LTC3538 Datasheet by Analog Devices Inc.

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LTLII‘IM TECHNOLOGY L7 LJUW
LTC3538
1
3538fb
TYPICAL APPLICATION
FEATURES
APPLICATIONS
DESCRIPTION
The LTC
®
3538 is a highly effi cient, 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 effi ciency 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 effi ciency.
Other features include <5μA shutdown current, internal
soft-start, short circuit protection and thermal shutdown.
The LTC3538 is available in a low profi le (0.75mm), ther-
mally enhanced 8-lead (2mm × 3mm) DFN package.
Li-Ion/Polymer to 3.3V at 800mA
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
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.
SW1
R2
200k
R1
464k
15k
10k
330pF
33pF
CIN
10μF
VIN
BURST
SW2
LTC3538
VIN
2.9V TO 4.2V
L1
3.3μH
GND
VOUT
FB
SD
VC
VOUT
3.3V
800mA
COUT
22μF
PWM BURST
3538 TA01
ON OFF *
*μP OPEN DRAIN I/O
Effi ciency vs VIN
VIN (V)
2.4
95
85
3.9 4.9
3538 TA01b
2.9 3.4 4.4 5.4
90
80
EFFICIENCY (%)
100 VOUT = 3.3V
ILOAD = 200mA
800mA Synchronous
Buck-Boost
DC/DC Converter
LTC3538
LTC3538
2
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PIN CONFIGURATION
ELECTRICAL CHARACTERISTICS
ABSOLUTE MAXIMUM RATINGS
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
(Note 1)
PARAMETER CONDITIONS MIN TYP MAX UNITS
Input Voltage 2.4 5.5 V
Feedback Voltage (Note 4) 0.980 1.00 1.020 V
Feedback Input Current (Note 4) 1 50 nA
VIN Quiescent Current – Shutdown VC = 0V, Not Including Switch Leakage 1.5 5 μA
VIN Quiescent Current – Active 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
Input Current Limit 1.4 2 A
Reverse Current Limit 0.5 A
Burst Mode Operational Peak Current 0.9 A
Maximum Duty Cycle Boost (%Switch C On)
Buck (% Switch A On)
Buck (% Switch D On)
70
100
100
88 %
%
%
TOP VIEW
VIN
SW1
SW2
VOUT
FB
VC
GND
BURST
DCB PACKAGE
8-LEAD (2mm × 3mm) PLASTIC DFN
9
3
4
2
1
6
5
7
8
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
The denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at TA = 25°C. VIN = VOUT = 3.6V, BURST = 0V, unless otherwise noted.
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 specifi ed 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 specifi cations, go to: http://www.linear.com/tapeandreel/
LTC3538 L7 LJUW
LTC3538
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LOAD CURRENT (mA)
0.1 1 10 100 1000
0.1
1
10
100
1000
EFFICIENCY (%)
POWER LOSS (mW)
100
80
60
40
20
0
90
70
50
30
10
3538 G02
Burst Mode
OPERATION
POWER LOSS
FIXED FREQUENCY
POWER LOSS BURST
FIXED FREQUENCY
LOAD CURRENT (mA)
0.1 1 10 100 1000
EFFICIENCY (%)
100
80
60
40
20
10
90
70
50
30
3538 G01
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
Burst Mode
OPERATION
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Duty Cycle FB = 1.2V 0%
Frequency Accuracy 0.8 1 1.2 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 0.25 V
VC Output Current in Shutdown VC = GND –1 –3 μA
BURST Threshold (High) 1.4 V
BURST Threshold (Low) 0.4 V
BURST Input Current VBURST = 3.6V 0.1 1 μA
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 specifi cations
from 0°C to 85°C. Specifi cations over –40°C to 85°C 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 125°C when over-temperature protection is active.
Continuous operation above the specifi ed 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 Effi ciency
Effi ciency and Power Loss vs
Load Current
Switch Pins Before Entering
Boost Mode
ELECTRICAL CHARACTERISTICS
The denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at TA = 25°C. VIN = VOUT = 3.6V, unless otherwise noted.
TA = 25°C unless otherwise noted
50ns/DIV
SW1
2V/DIV
SW2
2V/DIV
3538 G03
VIN = 2.9V
VOUT = 3.3V AT 500mA
LTC3538 WVN/N/TM 1H WWW / U ~————-"‘" // L7LJCUEN2
LTC3538
4
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Error Amplifi er Source Current vs
Temperature
Oscillator Frequency vs
Temperature Feedback Voltage vs Temperature
Feedback Voltage Line
Regulation Minimum Start-Up Voltage
Switch Pins Before Entering
Buck Mode
VOUT Ripple in Buck, Buck-Boost
and Boost Modes at 500mA Load
Burst Mode Sleep Current vs
Temperature
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C unless otherwise noted
Maximum Output Current
Capability vs VIN
50ns/DIV
SW1
2V/DIV
SW2
2V/DIV
3538 G04
VIN = 3.9V
VOUT = 3.3V AT 500mA
TEMPERATURE (°C)
–50
20
Burst Mode SLEEP CURRENT (μA)
25
30
35
40
45
–25 0 25 50
3538 G16
75 100
TEMPERATURE (°C)
–50
–15.0
VC SOURCE CURRENT (μA)
–14.5
–14.0
–13.5
–13.0
–12.5
–25 0 25 50
3538 G06
75 100
TEMPERATURE (°C)
–50
975
OSCILLATOR FREQUENCY (kHz)
1000
1025
–25 0 25 50
3538 G07
75 100
VIN = VOUT = 3.6V
TEMPERATURE (°C)
–50
0.990
FB VOLTAGE (V)
1.010
1.005
1.000
0.995
–25 0 25 50
3538 G08
75 100
VIN (V)
2.0 2.5 3.0 3.5 4.0 5.04.5
0
OUTPUT CURRENT CAPABILITY (mA)
1800
1600
1400
1200
1000
600
400
200
800
3538 G17
5.5
VOUT = 3.3V
VIN (V)
2.4
–0.2
FB LINE REGULATION (%)
0.4
0.2
0.1
0
–0.1
0.3
3.4 4.4
3538 G09
5.4
VOUT = 3.3V
TEMPERATURE (°C)
–50
2.3005
2.3010
2.3015
2.3020
2.3025
2.3030
2.3035
2.3040
VIN START VOLTAGE (V)
2.3045
–25 0 25 50
3538 G10
75 100
1μs/DIV 3538 G05
VOUT = 3.3V, AC-COUPLED
20mV/DIV
COUT = 22μF
ILOAD = 500mA
VIN = 2.5V
VIN = 3.3V
VIN = 4.2V
LTC3538 \N\\I\N\N\\/——— \ \ ‘ L7 LJUW
LTC3538
5
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Current Limit vs Temperature
Load Transient in Fixed
Frequency Mode
Burst Mode Operation
Transition From Burst Mode
Operation to Fixed Frequency
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C unless otherwise noted
VC On/Off Threshold vs
Temperature
TEMPERATURE (°C)
–50
0.40
VC ON/OFF THRESHOLD (V)
0.80
0.75
0.70
0.65
0.60
0.55
0.50
0.45
050
3538 G10
100
VC ON THRESHOLD
VC OFF THRESHOLD
TEMPERATURE (°C)
–50
1.5
CURRENT LIMIT (A)
2.0
2.5
3.0
4.0
3.5
–25 0 25 50
3538 G12
75 100
LINEAR CURRENT LIMIT
PEAK CURRENT LIMIT
VIN = VOUT = 3.6V
100μs/DIV
VOUT
100mV/DIV
ILOAD
200mA/DIV
3538 G13
VIN = 3.3V
VOUT = 3.3V
ILOAD = 0mA TO 500mA
COUT = 22μF X5R CERAMIC
10μs/DIV
VOUT
50mV/DIV
IL
500mA/DIV
3538 G14
VIN = 3.3V
VOUT = 3.3V
ILOAD = 10mA
COUT = 22μF X5R CERAMIC
50μs/DIV
VOUT
100mV/DIV
BURST
2V/DIV
3538 G15
VIN = 3.3V
VOUT = 3.3V
ILOAD = 30mA
COUT = 22μF X5R CERAMIC
LTC3538 L7LJCUEN2
LTC3538
6
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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:
V
OUT = 1V • (1 + R1/R2)
VC (Pin 2): Error Amplifi er Output. A frequency compensa-
tion 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 fi xed 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 effi ciency
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 effi ciency 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.
LTC3538 J -‘- T g M - :3 L! M L J fil’l 1 _. T —EJ—> » —’ _’ 1. E L7 LJUW
LTC3538
7
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BLOCK DIAGRAM
GATE DRIVERS
AND
ANTICROSS
CONDUCTION
0.5A
C
1V
B
SW2SW1
D
L1
ANTI-RING
A
PWM LOGIC
AND
OUTPUT PHASING
INTERNAL
SOFT-START
THERMAL
SHUTDOWN
OSC
1MHz
5μs
DELAY
BURST
MODE
CONTROL
TSD
UVLO
REVERSE
CURRENT
LIMIT
AVERAGE
CURRENT LIMIT
PEAK
CURRENT LIMIT
PWM
COMPARATORS
UVLO
2A
CIN
VIN
2.4V TO 5.5V
3.5A
2.3V CP1
SOFT-START
CZ1
RZR2
R1
CP2
COUT
VOUT
VOUT
VIN
3538 BD
+
+
+
SLEEP
SS DONE
FB
+
+
+
+
7
8
6
5
FB
1
VC
2
BURST
GND
1 = BurstMode OPERATION
0 = FIXED FREQUENCY
BURST
4
3
+
OFF ON
LTC3538 L7LJCUEN2
LTC3538
8
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OPERATION
The LTC3538 provides high effi ciency, 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 amplifi er
output voltage on VC determines the output duty cycle of the
switches. Since VC is a fi ltered 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 effi ciency. High effi ciency is achieved at light loads
when Burst Mode operation is selected.
LOW NOISE FIXED FREQUENCY OPERATION
Operating Frequency
The operating frequency is internally fi xed to 1MHz to
maximize overall converter effi ciency while minimizing
external component size.
Error Amplifi er
The error amplifi er controls the duty cycle of the internal
switches. The loop compensation components are con-
gured around the amplifi er to provide converter loop
stability. Pulling down the output of the error amplifi er
(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 fl oat.
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 fi rst
is a high speed peak current limit amplifi er that will shut
off switch A once the input current exceeds ~
3.5
A typical.
The delay to output of this amplifi er 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 fi xed 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
SW1 SW2
VIN VOUT
NMOS B NMOS C
PMOS A
L1
PMOS D
3538 FO1
8 5
67
Figure 1. Simplifi ed Diagram of Output Switches
Figure 1 shows a simplifi ed 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.
LTC3538 T i » T REGTIUN VOUT i L7 LJUW
LTC3538
9
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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.
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:
V
IN = VOUT(1 – D4SW) ≈ 0.85 • VOUT
The point at which the four-switch region ends is given
by:
VIN =VOUT
1D4SW
V1.18VOUT
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 volt-
age 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 consump-
tion of the LTC3538 at light loads and improves overall
conversion effi ciency, increasing battery life. During Burst
Mode operation the LTC3538 delivers energy to the out-
put 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).
3538 F02
A ON, B OFF
PWM C, D SWITCHES BOOST REGION
FOUR-SWITCH PWM BUCK-BOOST
REGION
D ON, C OFF
PWM A, B SWITCHES
DMIN
BOOST
DMAX
BUCK
DMAX
BOOST
0%
88%
DUTY
CYCLE
V3 (~1.8V)
V2 (~1.7V)
V4 (~2.2V)
V1 (~1.2V)
CONTROL
VOLTAGE, VC
BUCK REGION
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 volt-
age 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 volt-
age increases, the duty cycle of switch A increases until
the maximum duty cycle of the converter in buck mode
reaches DMAX_BUCK, given by:
D
MAX_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.
LTC3538 L7LJCUEN2
LTC3538
10
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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) =0.25VIN
VOUT
A; VOUT >VIN
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 specifi ed above.
INDUCTOR SELECTION
To achieve high effi ciency, a low ESR inductor should be
utilized for the converter. The inductor must have a satura-
tion rating greater than the worst case average inductor
current plus half the ripple current. The peak-to-peak cur-
rent 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 •V
IN –V
OUT
()
/V
IN
f•L A
ΔIL,P-P,BOOST =VOUT •V
OUT –V
IN
()
/V
OUT
f•L A
where f = frequency (1MHz typical), Hz
L = inductor, H
OPERATION
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.
For high effi ciency, choose a ferrite inductor with a high
frequency core material to reduce core loses. The induc-
tor 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 fi lter 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 fi lter capacitor, F
ILOAD = Output load current, A
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
Table 1. Inductor Vendor Information
LTC3538 L7 LJUW
LTC3538
11
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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 signifi cant
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 4.7μF, 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
effi ciency. Use a surface mount Schottky diode such as an
MBRM120T3 or equivalent. Do not use ordinary rectifi er
diodes since their slow recovery times will compromise
effi ciency.
Table 2. Capacitor Vendor Information
SUPPLIER PHONE FAX WEB SITE
AVX (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 pull-
down device or other suitable means can be used to put
the part in shutdown by pulling VC below 0.25V. Since
the error amplifi er sources 13μA typically when active
and 1.5μA in shutdown, a relatively high resistance pull-
down device can be used to pull VC below 0.25V. More
OPERATION
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.
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 recom-
mended 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.
LTC3538 \IL'COUT /°“T v.“ L7LJCUEN2
LTC3538
12
3538fb
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 fi lter exhibits a double pole response, as
given by:
ƒFILTER_POLE =1
2πLCOUT
Hz
(in buck mode)
ƒFILTER_POLE =VIN
2VOUT πLCOUT
Hz
(in boost mode)
where L is in Henries and COUT is in Farads.
The output fi lter zero is given by:
ƒFILTER_ ZERO =1
2πRESR COUT
Hz
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:
ƒRHPZ =VIN2
2πIOUT LVOUT
Hz
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 amplifi er
with the Type I compensation is given by:
ƒUG =1
2πR1CP1
Hz
1V
FB
CP1
VC
R2
3538 F04
+
1
2
VOUT
R1
Figure 4. Error Amplifi er with Type I Compensation
VIA TO GND PLANE
8
VIN
7
SW1
6
SW2
5
VOUT
1
FB
2
VC
3
GND
4
BURST
VOUT
3538 F03
Figure 3. LTC3538 Recommended PCB Layout
OPERATION
L7 LJUW FE LTC3538 4%
LTC3538
13
3538fb
VOUT
R1
1V
FB
CP2
CP1
RZ
VC
R2
CZ1
3538 F05
+
1
2
Figure 5. Error Amplifi er with Type III Compensation
OPERATION
Most applications demand an improved transient response
to allow a smaller output fi lter capacitor. To achieve a higher
bandwidth, Type III compensation is required, providing
two zeros to compensate for the double-pole response of
the output fi lter. Referring to Figure 5, the location of the
poles and zeros are given by:
ƒPOLE1 1
2π32e3R1CP1
Hz
(which is extremly close to DC)
ƒZERO1 =1
2πRZCP1
Hz
ƒZERO2 =1
2πR1CZ1
Hz
ƒPOLE2 =1
2πRZCP2
Hz
where resistance is in Ohms and capacitance is in
Farads.
LTC3538 L7LJCUEN2
LTC3538
14
3538fb
SW1
R2
200k
M1
R1
806k 10k
15k
CIN
10μF
CIN: TAIYO YUDEN JMK212BJ106MG
COUT: TAIYO YUDEN JMK325BJ226MM
L1: SUMIDA CDRH2D18/HP-3R3NC
M1: μP OPEN DRAIN I/O OR FAIRCHILD FDV301N
VIN
BURST
SW2
LTC3538
USB
4.35V TO 5.25V
L1
3.3μH
GND
VOUT
FB
VC
VOUT
5V, 500mA
COUT
22μF
330pF
33pF
PWM BURST
3538 TA03
ON OFF
TYPICAL APPLICATION
High Effi ciency 5V/500mA from USB Input
LTC3538 a» W L7 LJUW
LTC3538
15
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 representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
PACKAGE DESCRIPTION
3.00 ±0.10
(2 SIDES)
2.00 ±0.10
(2 SIDES)
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
0.40 ± 0.10
BOTTOM VIEW—EXPOSED PAD
0.75 ±0.05
R = 0.115
TYP
R = 0.05
TYP
1.35 REF
1
4
85
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
0.200 REF
0.00 – 0.05
(DCB8) DFN 0106 REV A
0.23 ± 0.05
0.45 BSC
PIN 1 NOTCH
R = 0.20 OR 0.25
× 45° CHAMFER
0.25 ± 0.05
1.35 REF
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
2.10 ±0.05
0.70 ±0.05
3.50 ±0.05
PACKAGE
OUTLINE
0.45 BSC
1.35 ±0.10
1.35 ±0.05
1.65 ± 0.10
1.65 ± 0.05
DCB Package
8-Lead Plastic DFN (2mm × 3mm)
(Reference LTC DWG # 05-08-1718 Rev A)
LTC3538 L7HߤN2
LTC3538
16
3538fb
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2007
LT 1007 REV B • PRINTED IN USA
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.

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