to be closer to the average. However, a larger value inductor within the same family will generally have a greater series resistance, thereby offsetting this efficiency advantage. Given a desired peak to peak current ripple, ΔIL, the required inductance can be calculated via the following expression, where f represents the switching frequency in MHz: L= ⎛ V ⎞ 1 VOUT ⎜1 – OUT ⎟ (μH) fΔIL VIN ⎠ ⎝ A reasonable choice for ripple current is ΔIL = 80mA which represents 40% of the maximum 200mA load current. The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current in order to prevent core saturation and loss of efficiency during operation. To optimize efficiency the inductor should have a low series resistance. In particularly space restricted applications it may be advantageous to use a much smaller value inductor at the expense of larger ripple current. In such cases, the converter will operate in discontinuous conduction for a wider range of output loads and efficiency will be reduced. In addition, there is a minimum inductor value required to maintain stability of the current loop (given the fixed internal slope compensation). Specifically, if the buck converter is going to be utilized at duty cycles over 40%, the inductance value must be at least LMIN as given by the following equation: LMIN = 2.5 • VOUT (μH) Table 1 depicts the minimum required inductance for several common output voltages. Table 1. Buck Minimum Inductance OUTPUT VOLTAGE MINIMUM INDUCTANCE 0.6V 1.5μH 0.8V 2.0μH 1.2V 3.0μH 2.0V 5.0μH 2.7V 6.8μH 3.3V 8.3μH Buck Output Capacitor Selection A low ESR output capacitor should be utilized at the buck output in order to minimize voltage ripple. Multi-layer ceramic capacitors are an excellent choice as they have low ESR and are available in small footprints. In addition to controlling the ripple magnitude, the value of the output capacitor also sets the loop crossover frequency and therefore can impact loop stability. There is both a minimum and maximum capacitance value required to ensure stability of the loop. If the output capacitance is too small, the loop cross-over frequency will increase to the point where switching delay and the high frequency parasitic poles of the error amplifier will degrade the phase margin. In addition, the wider bandwidth produced by a small output capacitor will make the loop more susceptible to switching noise. At the other extreme, if the output capacitor is too large, the cross-over frequency can decrease too far below the compensation zero and also lead to degraded phase margin. Table 2 provides a guideline for the range of allowable values of low ESR output capacitors. Larger value output capacitors can be accommodated provided they have sufficient ESR to stabilize the loop or by increasing the value of the feedforward capacitor in parallel with the upper resistor divider resistor. 3522f 13 LTC3522 APPLICATIONS INFORMATION Table 3. Buck Resistor Divider Values Table 2. Buck Output Capacitor Range VOUT CMIN CMAX VOUT R1 R2 CFF 0.6V 15μF 300μF 0.6V – 0 – 0.8V 15μF 230μF 0.8V 200k 69.8k 12pF 1.2V 10μF 150μF 1.0V 118k 80.6k 12pF 1.8V 6.8μF 90μF 1.2V 100k 102k 12pF 2.7V 6.8μF 70μF 1.5V 78.7k 121k 12pF 3.3V 6.8μF 50μF 1.8V 68.1k 137k 12pF 2.7V 63.4k 226k 18pF 3.3V 60.4k 274k 20pF Buck Input Capacitor Selection The PVIN2 pin provides current to the buck converter power switch and is also the supply pin for the IC’s internal circuitry. It is recommended that a low ESR ceramic capacitor with a value of at least 4.7μF be used to bypass this pin. The capacitor should be placed as close to the pin as possible and have a short return to ground. Buck Output Voltage Programming The output voltage is set by a resistive divider according to the following formula: ⎛ R2⎞ VOUT = 0.594V ⎜1+ ⎟ ⎝ R1⎠ The external divider is connected to the output as shown in Figure 3. It is recommended that a feedforward capacitor, CFF , be placed in parallel with resistor R2 in order to improve the noise immunity of the feedback node. Table 3 provides the recommended resistor and feedforward capacitor combinations for common output voltage options. Buck-Boost Output Voltage Programming The buck-boost output voltage is set by a resistive divider according to the following formula: ⎛ R2⎞ VOUT = 1V ⎜1+ ⎟ ⎝ R1⎠ The external divider is connected to the output as shown in Figure 4. The buck-boost converter utilizes voltage mode control and the value of R2 plays an integral role in the dynamics of the feedback loop. In general, a larger value for R2 will increase stability and reduce the speed of the transient response. A smaller value of R2 will reduce stability but increase the transient response speed. A good starting point is to choose R2 = 1M, and then calculate the required value of R1 to set the desired output voltage according to the formula given above. If a large output capacitor is used, the bandwidth of the converter is reduced. In such cases R2 can be reduced to improve the transient 2.2V ≤ VOUT ≤ 5.25V 0.6V ≤ VOUT ≤ 5.25V R2 R2 CFF FB1 FB2 LTC3522 R1 LTC3522 R1 GND GND 3522 F03 Figure 3. Setting the Buck Output Voltage 3522 F04 Figure 4. Setting the Buck-Boost Output Voltage 3522f 14 LTC3522 APPLICATIONS INFORMATION response. If a large inductor or small output capacitor is utilized the loop will be less stable and the phase margin can be improved by increasing the value of R2. Buck-Boost Inductor Selection To achieve high efficiency, a low ESR inductor should be utilized for the buck-boost converter. The inductor must have a saturation rating greater than the worst case average inductor current plus half the ripple current. The peak-topeak inductor current ripple will be larger in buck and boost mode than in the buck-boost region. The peak-to-peak inductor current ripple for each mode can be calculated from the following formulas, where f is the frequency in MHz and L is the inductance in μH: ΔIL,P-P,BUCK = 1 VOUT (VIN – VOUT ) • fL VIN ΔIL,P-P,BOOST = 1 VIN (VOUT – VIN) • fL VOUT In addition to affecting output current ripple, the size of the inductor can also affect the stability of the feedback loop. In boost mode, the converter transfer function has a right half plane zero at a frequency that is inversely proportional to the value of the inductor. As a result, a large inductor can move this zero to a frequency that is low enough to degrade the phase margin of the feedback loop. It is recommended that the inductor value be chosen less than 10μH if the buck-boost converter is to be used in the boost region. Buck-Boost Output Capacitor Selection A low ESR output capacitor should be utilized at the buckboost converter output in order to minimize output voltage ripple. Multi-layer ceramic capacitors are an excellent choice as they have low ESR and are available in small footprints. The capacitor should be chosen large enough to reduce the output voltage ripple to acceptable levels. Neglecting the capacitor ESR and ESL, the peak-to-peak output voltage ripple can be calculated by the following formulas, where f is the frequency in MHz, COUT is the capacitance in μF, L is the inductance in μH and ILOAD is the output current in Amps: ΔVP-P(BOOST) = ΔVP-P(BUCK) = ILOAD (VOUT – VIN) COUT • VOUT • f 1 8 • L • COUT • f 2 • (VIN – VOUT ) VOUT VIN Since the output current is discontinuous in boost mode, the ripple in this mode will generally be much larger than the magnitude of the ripple in buck mode. In addition to controlling the ripple magnitude, the value of the output capacitor also affects the location of the resonant frequency in the open loop converter transfer function. If the output capacitor is too small, the bandwidth of the converter will extend high enough to degrade the phase margin. To prevent this from happening, it is recommended that a minimum value of 4.7μF be used for the buck-boost output capacitor. Buck-Boost Input Capacitor Selection The supply current to the buck-boost converter is provided by the PVIN1 pin. It is recommended that a low ESR ceramic capacitor with a value of at least 4.7μF be located as close to this pin as possible. Inductor Style and Core Material Different inductor core materials and styles have an impact on the size and price of an inductor at any given peak current rating. Toroid or shielded pot cores in ferrite or permalloy materials are small and reduce emissions, but generally cost more than powdered iron core inductors with similar electrical characteristics. The choice of inductor style depends upon the price, sizing, and EMI requirements of a particular application. Table 4 provides a sampling 3522f 15 LTC3522 APPLICATIONS INFORMATION of inductors that are well suited to many LTC3522 buck converter applications. Table 4. Representative Surface Mount Inductors MAX MANUFACTURER PART NUMBER VALUE CURRENT Taiyo Yuden Coilcraft Sumida CooperBussmann DCR HEIGHT NP035B-4R7M 4.7μH 1.2A 0.047Ω 1.8mm NP035B-6R8M 6.8μH 1.0A 0.084Ω 1.8mm MSS6132-472ML 4.7μH 1.8A 0.056Ω 3.2mm MSS6132-822ML 8.2μH 1.35A 0.070Ω 3.2mm CDRH2D14NP4R7N 4.7μH 1.0A 0.135Ω 1.55mm CDRH2D18/ HPNP-4R7N 4.7μH 1.2A 0.110Ω 2.0mm CDRH3D16NP4R7N 4.7μH 0.9A 0.08Ω 1.8mm SD18-4R7 4.7μH 1.54A 0.082Ω 1.8mm SD10-4R7 4.7μH 1.08A 0.153Ω 1.0mm Capacitor Vendor Information Both the input and output capacitors used with the LTC3522 must be low ESR and designed to handle the large AC currents generated by switching converters. The vendors in Table 5 provide capacitors that are well suited to LTC3522 application circuits. Table 5. Capacitor Vendor Information MANUFACTURER WEB SITE REPRESENTATIVE PART NUMBERS Taiyo Yuden www.t-yuden.com JMK107BJ105MA 4.7μF, 6.3V TDK www.component. tdk.com C2012X5R0J475K 4.7μF, 6.3V Murata www.murata.com GRM219R61A475K 4.7μF AVX www.avxcorp.com SM055C475KHN480 4.7μF PCB Layout Considerations The LTC3522 switches large currents at high frequencies. Special care should be given to the PCB layout to ensure stable, noise-free operation. Figure 5 depicts the recommended PCB layout to be utilized for the LTC3522. A few key guidelines follow: 1. All circulating high current paths should be kept as short as possible. This can be accomplished by keeping the routes to all bold components in Figure 5 as short and as wide as possible. Capacitor ground connections should via down to the ground plane in the shortest route possible. The bypass capacitors on PVIN1 and PVIN2 should be placed as close to the IC as possible and should have the shortest possible paths to ground. 2. The small-signal ground pad (GND) should have a single point connection to the power ground. A convenient way to achieve this is to short the pin directly to the Exposed Pad as shown in Figure 5. 3. The components shown in bold and their connections should all be placed over a complete ground plane. 4. To prevent large circulating currents from disrupting the output voltage sensing, the ground for each resistor divider should be returned directly to the small signal ground pin (GND). 5. Use of vias in the die attach pad will enhance the thermal environment of the converter especially if the vias extend to a ground plane region on the exposed bottom surface of the PCB. 6. Keep the connection from the resistor dividers to the feedback pins FB1 and FB2 as short as possible and away from the switch pin connections. 3522f 16 LTC3522 APPLICATIONS INFORMATION KELVIN TO VOUT PAD BUCK VOUT UNINTERRUPTED GROUND PLANE MUST EXIST UNDER ALL COMPONENTS SHOWN IN BOLD AND UNDER TRACES CONNECTING TO THOSE COMPONENTS PGND1 (14) SW2 (14) SW1A (11) GND (3) SW1B (10) PGOOD2 (4) PGND2 (9) MINIMIZE TRACE LENGTH BUCK-BOOST VOUT KELVIN TO VOUT PAD VIA TO GROUND PLANE PVIN1 (8) PWM (2) SHDN1 (7) VOUT1 (12) PGOOD1 (6) FB2 (1) FB1 (5) DIRECT TIE BACK TO GND PIN SHDN2 (16) MINIMIZE TRACE LENGTH PVIN2 (15) VIA TO GROUND PLANE 3522 F05 Figure 5. LTC3522 Recommended PCB Layout 3522f 17 LTC3522 TYPICAL APPLICATION Li-Ion to 3V at 400mA and 1.2V at 200mA VIN 2.4V TO 4.2V + C3 4.7μF Li-Ion L1 6.8μH VOUT2 1.2V 200mA 499k COMBINED PGOOD OUTPUT C1 10μF 12pF L2 4.7μH PVIN1 PVIN2 SW1A SW2 102k FB2 SW1B LTC3522 VOUT1 1M 100k VOUT1 3V 300mA C2 4.7μF (400mA, VIN > 3V) FB1 PGOOD2 SHDN2 PGOOD1 SHDN1 ON 499k OFF PWM BURST C1: MURATA GRM219R61A475K (0805 PACKAGE) C2, C3: MURATA GRM21BR60J106K (0805 PACKAGE) L1: TAIYO YUDEN NPO35B-6R8M L2: TAIYO YUDEN NPO35B-4R7M PWM PGND1 GND1 PGND2 3522 TA02 Buck-Boost Converter Efficiency vs Load Current Buck Converter Efficiency vs Load Current 100 100 90 80 EFFICIENCY (%) EFFICIENCY (%) 80 Burst Mode OPERATION 90 Burst Mode OPERATION 70 60 PWM Mode 50 70 PWM Mode 60 50 40 30 VIN = 4.2V VIN = 2.7V 20 1 100 10 LOAD CURRENT (mA) 1000 3522 TA02b 40 VIN = 4.2V VIN = 2.7V 30 1 100 10 LOAD CURRENT (mA) 1000 3522 TA02c 3522f 18 LTC3522 PACKAGE DESCRIPTION UD Package 16-Lead Plastic QFN (3mm × 3mm) (Reference LTC DWG # 05-08-1691) 0.70 ±0.05 3.50 ± 0.05 1.45 ± 0.05 2.10 ± 0.05 (4 SIDES) PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 ± 0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD PIN 1 NOTCH R = 0.20 TYP OR 0.25 × 45° CHAMFER R = 0.115 TYP 0.75 ± 0.05 15 16 PIN 1 TOP MARK (NOTE 6) 0.40 ± 0.10 1 1.45 ± 0.10 (4-SIDES) 2 (UD16) QFN 0904 0.200 REF 0.00 – 0.05 NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WEED-2) 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 0.25 ± 0.05 0.50 BSC 3522f Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 19 LTC3522 TYPICAL APPLICATION 3V at 400mA and 1.8V at 200mA with Sequenced Start-Up VIN 2.4V TO 4.2V VOUT2 1.8V 200mA + C3 4.7μF Li-Ion L1 8.2μH 499k SW1A SW2 C1 6.8μF 12pF L2 4.7μH PVIN1 PVIN2 137k FB2 SW1B LTC3522 VOUT1 1M 68.1k VOUT1 3V 300mA C2 4.7μF (400mA, VIN > 3V) FB1 499k PGOOD1 PGOOD1 PWM BURST C1: TDK C3216X5R0J685M C2, C3: TAIYO YUDEN JMK212BJ106MG L1: COOPER BUSSMANN SD18-8R2 L2: COOPER BUSSMANN SD18-4R7 ON SHDN2 PGOOD2 SHDN1 PGND1 GND1 PGND2 OFF PWM 499k 3522 TA03A Sequenced Start-Up Waveforms VOUT2 1V/DIV VOUT1 2V/DIV PGOOD2 5V/DIV PGOOD1 5V/DIV 200μs/DIV 3522 TA03b RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC3410/LTC3410B 300mA (IOUT), 2.25MHz Synchronous Buck DC/DC Converter VIN: 2.5V to 5.5V, VOUT(RANGE) = 0.8V to VIN, IQ = 26μA, ISD < 1μA, SC70 Package LTC3440 600mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter VIN: 2.5V to 5.5V, VOUT(RANGE) = 2.5V to 5.5V, IQ = 25μA, ISD < 1μA, MS, DFN Packages LTC3441 600mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter VIN: 2.5V to 5.5V, VOUT(RANGE) = 2.4V to 5.25V, IQ = 25μA, ISD < 1μA, DFN Package LTC3442 1.2A (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter VIN: 2.4V to 5.5V, VOUT(RANGE) = 2.4V to 5.25V, IQ = 35μA, ISD < 1μA, DFN Package LTC3455 Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger 96% Efficiency, Seamless Transition Between Inputs, IQ = 110μA, ISD < 2μA, QFN Package LTC3456 2-Cell Multi-Output DC/DC Converter with USB Power Manager 92% Efficiency, Seamless Transition Between Inputs, IQ = 180μA, ISD < 1μA, QFN Package LTC3530 600mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter VIN: 1.8V to 5.5V, VOUT(RANGE) = 1.8V to 5.5V, IQ = 40μA, ISD < 1μA, DFN, MSOP Packages LTC3532 500mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter VIN: 2.4V to 5.5V, VOUT(RANGE) = 2.4V to 5.25V, IQ = 35μA, ISD < 1μA, DFN, MSOP Packages LTC3544/LTC3544B 300mA, 200mA ×2, 100mA, 2.25MHz Quad Output Synchronous Step-Down DC/DC Converter VIN: 2.25V to 5.5V, VOUT(MIN) = 0.8V, IQ = 70μA, ISD < 1μA, 3mm × 3mm QFN Packages 3522f 20 Linear Technology Corporation LT 0507 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2007