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LTC3538 800mA Synchronous Buck-Boost DC/DC Converter FEATURES

DESCRIPTION
The LTC(R)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 (HDD) applications with its 800mA current capability. Burst Mode(R) operation provides high efficiency at light loads. The LTC3538 includes two 0.17 N-channel and two 0.2 P-channel MOSFET switches. Operating frequency is internally set to 1MHz to minimize solution footprint while maximizing efficiency. Other features include <5A 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 x 3mm) DFN 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 5481178, 6304066, 6580258, 6166527, 6404251.

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 35A Quiesecent Current in Burst Mode Operation <5A Shutdown Current Internal Soft-Start Small, Thermally Enhanced 8-Lead (2mm x 3mm) DFN package
APPLICATIONS

Miniature Hard Disk Drives MP3 Players Digital Cameras Cellular Handsets PDAs, Handheld PC GPS Receivers
TYPICAL APPLICATION
Li-Ion/Polymer to 3.3V at 800mA
L1 3.3H VOUT 3.3V 800mA
Efficiency vs VIN
100 VOUT = 3.3V ILOAD = 200mA
LTC3538 SW1 VIN 2.9V TO 4.2V CIN 10F PWM BURST BURST GND ON OFF SD VIN SW2 VOUT FB VC 330pF
33pF COUT 22F 15k
EFFICIENCY (%)
R1 464k
10k
95
90
85
*
R2 200k
3538 TA01
80 2.4
2.9
3.4
4.4 3.9 VIN (V)
4.9
5.4
3538 TA01b
*P OPEN DRAIN I/O
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LTC3538 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW FB 1 VC 2 GND 3 BURST 4 9 8 VIN 7 SW1 6 SW2 5 VOUT
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)............... -40C to 85C Maximum Junction Temperature (Note 3)............. 125C Storage Temperature Range................... -65C to 125C
DCB PACKAGE 8-LEAD (2mm x 3mm) PLASTIC DFN TJMAX = 125C JA = 75C/W 4-LAYER BOARD, JC = 13.5C/W EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LTC3538EDCB#PBF LEAD BASED FINISH LTC3538EDCB TAPE AND REEL LTC3538EDCB#TRPBF TAPE AND REEL LTC3538EDCB#TR PART MARKING LCRB PART MARKING LCRB PACKAGE DESCRIPTION 8-Lead (2mm x 3mm) Plastic DFN PACKAGE DESCRIPTION 8-Lead (2mm x 3mm) Plastic DFN TEMPERATURE RANGE -40C to 85C TEMPERATURE RANGE -40C to 85C
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
PARAMETER Input Voltage Feedback Voltage Feedback Input Current VIN Quiescent Current - Shutdown VIN Quiescent Current - Active VIN Quiescent Current - Sleep NMOS Switch Leakage PMOS Switch Leakage NMOS Switch On-Resistance PMOS Switch On-Resistance Input Current Limit Reverse Current Limit Burst Mode Operational Peak Current Maximum Duty Cycle (Note 4) (Note 4) CONDITIONS
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = VOUT = 3.6V, BURST = 0V, unless otherwise noted.
MIN

TYP 1.00 1 1.5 1 35 0.1 0.1 0.17 0.2
MAX 5.5 1.020 50 5 1.8 60 7 10
UNITS V V nA A mA A A A A A A % % %
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2.4 0.980
VC = 0V, Not Including Switch Leakage FB = 0.8V FB = 1.2V, BURST = VIN Switches B and C Switches A and D Switches B and C Switches A and D
1.4
2 0.5 0.9
Boost (%Switch C On) Buck (% Switch A On) Buck (% Switch D On)

70 100 100
88
2
LTC3538
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = VOUT = 3.6V, unless otherwise noted.
PARAMETER Minimum Duty Cycle Frequency Accuracy Internal Soft-Start Time Error Amp AVOL Error Amp Source Current Error Amp Sink Current VC Shutdown Threshold (Off) VC Output Current in Shutdown BURST Threshold (High) BURST Threshold (Low) BURST Input Current VBURST = 3.6V VC = 1.5V, FB = OV VC = 1.5V, FB = 1.2V IC is Disabled VC = GND

ELECTRICAL CHARACTERISTICS
CONDITIONS FB = 1.2V
MIN

TYP 1 1.5 80 -13 130
MAX 0 1.2
UNITS % MHz ms dB A A
0.8
0.25 -1 1.4 0.4 0.1 1 -3
V A V V A
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 0C to 85C. Specifications over -40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls.
Note 3: This IC includes over-temperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125C 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
Li-Ion to 3.3V Efficiency
100 90 80 EFFICIENCY (%) EFFICIENCY (%) 70 60 50 40 30 20 10 0.1 1 VIN = 2.7V VIN = 3.6V VIN = 4.2V 10 100 LOAD CURRENT (mA) 1000
3538 G01
TA = 25C unless otherwise noted Switch Pins Before Entering Boost Mode
1000 SW1 2V/DIV
Efficiency and Power Loss vs Load Current
100 90 80 70 60 50 40 30 20 10 0 0.1 1 10 100 LOAD CURRENT (mA) 0.1 1000
3538 G02
FIXED FREQUENCY
Burst Mode OPERATION
100 POWER LOSS (mW) SW2 2V/DIV 50ns/DIV VIN = 2.9V VOUT = 3.3V AT 500mA
3538 G03
Burst Mode OPERATION
10 POWER LOSS FIXED FREQUENCY 1 POWER LOSS BURST
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LTC3538 TYPICAL PERFORMANCE CHARACTERISTICS
Switch Pins Before Entering Buck Mode
VIN = 2.5V Burst Mode SLEEP CURRENT (A) SW1 2V/DIV VIN = 3.3V SW2 2V/DIV VIN = 4.2V 50ns/DIV VIN = 3.9V VOUT = 3.3V AT 500mA
3538 G04
TA = 25C unless otherwise noted Burst Mode Sleep Current vs Temperature
45
VOUT Ripple in Buck, Buck-Boost and Boost Modes at 500mA Load
40
35
1s/DIV VOUT = 3.3V, AC-COUPLED 20mV/DIV COUT = 22F ILOAD = 500mA
3538 G05
30
25
20 -50
-25
0 25 50 TEMPERATURE (C)
75
100
3538 G16
Error Amplifier Source Current vs Temperature
-12.5 OSCILLATOR FREQUENCY (kHz) 1025
Oscillator Frequency vs Temperature
VIN = VOUT = 3.6V 1.010
Feedback Voltage vs Temperature
VC SOURCE CURRENT (A)
-13.0
1.005 FB VOLTAGE (V)
-13.5
1000
1.000
-14.0
0.995
-14.5
-15.0 -50
-25
0 25 50 TEMPERATURE (C)
75
100
3538 G06
975 -50
-25
0 25 50 TEMPERATURE (C)
75
100
3538 G07
0.990 -50
-25
0 25 50 TEMPERATURE (C)
75
100
3538 G08
Maximum Output Current Capability vs VIN
1800 OUTPUT CURRENT CAPABILITY (mA) 1600 FB LINE REGULATION (%) 1400 1200 1000 800 600 400 200 0 2.0 2.5 3.0 3.5 4.0 VIN (V) 4.5 5.0 5.5
3538 G17
Feedback Voltage Line Regulation
0.4 0.3 0.2 0.1 0 VIN START VOLTAGE (V) VOUT = 3.3V 2.3045
2.3040 2.3035 2.3030 2.3025 2.3020 2.3015 2.3010
Minimum Start-Up Voltage
VOUT = 3.3V
-0.1 -0.2 2.4 3.4 VIN (V) 4.4 5.4
3538 G09
2.3005 -50
-25
0 25 50 TEMPERATURE (C)
75
100
3538 G10
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LTC3538 TYPICAL PERFORMANCE CHARACTERISTICS
VC On/Off Threshold vs Temperature
0.80 0.75 VC ON/OFF THRESHOLD (V) 0.70 0.65 0.60 0.55 VC OFF THRESHOLD 0.50 0.45 0.40 -50 0 50 TEMPERATURE (C) 100
3538 G10
TA = 25C unless otherwise noted Load Transient in Fixed Frequency Mode
VOUT 100mV/DIV
Current Limit vs Temperature
4.0 VIN = VOUT = 3.6V PEAK CURRENT LIMIT ILOAD 200mA/DIV
3.5 VC ON THRESHOLD CURRENT LIMIT (A)
3.0
2.5 100s/DIV 2.0 LINEAR CURRENT LIMIT 1.5 -50 -25 0 25 50 TEMPERATURE (C) 75 100
3538 G12 3538 G13
VIN = 3.3V VOUT = 3.3V ILOAD = 0mA TO 500mA COUT = 22F X5R CERAMIC
Burst Mode Operation
BURST 2V/DIV
Transition From Burst Mode Operation to Fixed Frequency
VOUT 50mV/DIV
IL 500mA/DIV 10s/DIV VIN = 3.3V VOUT = 3.3V ILOAD = 10mA COUT = 22F X5R CERAMIC
3538 G14
VOUT 100mV/DIV
50s/DIV VIN = 3.3V VOUT = 3.3V ILOAD = 30mA COUT = 22F X5R CERAMIC
3538 G15
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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 (4.7F 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.
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LTC3538 BLOCK DIAGRAM
L1 SW1 7 ANTI-RING VIN VIN 2.4V TO 5.5V A GATE DRIVERS AND ANTICROSS CONDUCTION D VOUT 5 VOUT 6 SW2
+
CIN
8
B
C
0.5A
-+
REVERSE CURRENT LIMIT
R1
2A
+ - + - + -
AVERAGE CURRENT LIMIT
CZ1
COUT
UVLO
2.3V
THERMAL SHUTDOWN OSC 1MHz BURST 4 1 = BurstMode OPERATION 0 = FIXED FREQUENCY BURST 5s DELAY BURST MODE CONTROL SLEEP SS DONE TSD UVLO INTERNAL SOFT-START
FB 3
GND
3538 BD
+ -
PWM COMPARATORS
SOFT-START VC 2
- +
3.5A
PWM LOGIC AND OUTPUT PHASING
+ -
PEAK CURRENT LIMIT
1V 1
FB CP1 CP2 RZ R2
OFF ON
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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 1.5A 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 8 VOUT 5
PMOS A SW1 7 L1 SW2 6
PMOS D
NMOS B
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.
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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 A ON, B OFF PWM C, D SWITCHES DMIN BOOST DMAX BUCK BOOST REGION BUCK-BOOST REGION BUCK REGION V1 (~1.2V)
3538 F02
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
V4 (~2.2V)
V3 (~1.8V) V2 (~1.7V)
FOUR-SWITCH PWM D ON, C OFF PWM A, B SWITCHES
0% DUTY CYCLE
CONTROL VOLTAGE, VC
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 35A. 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).
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.
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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 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 10H. 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: VP-P, BOOST = ILOAD * (VOUT - VIN)/(COUT * VOUT * f)V VP-P,BUCK = (VIN - VOUT) * VOUT/(8 * L * VIN * COUT * f2)V where COUT = output filter capacitor, F ILOAD = Output load current, A A
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 H. IL,P-P,BUCK = VOUT * ( VIN - VOUT ) / VIN f *L VOUT * ( VOUT - VIN ) / VOUT f *L A
IL,P-P,BOOST =
where f = frequency (1MHz typical), Hz L = inductor, H
Table 1. Inductor Vendor Information
SUPPLIER Coilcraft CoEv Magnetics Murata Sumida TDK TOKO PHONE (847) 639-6400 (800) 227-7040 (814) 237-1431 (800) 831-9172 USA: (847) 956-0666 Japan: 81 (3) 3607-5111 (847) 803-6100 (847) 297-0070 FAX (847) 639-1469 (650) 361-2508 (814) 238-0490 USA: (847) 956-0702 Japan: 81(3) 3607-5144 (847) 803-6296 (847) 699-7864 WEB SITE www.coilcraft.com www.tycoelectronics.com www.murata.com www.sumida.com www.component.tdk.com www.tokoam.com
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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. Input Capacitor Selection Since VIN is the supply voltage for the IC it is recommended to place at least a 4.7F low ESR ceramic bypass capaci, tor close to VIN and GND. It is also important to minimize any stray resistance from the converter to the battery or other power source. Optional Schottky Diodes 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 Sanyo Taiyo Yuden TDK FAX WEB SITE (803) 448-9411 (803) 448-1943 www.avxcorp.com (619) 661-6322 (619) 661-1055 www.sanyovideo.com (408) 573-4150 (408) 573-4159 www.t-yuden.com (847) 803-6100 (847) 803-6296 www.component.tdk.com
importantly, leakage and parasitic capacitance need to be minimized. During start-up, 1.5A 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. PCB Layout Considerations 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: 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. 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.
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 13A typically when active and 1.5A in shutdown, a relatively high resistance pulldown device can be used to pull VC below 0.25V. More
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LTC3538 OPERATION
FILTER _ POLE =
1 FB 8 VIN
VIN 2* VOUT * * L * COUT
Hz
(in boost mode) where L is in Henries and COUT is in Farads. The output filter zero is given by: 1 FILTER _ ZERO = Hz 2* *RESR * COUT
VOUT
2 VC
7 SW1
3 GND
6 SW2
4 BURST
5 VOUT
where RESR is the equivalent series resistance of the output capacitor. A troublesome feature in boost mode is the right-half plane zero (RHP), given by: VIN 2 RHPZ = Hz 2* *IOUT *L * VOUT The loop gain is typically rolled off before the RHP zero frequency. 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 UG = Hz 2* *R1* CP1
3538 F03
VIA TO GND PLANE
Figure 3. LTC3538 Recommended PCB Layout
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: FILTER _ POLE = (in buck mode) 1 Hz 2* * L * COUT
VOUT
+ -
1V R1 FB 1 R2 VC 2 CP1
3538 F04
Figure 4. Error Amplifier with Type I Compensation
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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 POLE1 Hz 2* * 32e3 *R1* CP1 (which is extremly close to DC) 1 Hz 2* *R Z * CP1 1 ZERO2 = Hz 2* *R1* CZ1 1 POLE2 = Hz 2* *R Z * CP2 ZERO1 = where resistance is in Ohms and capacitance is in Farads.
VOUT
+ -
1V FB 1
R1
CZ1
CP2
R2
VC 2
RZ
CP1
3538 F05
Figure 5. Error Amplifier with Type III Compensation
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LTC3538 TYPICAL APPLICATION
High Efficiency 5V/500mA from USB Input
L1 3.3H VOUT 5V, 500mA LTC3538 SW1 USB 4.35V TO 5.25V CIN 10F PWM BURST BURST GND VIN SW2 VOUT FB VC 330pF M1 15k R2 200k R1 806k 10k 33pF COUT 22F
1
ON OFF
3538 TA03
CIN: TAIYO YUDEN JMK212BJ106MG COUT: TAIYO YUDEN JMK325BJ226MM L1: SUMIDA CDRH2D18/HP-3R3NC M1: P OPEN DRAIN I/O OR FAIRCHILD FDV301N
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LTC3538 PACKAGE DESCRIPTION
DCB Package 8-Lead Plastic DFN (2mm x 3mm)
(Reference LTC DWG # 05-08-1718 Rev A)
0.70 0.05
3.50 0.05 2.10 0.05
1.35 0.05 1.65 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 3.00 0.10 (2 SIDES) PIN 1 BAR TOP MARK (SEE NOTE 6) 4 0.200 REF 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 1 0.23 0.05 0.45 BSC 1.65 0.10 PIN 1 NOTCH R = 0.20 OR 0.25 x 45 CHAMFER
(DCB8) DFN 0106 REV A
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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 LTC3407 LTC3410 LTC3411 LTC3412 LTC3421 LTC3422 LTC3425 LTC3427 LTC3429 LTC3440 LTC3441/LTC3443 LTC3442 LTC3522 LTC3525 LTC3526/LTC3526B LTC3530 LTC3531 LTC3532 LTC3533 DESCRIPTION 600mA (IOUT), 1.5MHz Dual Synchronous Step-Up DC/DC Converter COMMENTS VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V IQ = 40A, ISD 1A, SC70 Package
300mA (ISW), 2.25MHz Synchronous Step-Down DC/DC Converter in SC70 VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V IQ = 26A, ISD 1A, MS Package 1.25A (IOUT), 4MHz Synchronous Step-Down DC/DC Converter 2.5A (IOUT), 4MHz Synchronous Step-Down DC/DC Converter 3A (ISW), 3MHz Synchronous Step-Up DC/DC Converter 1.5A (ISW), 3MHz Synchronous Step-Up DC/DC Converter with Output Disconnect 5A (ISW), 8MHz Multiphase Synchronous Step-Up DC/DC Converter 500mA (ISW), 1.25MHz Step-Up DC/DC Converter with Output Disconnect in 2mm x 2mm DFN 600mA (ISW), 500KHz Synchronous Step-Up DC/DC Converter 600mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter 1.2A (IOUT), Synchronous Buck-Boost DC/DC Converters, LTC3441(1MHz), LTC3443 (600kHz) 1.2A (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter 400mA, Synchronous Buck-Boost and 200mA Buck Converters 400mA (ISW), Synchronous Step-Up DC/DC Converter with Output Disconnect 500mA (ISW), 1MHz Synchronous Step-Up DC/DC Converter with Output Disconnect in 2mm x 2mm DFN 600mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter 200mA (IOUT) Synchronous Buck-Boost DC/DC Converter 500mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter 2A (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter VIN: 2.625V to 5.5V, VOUT(MIN) = 0.8V IQ = 62A, ISD 1A, MS Package VIN: 2.625V to 5.5V, VOUT(MIN) = 0.8V IQ = 62A, ISD 1A, TSSOP16E Package VIN: 0.5V to 4.5V, VOUT(MAX) = 5.25V IQ = 12A, ISD <1A, QFN Package VIN: 0.5V to 4.5V, VOUT(MAX) = 5.25V IQ = 25A, ISD <1A, DFN Package VIN: 0.5V to 4.5V, VOUT(MAX) = 5.25V IQ = 12A, ISD <1A, QFN Package VIN: 1.8V to 5V, VOUT(MAX) = 5.25V, IQ = 350A, ISD <1A, DFN Package VIN: 0.5V to 4.4V, VOUT(MAX) = 5V IQ = 20A, ISD <1A, ThinSOTTM Package VIN: 2.5V to 5.5V, VOUT: 2.5V to 5.5V IQ = 25A, ISD <1A, MS, DFN Package VIN: 2.5V to 5.5V, VOUT: 2.4V to 5.25V IQ = 25A, ISD <1A, DFN Package VIN: 2.4V to 5.5V, VOUT: 2.4V to 5.25V IQ = 28A, ISD <1A, MS Package VIN: 2.4V to 5.5V, VOUT Buck-Boost: 2.2V to 5.25V, IQ = 25A, ISD <1A, DFN Package VIN: 0.5V to 4.5V, VOUT = 3, 3.3, 5V IQ = 7A, ISD <1A, SC70 Package VIN: 0.5V to 4.5V, VOUT: 1.6V to 5.25V IQ = 9A, ISD <1A, DFN Package VIN: 1.8V to 5.5V, VOUT: 1.6V to 5.25V IQ = 40A, ISD <1A, DFN, MS Packages VIN: 1.8V to 5.5V, VOUT: 2V to 5V IQ = 16A, ISD <1A, DFN, ThinSOT Packages VIN: 2.4V to 5.5V, VOUT: 2.2V to 5.25V IQ = 35A, ISD <1A, DFN, MS Packages VIN: 1.8V to 5.5V, VOUT: 1.6V to 5.25V IQ = 40A, ISD <1A, DFN Package
ThinSOT is a trademark of Linear Technology Corporation.
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16 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
LT 1007 REV B * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2007

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