LTC3538EDCB-TRPBF [Linear]
800mA Synchronous Buck-Boost DC/DC Converter; 800毫安同步降压 - 升压型DC / DC转换器型号: | LTC3538EDCB-TRPBF |
厂家: | Linear |
描述: | 800mA Synchronous Buck-Boost DC/DC Converter |
文件: | 总16页 (文件大小:207K) |
中文: | 中文翻译 | 下载: | 下载PDF数据表文档文件 |
LTC3538
800mA Synchronous
Buck-Boost
DC/DC Converter
FEATURES
DESCRIPTION
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.
■
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 V
■
Range
OUT
IN
■
■
■
■
■
■
■
2.4V to 5.5V V Range
1MHz Fixed Frequency Operation
Output Disconnect in Shutdown
35μA Quiesecent Current in Burst Mode Operation
<5μA Shutdown Current
The LTC3538 is suited for use in Micro Hard Disk Drive
(μHDD)applicationswithits800mAcurrentcapability.Burst
Mode® operation provides high efficiency at light loads.
Internal Soft-Start
Small, Thermally Enhanced 8-Lead (2mm x 3mm)
DFN package
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), ther-
mally enhanced 8-lead (2mm × 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.
■
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
Efficiency vs VIN
100
L1
3.3μH
V
LOAD
= 3.3V
= 200mA
OUT
I
V
OUT
3.3V
800mA
95
90
85
80
LTC3538
R1
464k
10k
SW1
SW2
V
IN
2.9V TO 4.2V
V
V
IN
OUT
FB
33pF
C
IN
C
OUT
10μF
22μF
PWM
BURST
BURST
GND
V
C
15k
330pF
SD
*
R2
200k
ON OFF
2.4
2.9
3.4
3.9
(V)
4.4
4.9
5.4
V
IN
3538 TA01
3538 TA01b
*μP OPEN DRAIN I/O
3538fb
1
LTC3538
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
TOP VIEW
V ,V
Voltage.......................................... –0.3V to 6V
IN OUT
SW1,SW2 Voltage
8
7
6
5
FB
VC
1
2
3
4
V
IN
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
SW1
SW2
9
GND
BURST
V
OUT
DCB PACKAGE
8-LEAD (2mm × 3mm) PLASTIC DFN
T
= 125°C
JMAX
θ
= 75°C/W 4-LAYER BOARD, θ = 13.5°C/W
JA
JC
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
PART MARKING
LCRB
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3538EDCB#TRPBF
TAPE AND REEL
8-Lead (2mm × 3mm) Plastic DFN
PACKAGE DESCRIPTION
–40°C to 85°C
PART MARKING
LCRB
TEMPERATURE RANGE
–40°C to 85°C
LTC3538EDCB#TR
8-Lead (2mm × 3mm) Plastic DFN
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
MIN
2.4
TYP
MAX
5.5
1.020
50
UNITS
V
●
●
Input Voltage
Feedback Voltage
Feedback Input Current
(Note 4)
(Note 4)
0.980
1.00
1
V
nA
μA
mA
μA
μA
μA
Ω
V
V
V
Quiescent Current – Shutdown
Quiescent Current – Active
Quiescent Current – Sleep
V = 0V, Not Including Switch Leakage
C
1.5
1
5
IN
IN
IN
FB = 0.8V
1.8
60
FB = 1.2V, BURST = V
Switches B and C
Switches A and D
Switches B and C
Switches A and D
35
IN
NMOS Switch Leakage
0.1
0.1
0.17
0.2
2
7
PMOS Switch Leakage
10
NMOS Switch On-Resistance
PMOS Switch On-Resistance
Input Current Limit
Ω
●
1.4
A
Reverse Current Limit
0.5
0.9
88
A
Burst Mode Operational Peak Current
Maximum Duty Cycle
A
●
●
●
Boost (%Switch C On)
Buck (% Switch A On)
Buck (% Switch D On)
70
100
100
%
%
%
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
MIN
TYP
MAX
0
UNITS
%
●
●
Minimum Duty Cycle
Frequency Accuracy
Internal Soft-Start Time
FB = 1.2V
0.8
1
1.2
MHz
ms
dB
μA
μA
V
1.5
80
Error Amp A
VOL
Error Amp Source Current
Error Amp Sink Current
V = 1.5V, FB = OV
C
–13
130
V = 1.5V, FB = 1.2V
C
●
V Shutdown Threshold (Off)
C
IC is Disabled
0.25
–3
V Output Current in Shutdown
C
V = GND
C
–1
μA
V
●
●
BURST Threshold (High)
BURST Threshold (Low)
BURST Input Current
1.4
0.4
1
V
V
= 3.6V
0.1
μA
BURST
Note 3: This IC includes over-temperature protection that is intended
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.
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 TA = 25°C unless otherwise noted
Efficiency and Power Loss vs
Load Current
Switch Pins Before Entering
Boost Mode
Li-Ion to 3.3V Efficiency
100
90
80
70
60
50
40
30
20
10
100
90
80
70
60
50
40
30
20
10
0
1000
100
10
FIXED FREQUENCY
SW1
2V/DIV
Burst Mode
OPERATION
SW2
2V/DIV
Burst Mode
OPERATION
3538 G03
POWER LOSS
FIXED FREQUENCY
50ns/DIV
= 3.3V AT 500mA
V
V
= 2.9V
IN
OUT
1
V
V
V
= 2.7V
= 3.6V
= 4.2V
IN
IN
IN
POWER LOSS BURST
0.1
1000
0.1
1
10
100
1000
0.1
1
10
100
LOAD CURRENT (mA)
LOAD CURRENT (mA)
3538 G01
3538 G02
3538fb
3
LTC3538
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise noted
Switch Pins Before Entering
Buck Mode
VOUT Ripple in Buck, Buck-Boost
and Boost Modes at 500mA Load
Burst Mode Sleep Current vs
Temperature
45
40
35
30
V
V
V
= 2.5V
= 3.3V
= 4.2V
IN
IN
IN
SW1
2V/DIV
SW2
2V/DIV
3538 G05
3538 G04
1μs/DIV
50ns/DIV
V
= 3.3V, AC-COUPLED
V
V
= 3.9V
OUT
IN
OUT
20mV/DIV
= 3.3V AT 500mA
25
20
C
= 22μF
= 500mA
OUT
I
LOAD
–50
–25
0
25
50
75
100
TEMPERATURE (°C)
3538 G16
Error Amplifier Source Current vs
Temperature
Oscillator Frequency vs
Temperature
Feedback Voltage vs Temperature
–12.5
–13.0
–13.5
–14.0
1025
1000
975
1.010
1.005
1.000
0.995
0.990
V
IN
= V
= 3.6V
OUT
–14.5
–15.0
–50
–25
0
25
50
75
100
–50
–25
0
25
50
75
100
–50
–25
0
25
50
75
100
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
3538 G06
3538 G07
3538 G08
Maximum Output Current
Capability vs VIN
Feedback Voltage Line
Regulation
Minimum Start-Up Voltage
0.4
0.3
0.2
0.1
0
2.3045
2.3040
2.3035
2.3030
2.3025
2.3020
2.3015
2.3010
2.3005
1800
1600
1400
1200
1000
800
600
400
200
0
V
OUT
= 3.3V
V
= 3.3V
OUT
–0.1
–0.2
2.4
3.4
4.4
5.4
–50
–25
0
25
50
75
100
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
(V)
V
(V)
TEMPERATURE (°C)
V
IN
IN
3538 G09
3538 G10
3538 G17
3538fb
4
LTC3538
TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise noted
VC On/Off Threshold vs
Temperature
Load Transient in Fixed
Frequency Mode
Current Limit vs Temperature
0.80
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.40
4.0
3.5
3.0
2.5
2.0
1.5
V
= V
= 3.6V
OUT
IN
V
OUT
100mV/DIV
PEAK CURRENT LIMIT
V
ON THRESHOLD
C
I
LOAD
200mA/DIV
3538 G13
100μs/DIV
V
OFF THRESHOLD
C
V
V
I
= 3.3V
IN
OUT
= 3.3V
= 0mA TO 500mA
= 22μF X5R CERAMIC
LINEAR CURRENT LIMIT
LOAD
OUT
C
–50
0
50
100
–50
–25
0
25
50
75
100
TEMPERATURE (°C)
TEMPERATURE (°C)
3538 G10
3538 G12
Transition From Burst Mode
Operation to Fixed Frequency
Burst Mode Operation
BURST
2V/DIV
V
OUT
50mV/DIV
V
OUT
100mV/DIV
I
L
500mA/DIV
3538 G14
3538 G15
10μs/DIV
50μs/DIV
V
V
I
= 3.3V
V
V
I
= 3.3V
IN
OUT
IN
OUT
= 3.3V
= 3.3V
= 10mA
= 30mA
LOAD
LOAD
C
= 22μF X5R CERAMIC
C
= 22μF X5R CERAMIC
OUT
OUT
3538fb
5
LTC3538
PIN FUNCTIONS
FB (Pin 1): Feedback Input to Error Amplifer. Connect
SW2 (Pin 6): Switch Pin where the Internal Switches
resistive divider tap from V
to this pin to set the output
C and D are Connected. An optional Schottky diode can
OUT
voltage. The output voltage can be adjusted from 1.8V to
5.25V. Referring to the Block Diagram the output voltage
is given by:
be connected from SW2 to V
for a moderate efficiency
OUT
improvement. Keep the trace length as short as possible
to minimize EMI.
V
= 1V • (1 + R1/R2)
SW1(Pin7):SwitchPinwheretheInternalSwitchesAand
B are Connected. Connect an inductor from SW1 to SW2.
AnoptionalSchottkydiodecanbeconnectedfromSW1to
ground for a moderate efficiency improvement. Keep the
trace length as short as possible to minimize EMI.
OUT
V (Pin2):ErrorAmplifierOutput.Afrequencycompensa-
C
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
V below 0.25V disables the LTC3538.
V
IN
(Pin 8): Input Supply. This input provides power to
C
the IC and also supplies current to switch A. A ceramic
bypass capacitor (4.7μF or larger) is recommended as
GND (Pin 3): Ground.
BURST (Pin 4): Burst Mode Select Input.
BURST = Low for fixed frequency PWM operation
BURST = High for Burst Mode operation
close to V and GND as possible.
IN
Exposed Pad (Pin 9): GND. The exposed pad must be
electrically connected to the board ground for proper
electrical and thermal performance.
V
(Pin 5): Power Supply Output. This pin should be
OUT
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.
3538fb
6
LTC3538
BLOCK DIAGRAM
L1
SW1
SW2
7
6
ANTI-RING
V
OUT
V
IN
V
OUT
A
D
8
5
+
V
IN
2.4V TO 5.5V
GATE DRIVERS
AND
C
IN
0.5A
ANTICROSS
CONDUCTION
C
B
+
–
R1
REVERSE
CURRENT
LIMIT
C
Z1
C
OUT
AVERAGE
CURRENT LIMIT
+
–
2A
3.5A
2.3V
PEAK
–
+
+
–
CURRENT LIMIT
+
–
PWM LOGIC
AND
OUTPUT PHASING
1V
PWM
COMPARATORS
FB
–
+
1
2
+
–
UVLO
C
P1
C
P2
R
SOFT-START
R2
Z
V
C
THERMAL
SHUTDOWN
OSC
1MHz
OFF ON
BURST
BURST
INTERNAL
SOFT-START
TSD
UVLO
SLEEP
BURST
5μs
DELAY
4
MODE
SS DONE
CONTROL
1 = BurstMode OPERATION
0 = FIXED FREQUENCY
FB
GND
3
3538 BD
3538fb
7
LTC3538
OPERATION
Internal Current Limit
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
TherearetwocurrentlimitcircuitsintheLTC3538.Thefirst
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.
outputvoltageonV determinestheoutputdutycycleofthe
C
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
switches. Since V is a filtered signal, it provides rejection
C
of frequencies from well below the switching frequency.
The low R
, low gate charge synchronous switches
DS(ON)
provide high frequency pulse width modulation control at
high efficiency. High efficiency is achieved at light loads
when Burst Mode operation is selected.
when V
is near ground, such as during a short circuit
OUT
or during start-up, this threshold is cut to 1A typical,
providingafoldbackfeaturetolimitpowerdissipation. 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.
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.
Reverse Current Limit
Duringfixedfrequencyoperation,theLTC3538operatesin
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.
Error Amplifier
The error amplifier controls the duty cycle of the internal
switches. The loop compensation components are con-
figured around the amplifier to provide converter loop
stability. Pulling down the output of the error amplifier
Four-Switch Control
(V ) below 0.25V will disable the LTC3538. In shutdown
C
V
V
IN
OUT
5
the LTC3538 will draw only 1.5μA typical from the input
8
supply. During normal operation the V pin should be
C
allowed to float.
PMOS A
PMOS D
SW1
7
SW2
6
L1
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.
NMOS B
NMOS C
3538 FO1
Figure 1. Simplified Diagram of Output Switches
Figure1showsasimplifieddiagramofhowthefourinternal
switchesareconnectedtotheinductor,V ,V andGND.
IN OUT
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 V voltage, the LTC3538 will operate in
Buck-Boost or Four Switch (V ~ V
)
OUT
C
IN
either buck, buck-boost or boost mode. The four power
switches are properly phased so the transfer between
operatingmodesiscontinuous,smoothandtransparentto
When the control voltage, V , is above voltage V2, switch
C
pairADremainsonfordutycycleD
,andtheswitch
MAX_BUCK
pair AC begins to phase in. As switch pair AC phases in,
theuser.WhenV approachesV thebuck-boostregion
IN
OUT
switch pair BD phases out accordingly. When V reaches
C
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.
the edge of the buck-boost range, at voltage V3, the AC
switchpaircompletelyphaseouttheBDpair,andtheboost
phase begins at duty cycle D4 . The input voltage, V ,
SW
IN
where the four switch region begins is given by:
88%
D
MAX
V4 (~2.2V)
V = V (1 – D4 ) ≈ 0.85 • V
OUT
BOOST
IN
OUT
SW
A ON, B OFF
PWM C, D SWITCHES
BOOST REGION
The point at which the four-switch region ends is given
by:
D
MIN
V3 (~1.8V)
V2 (~1.7V)
BOOST
BUCK-BOOST
REGION
FOUR-SWITCH PWM
D
MAX
VOUT
1−D4SW
BUCK
V =
V ≈1.18• VOUT
IN
D ON, C OFF
PWM A, B SWITCHES
BUCK REGION
0%
V1 (~1.2V)
Boost Region (V < V
)
OUT
IN
DUTY
CYCLE
CONTROL
VOLTAGE, V
3538 F02
C
Switch A is always on and switch B is always off during
this mode. When the control voltage, V , is above volt-
Figure 2. Switch Control vs Control Voltage, VC
C
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
Buck Region (V > V
)
OUT
IN
Switch D is always on and switch C is always off during
this mode. When the control voltage, V , is above volt-
C
when V is above V4.
C
age V1, output A begins to switch. During the off time of
switchA,synchronousswitchBturnsonfortheremainder
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
Burst Mode OPERATION
BurstModeoperationreducesquiescentcurrentconsump-
tion of the LTC3538 at light loads and improves overall
conversionefficiency, increasingbatterylife. DuringBurst
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
reaches D , given by:
MAX_BUCK
D
= 100 – D4
%
MAX_BUCK
SW
where D4 = duty cycle % of the four switch range.
SW
D4 = (150ns • f) • 100 %
SW
where f = operating frequency, Hz.
Beyond this point the four switch, or buck-boost region
is reached.
capacitor across the upper resistor in the V
feedback
OUT
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:
Inadditiontoaffectingoutputcurrentripple, thesizeofthe
inductor can also affect the stability of the feedback loop.
In boost mode, the converter transfer function has a right
halfplanezeroatafrequencythatisinverselyproportional
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.
V
IN
I
OUT _BURST(BOOST) = 0.25•
A; VOUT > V
IN
VOUT
OUT _BURST(BUCK) = 0.27A; VOUT < V
For high efficiency, 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
I
IN
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.
2
reduce the I R losses, and must be able to handle the peak
inductorcurrentwithoutsaturating.Moldedchokesorchip
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.
INDUCTOR SELECTION
To achieve high efficiency, 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.
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:
ΔV
ΔV
I
• (V
– V )/(C
• V
• f)V
OUT
P-P, BOOST = LOAD
OUT
IN
OUT
2
= (V – V ) • V /(8 • L • V • C • f )V
OUT
P-P,BUCK
IN
OUT
OUT
IN
where C
= output filter capacitor, F
VOUT • V – V
/ V
IN
OUT
(
)
IN
OUT
ΔIL,P-P,BUCK
=
A
I
= Output load current, A
f •L
LOAD
VOUT • VOUT – V / V
(
)
IN
OUT
ΔIL,P-P,BOOST
=
A
f •L
where f = frequency (1MHz typical), Hz
L = inductor, H
Table 1. Inductor Vendor Information
SUPPLIER
Coilcraft
PHONE
FAX
WEB SITE
(847) 639-6400
(800) 227-7040
(847) 639-1469
(650) 361-2508
(814) 238-0490
www.coilcraft.com
CoEv Magnetics
Murata
www.tycoelectronics.com
www.murata.com
(814) 237-1431
(800) 831-9172
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
(847) 699-7864
www.component.tdk.com
www.tokoam.com
TOKO
(847) 297-0070
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.
importantly, leakage and parasitic capacitance need to
be minimized. During start-up, 1.5μA is typically sourced
from V . The leakage of an external pull-down device and
C
compensation components tied to V , must therefore be
C
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.
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
SinceV isthesupplyvoltagefortheICitisrecommended
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:
IN
to place at least a 4.7μF, low ESR ceramic bypass capaci-
tor close to V and GND. It is also important to minimize
IN
any stray resistance from the converter to the battery or
other power source.
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
Figure3asshortandaswideaspossible.Capacitorground
connections should via down to the ground plane in the
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.
shortestroutepossible.ThebypasscapacitoronV should
IN
be placed as close to the IC as possible and should have
the shortest possible paths to ground.
2. Thesmallsignalgroundpad(GND)shouldhaveasingle
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.
Table 2. Capacitor Vendor Information
SUPPLIER PHONE
FAX
WEB SITE
AVX
(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
Sanyo
3. The components in bold and their connections should
all be placed over a complete ground plane.
Taiyo
Yuden
TDK
(847) 803-6100 (847) 803-6296 www.component.tdk.com
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.
Shutdown MOSFET Selection
A discrete external N-channel MOSFET, open-drain pull-
down device or other suitable means can be used to put
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.
the part in shutdown by pulling V below 0.25V. Since
C
the error amplifier sources 13μA typically when active
and 1.5μA in shutdown, a relatively high resistance pull-
down device can be used to pull V below 0.25V. More
C
3538fb
11
LTC3538
OPERATION
V
IN
ƒFILTER _POLE
=
Hz
2• VOUT •π• L•COUT
(in boost mode)
where L is in Henries and C
1
FB
8
IN
V
is in Farads.
OUT
The output filter zero is given by:
2
7
V
C
SW1
1
ƒFILTER _ ZERO
=
Hz
2•π•RESR •COUT
3
GND
6
SW2
where R
is the equivalent series resistance of the
ESR
V
4
5
OUT
OUT
output capacitor.
BURST
V
Atroublesomefeatureinboostmodeistheright-halfplane
zero (RHP), given by:
2
3538 F03
V
IN
ƒRHPZ
=
Hz
VIA TO GND PLANE
2•π•IOUT •L• VOUT
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:
AsimpleTypeIcompensationnetworkcanbeincorporated
tostabilizetheloop,butatacostofreducedbandwidthand
slowertransientresponse.Toensureproperphasemargin
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
ƒFILTER _POLE
=
Hz
2•π• L•COUT
1
ƒUG
=
Hz
(in buck mode)
2•π•R1•CP1
V
OUT
1V
+
–
R1
FB
1
R2
C
P1
V
C
2
3538 F04
Figure 4. Error Amplifier with Type I Compensation
3538fb
12
LTC3538
OPERATION
Mostapplicationsdemandanimprovedtransientresponse
toallowasmalleroutputfiltercapacitor.Toachieveahigher
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
ƒZERO1
ƒZERO2
ƒPOLE2
=
=
=
Hz
Hz
Hz
2•π•RZ •CP1
1
2•π•R1•CZ1
1
2•π•RZ •CP2
1
ƒPOLE1
≅
Hz
2•π•32e3 •R1•CP1
where resistance is in Ohms and capacitance is in
Farads.
(which is extremly close to DC)
V
OUT
R1
C
Z1
1V
FB
+
–
1
R2
C
P2
C
P1
V
C
RZ
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
V
OUT
5V, 500mA
LTC3538
R1
806k
10k
33pF
SW1
SW2
USB
4.35V TO 5.25V
V
IN
V
OUT
C
IN
C
OUT
10μF
FB
22μF
PWM
BURST
BURST
V
C
15k
GND
330pF
R2
200k
ON OFF
M1
1Ω
3538 TA03
C
C
: TAIYO YUDEN JMK212BJ106MG
IN
: TAIYO YUDEN JMK325BJ226MM
OUT
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
1.65 0.05
3.50 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
2.00 0.10
(2 SIDES)
0.40 0.10
TYP
5
R = 0.05
TYP
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
1
0.23 0.05
0.45 BSC
0.75 0.05
0.200 REF
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 representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTC3538
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
V : 2.5V to 5.5V, V = 0.8V
OUT(MIN)
LTC3407
600mA (I ), 1.5MHz Dual Synchronous Step-Up DC/DC Converter
OUT
IN
I = 40μA, I ≤1μA, SC70 Package
Q
SD
LTC3410
300mA (I ), 2.25MHz Synchronous Step-Down DC/DC Converter in SC70 V : 2.5V to 5.5V, V
= 0.8V
SW
IN
OUT(MIN)
I = 26μA, I ≤1μA, MS Package
Q
SD
LTC3411
1.25A (I ), 4MHz Synchronous Step-Down DC/DC Converter
V : 2.625V to 5.5V, V
= 0.8V
OUT
IN
OUT(MIN)
I = 62μA, I ≤1μA, MS Package
Q
SD
LTC3412
2.5A (I ), 4MHz Synchronous Step-Down DC/DC Converter
V : 2.625V to 5.5V, V
= 0.8V
OUT
IN
OUT(MIN)
I = 62μA, I ≤1μA, TSSOP16E Package
Q
SD
LTC3421
3A (I ), 3MHz Synchronous Step-Up DC/DC Converter
V : 0.5V to 4.5V, V
= 5.25V
SW
IN
OUT(MAX)
I = 12μA, I <1μA, QFN Package
Q
SD
LTC3422
1.5A (I ), 3MHz Synchronous Step-Up DC/DC Converter with
V : 0.5V to 4.5V, V
= 5.25V
SW
IN
OUT(MAX)
Output Disconnect
I = 25μA, I <1μA, DFN Package
Q SD
LTC3425
5A (I ), 8MHz Multiphase Synchronous Step-Up DC/DC Converter
V : 0.5V to 4.5V, V
= 5.25V
SW
IN
OUT(MAX)
I = 12μA, I <1μA, QFN Package
Q
SD
LTC3427
500mA (I ), 1.25MHz Step-Up DC/DC Converter with
V : 1.8V to 5V, V
= 5.25V,
SW
IN
OUT(MAX)
Output Disconnect in 2mm × 2mm DFN
I = 350μA, I <1μA, DFN Package
Q SD
LTC3429
600mA (I ), 500KHz Synchronous Step-Up DC/DC Converter
V : 0.5V to 4.4V, V
= 5V
SW
IN
OUT(MAX)
I = 20μA, I <1μA, ThinSOT™ Package
Q
SD
LTC3440
600mA (I ), 2MHz Synchronous Buck-Boost DC/DC Converter
V : 2.5V to 5.5V, V : 2.5V to 5.5V
IN OUT
OUT
I = 25μA, I <1μA, MS, DFN Package
Q
SD
LTC3441/LTC3443
LTC3442
1.2A (I ), Synchronous Buck-Boost DC/DC Converters, LTC3441(1MHz), V : 2.5V to 5.5V, V : 2.4V to 5.25V
OUT IN OUT
LTC3443 (600kHz)
I = 25μA, I <1μA, DFN Package
Q SD
1.2A (I ), 2MHz Synchronous Buck-Boost DC/DC Converter
V : 2.4V to 5.5V, V : 2.4V to 5.25V
IN OUT
I = 28μA, I <1μA, MS Package
OUT
Q
SD
LTC3522
400mA, Synchronous Buck-Boost and 200mA Buck Converters
V : 2.4V to 5.5V, V
Buck-Boost: 2.2V to 5.25V,
IN
OUT
I = 25μA, I <1μA, DFN Package
Q
SD
LTC3525
400mA (I ), Synchronous Step-Up DC/DC Converter with
V : 0.5V to 4.5V, V
= 3, 3.3, 5V
SW
IN
OUT
Output Disconnect
I = 7μA, I <1μA, SC70 Package
Q SD
LTC3526/LTC3526B
LTC3530
500mA (I ), 1MHz Synchronous Step-Up DC/DC Converter with
V : 0.5V to 4.5V, V : 1.6V to 5.25V
IN OUT
SW
Output Disconnect in 2mm × 2mm DFN
I = 9μA, I <1μA, DFN Package
Q SD
600mA (I ), 2MHz Synchronous Buck-Boost DC/DC Converter
V : 1.8V to 5.5V, V : 1.6V to 5.25V
IN OUT
OUT
I = 40μA, I <1μA, DFN, MS Packages
Q
SD
LTC3531
200mA (I ) Synchronous Buck-Boost DC/DC Converter
V : 1.8V to 5.5V, V : 2V to 5V
IN OUT
OUT
I = 16μA, I <1μA, DFN, ThinSOT Packages
Q
SD
LTC3532
500mA (I ), 2MHz Synchronous Buck-Boost DC/DC Converter
V : 2.4V to 5.5V, V : 2.2V to 5.25V
IN OUT
OUT
I = 35μA, I <1μA, DFN, MS Packages
Q
SD
LTC3533
2A (I ), 2MHz Synchronous Buck-Boost DC/DC Converter
V : 1.8V to 5.5V, V : 1.6V to 5.25V
IN OUT
OUT
I = 40μA, I <1μA, DFN Package
Q
SD
ThinSOT is a trademark of Linear Technology Corporation.
3538fb
LT 1007 REV B • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
16
●
●
© LINEAR TECHNOLOGY CORPORATION 2007
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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