LTC3538 800mA Synchronous Buck-Boost DC/DC Converter FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTION Regulated Output with Input Voltages Above, Below, or Equal to the Output 800mA Continuous Output Current from a Single Lithium-Ion/Polymer Cell Single Inductor 1.8V to 5.25V VOUT Range 2.4V to 5.5V VIN Range 1MHz Fixed Frequency Operation Output Disconnect in Shutdown 35μA Quiesecent Current in Burst Mode Operation <5μA Shutdown Current Internal Soft-Start Small, Thermally Enhanced 8-Lead (2mm x 3mm) DFN package The LTC®3538 is a highly efficient, low noise, buck-boost DC/DC converter that operates from input voltages above, below, and equal to the output voltage. The topology incorporated in the IC provides a continuous transfer function through all operating modes, making the product ideal for single Lithium Ion or multicell Alkaline or NiMH applications where the output voltage is within the battery voltage range. The LTC3538 is suited for use in Micro Hard Disk Drive (μHDD) applications with its 800mA current capability. Burst Mode® operation provides high efficiency at light loads. The LTC3538 includes two 0.17Ω N-channel and two 0.2Ω P-channel MOSFET switches. Operating frequency is internally set to 1MHz to minimize solution footprint while maximizing efficiency. APPLICATIONS ■ ■ ■ ■ ■ ■ Other features include <5μA shutdown current, internal soft-start, short circuit protection and thermal shutdown. The LTC3538 is available in a low profile (0.75mm), thermally enhanced 8-lead (2mm × 3mm) DFN package. Miniature Hard Disk Drives MP3 Players Digital Cameras Cellular Handsets PDAs, Handheld PC 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, 6304066, 6580258, 6166527, 6404251. TYPICAL APPLICATION Li-Ion/Polymer to 3.3V at 800mA Efficiency vs VIN 100 L1 3.3μH VIN 2.9V TO 4.2V SW2 VIN VOUT CIN 10μF PWM R1 464k 10k 33pF COUT 22μF FB BURST BURST VC GND ON OFF 330pF SD * 90 85 15k R2 200k 80 2.4 3538 TA01 *μP OPEN DRAIN I/O 95 EFFICIENCY (%) LTC3538 SW1 VOUT 3.3V 800mA VOUT = 3.3V ILOAD = 200mA 2.9 3.4 4.4 3.9 VIN (V) 4.9 5.4 3538 TA01b 3538fb 1 LTC3538 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) VIN,VOUT Voltage .......................................... –0.3V to 6V SW1,SW2 Voltage DC............................................................ –0.3V to 6V Pulsed < 100ns ........................................ –0.3V to 7V BURST, FB, VC Voltage ................................. –0.3V to 6V Operating Temperature (Note 2)............... –40°C to 85°C Maximum Junction Temperature (Note 3)............. 125°C Storage Temperature Range................... –65°C to 125°C TOP VIEW 8 VIN FB 1 VC 2 9 7 SW1 GND 3 6 SW2 BURST 4 5 VOUT DCB PACKAGE 8-LEAD (2mm × 3mm) PLASTIC DFN TJMAX = 125°C θJA = 75°C/W 4-LAYER BOARD, θJC = 13.5°C/W EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LTC3538EDCB#PBF LTC3538EDCB#TRPBF LCRB 8-Lead (2mm × 3mm) Plastic DFN –40°C to 85°C LEAD BASED FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LTC3538EDCB LTC3538EDCB#TR LCRB 8-Lead (2mm × 3mm) Plastic DFN –40°C to 85°C Consult LTC Marketing for parts specified with wider operating temperature ranges. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = VOUT = 3.6V, BURST = 0V, unless otherwise noted. PARAMETER CONDITIONS Input Voltage Feedback Voltage (Note 4) Feedback Input Current (Note 4) VIN Quiescent Current – Shutdown VC = 0V, Not Including Switch Leakage VIN Quiescent Current – Active MIN ● 2.4 ● 0.980 TYP MAX UNITS 5.5 V 1.00 1.020 V 1 50 nA 1.5 5 μA FB = 0.8V 1 1.8 mA VIN Quiescent Current – Sleep FB = 1.2V, BURST = VIN 35 60 μA NMOS Switch Leakage Switches B and C 0.1 7 μA PMOS Switch Leakage Switches A and D 0.1 10 μA NMOS Switch On-Resistance Switches B and C 0.17 Ω PMOS Switch On-Resistance Switches A and D 0.2 Ω 2 A 0.5 A ● Input Current Limit 1.4 Reverse Current Limit Burst Mode Operational Peak Current Maximum Duty Cycle Boost (%Switch C On) Buck (% Switch A On) Buck (% Switch D On) ● ● ● 70 100 100 0.9 A 88 % % % 3538fb 2 LTC3538 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = VOUT = 3.6V, unless otherwise noted. PARAMETER CONDITIONS Minimum Duty Cycle FB = 1.2V MIN TYP MAX ● ● Frequency Accuracy 0 0.8 1 1.2 UNITS % MHz Internal Soft-Start Time 1.5 ms Error Amp AVOL 80 dB Error Amp Source Current VC = 1.5V, FB = OV –13 μA Error Amp Sink Current VC = 1.5V, FB = 1.2V 130 μA VC Shutdown Threshold (Off) IC is Disabled VC Output Current in Shutdown VC = GND ● ● BURST Threshold (Low) ● VBURST = 3.6V 80 80 60 50 EFFICIENCY (%) 70 Burst Mode OPERATION 40 VIN = 2.7V VIN = 3.6V VIN = 4.2V 20 1 10 100 LOAD CURRENT (mA) 1000 3538 G01 V 1 μA 1000 FIXED FREQUENCY SW1 2V/DIV 100 Burst Mode OPERATION 60 10 50 40 POWER LOSS FIXED FREQUENCY 30 30 10 0.1 70 0.4 Switch Pins Before Entering Boost Mode 1 20 POWER LOSS (mW) EFFICIENCY (%) 90 90 V TA = 25°C unless otherwise noted Efficiency and Power Loss vs Load Current 100 μA Note 3: This IC includes over-temperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125°C when over-temperature protection is active. Continuous operation above the specified maximum operating junction temperature may result in device degradation or failure. Note 4: The IC is tested in a feedback loop to make the measurement. TYPICAL PERFORMANCE CHARACTERISTICS 100 –3 1.4 0.1 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 LTC3538 is guaranteed to meet performance specifications from 0°C to 85°C. Specifications over –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. Li-Ion to 3.3V Efficiency V –1 BURST Threshold (High) BURST Input Current 0.25 SW2 2V/DIV 50ns/DIV VIN = 2.9V VOUT = 3.3V AT 500mA 3538 G03 POWER LOSS BURST 10 0 0.1 1 10 100 LOAD CURRENT (mA) 0.1 1000 3538 G02 3538fb 3 LTC3538 TYPICAL PERFORMANCE CHARACTERISTICS Switch Pins Before Entering Buck Mode TA = 25°C unless otherwise noted VOUT Ripple in Buck, Buck-Boost and Boost Modes at 500mA Load Burst Mode Sleep Current vs Temperature 45 VIN = 2.5V Burst Mode SLEEP CURRENT (μA) SW1 2V/DIV VIN = 3.3V SW2 2V/DIV VIN = 4.2V 3538 G04 50ns/DIV VIN = 3.9V VOUT = 3.3V AT 500mA 3538 G05 1μs/DIV VOUT = 3.3V, AC-COUPLED 20mV/DIV COUT = 22μF ILOAD = 500mA 40 35 30 25 20 –50 –25 0 25 50 TEMPERATURE (°C) 75 100 3538 G16 Error Amplifier Source Current vs Temperature Oscillator Frequency vs Temperature 1025 –13.0 –13.5 –14.0 –14.5 –15.0 –50 –25 0 25 50 TEMPERATURE (°C) 75 VIN = VOUT = 3.6V 1.005 1000 975 –50 100 –25 0 25 50 TEMPERATURE (°C) 0.990 –50 100 0.4 VOUT = 3.3V 1600 1000 800 600 400 VOUT = 3.3V 0.2 0.1 0 –0.2 3.0 3.5 4.0 VIN (V) 4.5 5.0 5.5 2.4 3.4 4.4 VIN (V) 3538 G17 2.3035 2.3030 2.3025 2.3020 2.3015 2.3010 200 2.5 100 2.3040 –0.1 0 75 Minimum Start-Up Voltage VIN START VOLTAGE (V) 1200 0 25 50 TEMPERATURE (°C) 2.3045 0.3 1400 –25 3538 G08 Feedback Voltage Line Regulation FB LINE REGULATION (%) OUTPUT CURRENT CAPABILITY (mA) 75 3538 G07 Maximum Output Current Capability vs VIN 2.0 1.000 0.995 3538 G06 1800 Feedback Voltage vs Temperature 1.010 FB VOLTAGE (V) OSCILLATOR FREQUENCY (kHz) VC SOURCE CURRENT (μA) –12.5 5.4 3538 G09 2.3005 –50 –25 0 25 50 TEMPERATURE (°C) 75 100 3538 G10 3538fb 4 LTC3538 TYPICAL PERFORMANCE CHARACTERISTICS VC On/Off Threshold vs Temperature TA = 25°C unless otherwise noted Load Transient in Fixed Frequency Mode Current Limit vs Temperature 4.0 0.80 VIN = VOUT = 3.6V VOUT 100mV/DIV 3.5 0.70 0.65 CURRENT LIMIT (A) VC ON/OFF THRESHOLD (V) 0.75 VC ON THRESHOLD 0.60 0.55 VC OFF THRESHOLD 0.50 3.0 ILOAD 200mA/DIV 2.5 100μs/DIV LINEAR CURRENT LIMIT 0 50 TEMPERATURE (°C) 100 1.5 –50 –25 0 25 50 TEMPERATURE (°C) 3538 G10 3538 G13 VIN = 3.3V VOUT = 3.3V ILOAD = 0mA TO 500mA COUT = 22μF X5R CERAMIC 2.0 0.45 0.40 –50 PEAK CURRENT LIMIT 75 100 3538 G12 Transition From Burst Mode Operation to Fixed Frequency Burst Mode Operation BURST 2V/DIV VOUT 50mV/DIV VOUT 100mV/DIV IL 500mA/DIV 10μs/DIV VIN = 3.3V VOUT = 3.3V ILOAD = 10mA COUT = 22μF X5R CERAMIC 3538 G14 50μs/DIV VIN = 3.3V VOUT = 3.3V ILOAD = 30mA COUT = 22μF X5R CERAMIC 3538 G15 3538fb 5 LTC3538 PIN FUNCTIONS FB (Pin 1): Feedback Input to Error Amplifer. Connect resistive divider tap from VOUT to this pin to set the output voltage. The output voltage can be adjusted from 1.8V to 5.25V. Referring to the Block Diagram the output voltage is given by: VOUT = 1V • (1 + R1/R2) VC (Pin 2): Error Amplifier Output. A frequency compensation network should be connected between this pin and FB to compensate the loop. See Closing the Feedback Loop section of the datasheet for further information. Pulling VC below 0.25V disables the LTC3538. GND (Pin 3): Ground. BURST (Pin 4): Burst Mode Select Input. BURST = Low for fixed frequency PWM operation BURST = High for Burst Mode operation VOUT (Pin 5): Power Supply Output. 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 GND. SW2 (Pin 6): Switch Pin where the Internal Switches C and D are Connected. An optional Schottky diode can be connected from SW2 to VOUT for a moderate efficiency improvement. Keep the trace length as short as possible to minimize EMI. SW1 (Pin 7): Switch Pin where the Internal Switches A and B are Connected. Connect an inductor from SW1 to SW2. An optional Schottky diode can be connected from SW1 to ground for a moderate efficiency improvement. Keep the trace length as short as possible to minimize EMI. VIN (Pin 8): Input Supply. This input provides power to the IC and also supplies current to switch A. A ceramic bypass capacitor (4.7μF or larger) is recommended as close to VIN and GND as possible. Exposed Pad (Pin 9): GND. The exposed pad must be electrically connected to the board ground for proper electrical and thermal performance. 3538fb 6 LTC3538 BLOCK DIAGRAM L1 SW1 SW2 7 6 ANTI-RING GATE DRIVERS AND ANTICROSS CONDUCTION CIN B 3.5A + – 2.3V + – C 0.5A – + R1 REVERSE CURRENT LIMIT AVERAGE CURRENT LIMIT PEAK CURRENT LIMIT PWM LOGIC AND OUTPUT PHASING PWM COMPARATORS UVLO CZ1 1V FB CP1 CP2 RZ 2 OSC 1MHz OFF ON BURST 4 1 = BurstMode OPERATION 0 = FIXED FREQUENCY 5μs DELAY R2 VC THERMAL SHUTDOWN BURST MODE CONTROL COUT 1 SOFT-START BURST VOUT 5 + – 2A + – VOUT D + – VIN 2.4V TO 5.5V A 8 + – + VIN TSD UVLO SLEEP INTERNAL SOFT-START SS DONE FB 3 GND 3538 BD 3538fb 7 LTC3538 OPERATION The LTC3538 provides high efficiency, low noise power for a wide variety of handheld electronic devices. The LTC proprietary topology allows input voltages above, below and equal to the output voltage through proper phasing of the four on-chip MOSFET switches. The error amplifier output voltage on VC determines the output duty cycle of the switches. Since VC is a filtered signal, it provides rejection of frequencies from well below the switching frequency. The low RDS(ON), low gate charge synchronous switches provide high frequency pulse width modulation control at high efficiency. High efficiency is achieved at light loads when Burst Mode operation is selected. LOW NOISE FIXED FREQUENCY OPERATION Operating Frequency The operating frequency is internally fixed to 1MHz to maximize overall converter efficiency while minimizing external component size. Error Amplifier The error amplifier controls the duty cycle of the internal switches. The loop compensation components are configured around the amplifier to provide converter loop stability. Pulling down the output of the error amplifier (VC) below 0.25V will disable the LTC3538. In shutdown the LTC3538 will draw only 1.5μA typical from the input supply. During normal operation the VC pin should be allowed to float. Soft-Start The converter has an internal voltage mode soft-start circuit with a nominal duration of 1.5ms. 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 risetime has minimal dependency on the size of the output capacitor or load. During soft-start, the converter is forced into PWM operation regardless of the state of the BURST pin. Internal Current Limit There are two current limit circuits in the LTC3538. The first is a high speed peak current limit amplifier that will shut off switch A once the input current exceeds ~ 3.5A typical. The delay to output of this amplifier is typically 50ns. The second current limit sources current out of the FB pin to drop the output voltage once the input average current exceeds 2A typical. This method provides a closed loop means of clamping the input current. During conditions when VOUT is near ground, such as during a short circuit or during start-up, this threshold is cut to 1A typical, providing a foldback feature to limit power dissipation. For this current limit feature to be most effective, the Thevenin resistance (typically the parallel combination of R1 and R2) from FB to ground should be greater than 100k. Reverse Current Limit During fixed frequency operation, the LTC3538 operates in forced continuous conduction mode. The reverse current limit comparator monitors the inductor current from the output through switch D. Should this negative inductor current exceed 500mA typical, the LTC3538 shuts off switch D. Four-Switch Control VIN VOUT 8 5 PMOS A SW1 7 NMOS B PMOS D L1 SW2 6 NMOS C 3538 FO1 Figure 1. Simplified Diagram of Output Switches Figure 1 shows a simplified diagram of how the four internal switches are connected to the inductor, VIN, VOUT and GND. Figure 2 shows the regions of operation for the LTC3538 as a function of the internal control voltage. 3538fb 8 LTC3538 OPERATION Depending on the VC voltage, the LTC3538 will operate in either buck, buck-boost or boost mode. The four power switches are properly phased so the transfer between operating modes is continuous, smooth and transparent to the user. When VIN approaches VOUT the buck-boost region is entered, where the conduction time of the four-switch region is typically 150ns. Referring to Figures 1 and 2, the various regions of operation will now be described. 88% DMAX BOOST V4 (~2.2V) A ON, B OFF PWM C, D SWITCHES DMIN BOOST DMAX BUCK BOOST REGION BUCK-BOOST REGION FOUR-SWITCH PWM D ON, C OFF PWM A, B SWITCHES V3 (~1.8V) V2 (~1.7V) BUCK REGION 0% DUTY CYCLE Buck-Boost or Four Switch (VIN ~ VOUT) When the control voltage, VC, is above voltage V2, switch pair AD remains on for duty cycle DMAX_BUCK, and the switch pair AC begins to phase in. As switch pair AC phases in, switch pair BD phases out accordingly. When VC reaches the edge of the buck-boost range, at voltage V3, the AC switch pair completely phase out the BD pair, and the boost phase begins at duty cycle D4SW. The input voltage, VIN, where the four switch region begins is given by: VIN = VOUT(1 – D4SW) ≈ 0.85 • VOUT The point at which the four-switch region ends is given by: VIN = VOUT V ≈ 1.18 • VOUT 1− D4SW V1 (~1.2V) 3538 F02 CONTROL VOLTAGE, VC Figure 2. Switch Control vs Control Voltage, VC Buck Region (VIN > VOUT) Switch D is always on and switch C is always off during this mode. When the control voltage, VC, is above voltage V1, output A begins to switch. During the off time of switch A, synchronous switch B turns on for the remainder of the period. Switches A and B will alternate similar to a typical synchronous buck regulator. As the control voltage increases, the duty cycle of switch A increases until the maximum duty cycle of the converter in buck mode reaches DMAX_BUCK, given by: DMAX_BUCK = 100 – D4SW % where D4SW = duty cycle % of the four switch range. D4SW = (150ns • f) • 100 % where f = operating frequency, Hz. Beyond this point the four switch, or buck-boost region is reached. Boost Region (VIN < VOUT) Switch A is always on and switch B is always off during this mode. When the control voltage, VC, is above voltage V3, switch pair CD will alternately switch to provide a boosted output voltage. This operation is typical to a synchronous boost regulator. The maximum duty cycle of the converter is limited to 88% typical and is reached when VC is above V4. Burst Mode OPERATION Burst Mode operation reduces quiescent current consumption of the LTC3538 at light loads and improves overall conversion efficiency, increasing battery life. During Burst Mode operation the LTC3538 delivers energy to the output until it is regulated and then goes into a sleep mode where the outputs are off and the quiescent current drops to 35μA. In this mode the output ripple has a variable frequency component that depends upon load current, and will typically be about 2% peak-to-peak. Burst Mode operation ripple can be reduced slightly by using more output capacitance. Another method of reducing Burst Mode operation ripple is to place a small feed-forward capacitor across the upper resistor in the VOUT feedback divider network (as in Type III compensation). 3538fb 9 LTC3538 OPERATION During the period when the LTC3538 is delivering energy to the output, the peak inductor current will be equal to 800mA typical and the inductor current will terminate each cycle at zero current. In Burst Mode operation the maximum average output current that can be delivered while maintaining output regulation is given by: IOUT _ BURST(BOOST) = 0.25• VIN A; VOUT > VIN VOUT IOUT _ BURST(BUCK) = 0.27A; VOUT < VIN The maximum average Burst Mode output current that can be delivered in the four-switch buck-boost region is limited to the boost equation specified above. INDUCTOR SELECTION To achieve high efficiency, a low ESR inductor should be utilized for the converter. The inductor must have a saturation rating greater than the worst case average inductor current plus half the ripple current. The peak-to-peak 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 (1MHz typical) and L is the inductance in μH. ΔIL,P-P,BUCK = VOUT • ( VIN – VOUT ) / VIN ΔIL,P-P,BOOST = f •L For high efficiency, choose a ferrite inductor with a high frequency core material to reduce core loses. The inductor should have low ESR (equivalent series resistance) to reduce the I2R losses, and must be able to handle the peak inductor current without saturating. Molded chokes or chip inductors usually do not have enough core to support the peak inductor currents in the 1A to 2A region. To minimize radiated noise, use a shielded inductor. See Table 1 for a suggested list of inductor suppliers. Output Capacitor Selection The bulk value of the output filter capacitor is selected to reduce the ripple due to charge into the capacitor each cycle. The steady state ripple due to charge is given by: ΔVP-P, BOOST = ILOAD • (VOUT – VIN)/(COUT • VOUT • f)V ΔVP-P,BUCK = (VIN – VOUT) • VOUT/(8 • L • VIN • COUT • f2)V where COUT = output filter capacitor, F A f •L VOUT • ( VOUT – VIN ) / 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 low enough to degrade the phase margin of the feedback loop. It is recommended that the inductor value be chosen less than 10μH. ILOAD = Output load current, A A where f = frequency (1MHz typical), Hz L = inductor, H Table 1. Inductor Vendor Information SUPPLIER PHONE FAX WEB SITE Coilcraft (847) 639-6400 (847) 639-1469 www.coilcraft.com CoEv Magnetics (800) 227-7040 (650) 361-2508 www.tycoelectronics.com Murata (814) 237-1431 (800) 831-9172 (814) 238-0490 www.murata.com Sumida USA: (847) 956-0666 Japan: 81 (3) 3607-5111 USA: (847) 956-0702 Japan: 81(3) 3607-5144 www.sumida.com TDK (847) 803-6100 (847) 803-6296 www.component.tdk.com TOKO (847) 297-0070 (847) 699-7864 www.tokoam.com 3538fb 10 LTC3538 OPERATION 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. Minimizing solution size is usually a priority. Please be aware that ceramic capacitors can exhibit a significant reduction in effective capacitance when a bias is applied. The capacitors exhibiting the highest reduction are those packaged in the smallest case size. importantly, leakage and parasitic capacitance need to be minimized. During start-up, 1.5μA is typically sourced from VC. The leakage of an external pull-down device and compensation components tied to VC, must therefore be minimized to ensure proper start-up. Capacitance from the pull-down device should also be minimized as it can affect converter stability. An N-channel MOSFET such as the FDV301N or similar is recommended if an external discrete N-channel MOSFET is needed. Input Capacitor Selection PCB Layout Considerations Since VIN is the supply voltage for the IC it is recommended to place at least a 4.7μF, low ESR ceramic bypass capacitor close to VIN and GND. It is also important to minimize any stray resistance from the converter to the battery or other power source. The LTC3538 switches large currents at high frequencies. Special care should be given to the PCB layout to ensure stable, noise-free operation. Figure 3 depicts the recommended PCB layout to be utilized for the LTC3538. A few key guidelines follow: Optional Schottky Diodes 1. All circulating current paths should be kept as short as possible. This can be accomplished by keeping the routes to all components (except the FB divider network) in Figure 3 as short and as wide as possible. Capacitor ground connections should via down to the ground plane in the shortest route possible. The bypass capacitor on VIN should be placed as close to the IC as possible and should have the shortest possible paths to ground. Schottky diodes across the synchronous switches B and D are not required, but do provide a lower drop during the break-before-make time (typically 15ns), thus improving efficiency. Use a surface mount Schottky diode such as an MBRM120T3 or equivalent. Do not use ordinary rectifier diodes since their slow recovery times will compromise efficiency. Table 2. Capacitor Vendor Information SUPPLIER PHONE AVX FAX WEB SITE (803) 448-9411 (803) 448-1943 www.avxcorp.com Sanyo (619) 661-6322 (619) 661-1055 www.sanyovideo.com Taiyo Yuden (408) 573-4150 (408) 573-4159 www.t-yuden.com TDK (847) 803-6100 (847) 803-6296 www.component.tdk.com Shutdown MOSFET Selection A discrete external N-channel MOSFET, open-drain pulldown device or other suitable means can be used to put the part in shutdown by pulling VC below 0.25V. Since the error amplifier sources 13μA typically when active and 1.5μA in shutdown, a relatively high resistance pulldown device can be used to pull VC below 0.25V. More 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 this pin directly to the Exposed Pad as shown in Figure 3. 3. The components 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 the resistor divider should be returned directly to the small signal ground (GND) as shown. 5. Use of vias in the 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. 3538fb 11 LTC3538 OPERATION ƒ FILTER _ POLE = VIN 2• VOUT • π • L • COUT Hz (in boost mode) 1 FB 8 VIN 2 VC 7 SW1 3 GND 6 SW2 4 BURST 5 where L is in Henries and COUT is in Farads. The output filter zero is given by: 1 ƒ FILTER _ ZERO = Hz 2• π •RESR • COUT where RESR is the equivalent series resistance of the output capacitor. VOUT VOUT A troublesome feature in boost mode is the right-half plane zero (RHP), given by: VIN 2 ƒ RHPZ = Hz 2• π •IOUT •L • VOUT 3538 F03 VIA TO GND PLANE Figure 3. LTC3538 Recommended PCB Layout The loop gain is typically rolled off before the RHP zero frequency. Closing the Feedback Loop The LTC3538 incorporates voltage mode PWM control. The control to output gain varies with operation region (buck, boost, buck-boost), but is usually no greater than 15. The output filter exhibits a double pole response, as given by: ƒ FILTER _ POLE = A simple Type I compensation network can be incorporated to stabilize the loop, but at a cost of reduced bandwidth and slower transient response. To ensure proper phase margin using Type I compensation, the loop must be crossed over a decade before the LC double pole. Referring to Figure 4, the unity-gain frequency of the error amplifier with the Type I compensation is given by: 1 ƒ UG = Hz 2• π •R1• CP1 1 Hz 2• π • L • COUT (in buck mode) VOUT + – 1V R1 FB 1 R2 VC 2 CP1 3538 F04 Figure 4. Error Amplifier with Type I Compensation 3538fb 12 LTC3538 OPERATION Most applications demand an improved transient response to allow a smaller output filter capacitor. To achieve a higher bandwidth, Type III compensation is required, providing two zeros to compensate for the double-pole response of the output filter. Referring to Figure 5, the location of the poles and zeros are given by: 1 ƒ POLE1 ≅ Hz 2• π • 32e3 •R1• CP1 (which is extremly close to DC) 1 Hz 2• π •R Z • CP1 1 ƒ ZERO2 = Hz 2• π •R1• CZ1 1 ƒ POLE2 = Hz 2• π •R Z • CP2 ƒ ZERO1 = where resistance is in Ohms and capacitance is in Farads. VOUT + – R1 1V CZ1 FB 1 R2 CP2 VC RZ CP1 2 3538 F05 Figure 5. Error Amplifier with Type III Compensation 3538fb 13 LTC3538 TYPICAL APPLICATION High Efficiency 5V/500mA from USB Input L1 3.3μH VOUT 5V, 500mA LTC3538 USB 4.35V TO 5.25V SW1 SW2 VIN VOUT CIN 10μF PWM 10k 33pF COUT 22μF FB BURST BURST VC GND ON OFF 1Ω R1 806k 330pF M1 15k R2 200k 3538 TA03 CIN: TAIYO YUDEN JMK212BJ106MG COUT: TAIYO YUDEN JMK325BJ226MM L1: SUMIDA CDRH2D18/HP-3R3NC M1: μP OPEN DRAIN I/O OR FAIRCHILD FDV301N 3538fb 14 LTC3538 PACKAGE DESCRIPTION DCB Package 8-Lead Plastic DFN (2mm × 3mm) (Reference LTC DWG # 05-08-1718 Rev A) 0.70 ±0.05 1.35 ±0.05 3.50 ±0.05 1.65 ± 0.05 2.10 ±0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.45 BSC 1.35 REF RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED R = 0.115 TYP R = 0.05 5 TYP 2.00 ±0.10 (2 SIDES) 0.40 ± 0.10 8 1.35 ±0.10 1.65 ± 0.10 3.00 ±0.10 (2 SIDES) PIN 1 NOTCH R = 0.20 OR 0.25 × 45° CHAMFER PIN 1 BAR TOP MARK (SEE NOTE 6) (DCB8) DFN 0106 REV A 4 0.200 REF 1 0.23 ± 0.05 0.45 BSC 0.75 ±0.05 1.35 REF BOTTOM VIEW—EXPOSED PAD 0.00 – 0.05 NOTE: 1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE 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 3538fb 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. 15 LTC3538 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC3407 600mA (IOUT), 1.5MHz Dual Synchronous Step-Up DC/DC Converter VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V IQ = 40μA, ISD ≤1μA, SC70 Package LTC3410 300mA (ISW), 2.25MHz Synchronous Step-Down DC/DC Converter in SC70 VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V IQ = 26μA, ISD ≤1μA, MS Package LTC3411 1.25A (IOUT), 4MHz Synchronous Step-Down DC/DC Converter VIN: 2.625V to 5.5V, VOUT(MIN) = 0.8V IQ = 62μA, ISD ≤1μA, MS Package LTC3412 2.5A (IOUT), 4MHz Synchronous Step-Down DC/DC Converter VIN: 2.625V to 5.5V, VOUT(MIN) = 0.8V IQ = 62μA, ISD ≤1μA, TSSOP16E Package LTC3421 3A (ISW), 3MHz Synchronous Step-Up DC/DC Converter VIN: 0.5V to 4.5V, VOUT(MAX) = 5.25V IQ = 12μA, ISD <1μA, QFN Package LTC3422 1.5A (ISW), 3MHz Synchronous Step-Up DC/DC Converter with Output Disconnect VIN: 0.5V to 4.5V, VOUT(MAX) = 5.25V IQ = 25μA, ISD <1μA, DFN Package LTC3425 5A (ISW), 8MHz Multiphase Synchronous Step-Up DC/DC Converter VIN: 0.5V to 4.5V, VOUT(MAX) = 5.25V IQ = 12μA, ISD <1μA, QFN Package LTC3427 500mA (ISW), 1.25MHz Step-Up DC/DC Converter with Output Disconnect in 2mm × 2mm DFN VIN: 1.8V to 5V, VOUT(MAX) = 5.25V, IQ = 350μA, ISD <1μA, DFN Package LTC3429 600mA (ISW), 500KHz Synchronous Step-Up DC/DC Converter VIN: 0.5V to 4.4V, VOUT(MAX) = 5V IQ = 20μA, ISD <1μA, ThinSOT™ Package LTC3440 600mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter VIN: 2.5V to 5.5V, VOUT: 2.5V to 5.5V IQ = 25μA, ISD <1μA, MS, DFN Package LTC3441/LTC3443 1.2A (IOUT), Synchronous Buck-Boost DC/DC Converters, LTC3441(1MHz), LTC3443 (600kHz) VIN: 2.5V to 5.5V, VOUT: 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: 2.4V to 5.25V IQ = 28μA, ISD <1μA, MS Package LTC3522 400mA, Synchronous Buck-Boost and 200mA Buck Converters VIN: 2.4V to 5.5V, VOUT Buck-Boost: 2.2V to 5.25V, IQ = 25μA, ISD <1μA, DFN Package LTC3525 400mA (ISW), Synchronous Step-Up DC/DC Converter with Output Disconnect VIN: 0.5V to 4.5V, VOUT = 3, 3.3, 5V IQ = 7μA, ISD <1μA, SC70 Package LTC3526/LTC3526B 500mA (ISW), 1MHz Synchronous Step-Up DC/DC Converter with Output Disconnect in 2mm × 2mm DFN VIN: 0.5V to 4.5V, VOUT: 1.6V to 5.25V IQ = 9μA, ISD <1μA, DFN Package LTC3530 600mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter VIN: 1.8V to 5.5V, VOUT: 1.6V to 5.25V IQ = 40μA, ISD <1μA, DFN, MS Packages LTC3531 200mA (IOUT) Synchronous Buck-Boost DC/DC Converter VIN: 1.8V to 5.5V, VOUT: 2V to 5V IQ = 16μA, ISD <1μA, DFN, ThinSOT Packages LTC3532 500mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter VIN: 2.4V to 5.5V, VOUT: 2.2V to 5.25V IQ = 35μA, ISD <1μA, DFN, MS Packages LTC3533 2A (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter VIN: 1.8V to 5.5V, VOUT: 1.6V to 5.25V IQ = 40μA, ISD <1μA, DFN Package ThinSOT is a trademark of Linear Technology Corporation. 3538fb 16 Linear Technology Corporation LT 1007 REV B • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2007