LTC3251/LTC3251-1.2/LTC3251-1.5 - 500mA High Efficiency, Low Noise, Inductorless Step-Down DC/DC Converters

LTC3251/
LTC3251-1.2/LTC3251-1.5
500mA High Efficiency,
Low Noise, Inductorless
Step-Down DC/DC Converter
DESCRIPTIO
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FEATURES
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Up to 500mA Output Current
No Inductors
2.7V to 5.5V Input Voltage Range
2x Efficiency Improvement Over LDOs
2-Phase, Spread Spectrum Operation
for Low Input and Output Noise
Shutdown Disconnects Load from VIN
Adjustable Output Voltage Range: 0.9V to 1.6V
Fixed Output Voltages: 1.2V, 1.5V
Super Burst, Burst and Burst Defeat Operating Modes
Low Operating Current: IIN = 35µA (Burst Mode®
Operation)
Super Burst Operating Current: IIN = 10µA
Low Shutdown Current: IIN = 0.01µA Typ
Soft-Start Limits Inrush Current at Turn-On
Short-Circuit and Overtemperature Protected
Available in a Thermally Enhanced
10-Pin MSOP Package
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APPLICATIO S
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Handheld Devices
Cellular Phones
Portable Electronic Equipment
DSP Power Supplies
A unique 2-phase spread spectrum architecture provides
a very low noise regulated output as well as low noise at the
input.* The parts have four operating modes: Continuous
Spread Spectrum, Spread Spectrum with Burst Mode
operation, Super BurstTM mode operation and shutdown.
Low operating current (35µA in Burst Mode operation,
10µA in Super Burst mode operation) and low external
parts count make the LTC3251/LTC3251-1.2/LTC3251-1.5
ideally suited for space-constrained battery-powered
applications. The parts are short-circuit and overtemperature protected, and are available in a thermally enhanced
10-pin MSOP package.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
Super Burst is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by US Patents including 6411531.
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The LTC®3251/LTC3251-1.2/LTC3251-1.5 are 2-phase
charge pump step-down DC/DC converters that produce a
regulated output from a 2.7V to 5.5V input. The parts use
switched capacitor fractional conversion to achieve twice
the typical efficiency of a linear regulator. No inductors are
required. VOUT is resistor programmable from 0.9V to 1.6V
or fixed at 1.2V or 1.5V, with up to 500mA of load current
available.
1.5V Efficiency vs Input Voltage
(Burst Mode Operation)
TYPICAL APPLICATIO
100
Spread Spectrum Step-Down Converter
LTC3251-1.5
2
7
VOUT
VIN
3
8
C1+
C2+
1µF 4
6
C1–
C2–
5, 11
10
GND
MODE
VOUT = 1.5V
500mA
10µF
1µF
EFFICIENCY (%)
1
9
MD0 MD1
1µF
LTC3251-1.5
80
OFF ON
1-CELL Li-Ion
OR
3-CELL NiMH
IOUT = 200mA
90
70
60
50
LDO
40
30
20
10
3251 TA01
0
3
3.5
4.5
4
INPUT VOLTAGE (V)
5
5.5
3251 TA02
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LTC3251/
LTC3251-1.2/LTC3251-1.5
W W
W
AXI U
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ABSOLUTE
RATI GS
(Notes 1, 7)
VIN to GND ................................................... –0.3V to 6V
MD0, MD1, MODE and FB to GND . – 0.3V to (VIN + 0.3V)
IOUT (Note 2) ...................................................... 650mA
Operating Temperature Range (Note 3) ... –40°C to 85°C
Storage Temperature Range .................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................... 300°C
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PACKAGE/ORDER I FOR ATIO
TOP VIEW
MD0
VIN
C1 +
C1–
GND
1
2
3
4
5
11
10
9
8
7
6
MODE
MD1
C2+
VOUT
C2–
MSE PACKAGE
10-LEAD PLASTIC MSOP
EXPOSED PAD IS GND (PIN 11),
MUST BE SOLDERED TO PCB
TJMAX = 125°C, θJA = 40°C/W, θJC = 10°C/W
ORDER PART
NUMBER
LTC3251EMSE
TOP VIEW
MD0
VIN
C1 +
C1–
GND
11
10
9
8
7
6
MODE
MD1
C2+
VOUT
C2–
MSE PACKAGE
10-LEAD PLASTIC MSOP
MSE PART MARKING
LTB4
1
2
3
4
5
EXPOSED PAD IS GND (PIN 11),
MUST BE SOLDERED TO PCB
TJMAX = 125°C, θJA = 40°C/W, θJC = 10°C/W
ORDER PART
NUMBER
LTC3251EMSE-1.2
LTC3251EMSE-1.5
MSE PART MARKING
LTAGM
LTABE
Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking: http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.6V, C1 = C2 = 1µF, CIN = 1µF, COUT = 10µF,
VMODE = 0V for LTC3251-1.2V or LTC3251-1.5, VOUT = 1.5V for LTC3251, all capacitors ceramic, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
VIN Minimum Operating Voltage
(Notes 4,5)
●
VIN Maximum Operating Voltage
(Note 5)
●
VIN Continuous Mode Operating Current
IOUT = 0mA, VMD0 = 0, VMD1 = VIN
Spread Spectrum Disabled MODE = VIN
●
●
3
3.75
5
6
mA
mA
VIN Burst Mode Operating Current
IOUT = 0mA, VMD0 = VIN, VMD1 = 0
Spread Spectrum Disabled MODE = VIN
●
●
35
35
60
60
µA
µA
VIN Super Burst Mode Operating Current
IOUT = 0mA, VMD0 = VIN, VMD1 = VIN
Spread Spectrum Disabled MODE = VIN
●
●
10
10
15
15
µA
µA
VIN Shutdown Current
VMD0 = 0V, VMD1 = 0V (Note 5)
●
0.01
1
µA
VFB Regulation Voltage (LTC3251)
IOUT = 0mA, 2.7V ≤ VIN ≤ 5.5V
●
0.78
0.8
0.82
V
VOUT Regulation Voltage (LTC3251-1.2)
Continuous Mode or Burst Mode Operation
IOUT ≤ 200mA, 2.7V ≤ VIN ≤ 5.5V (Note 5)
IOUT ≤ 300mA, 2.8V ≤ VIN ≤ 5.5V (Note 5)
IOUT ≤ 500mA, 3V ≤ VIN ≤ 5.5V (Note 5)
●
●
1.15
1.15
1.15
1.2
1.2
1.2
1.25
1.25
1.25
V
V
V
VOUT Regulation Voltage (LTC3251-1.2)
Super Burst Operation
IOUT ≤ 40mA
●
1.15
1.2
1.25
V
VOUT Regulation Voltage (LTC3251-1.5)
Continuous Mode or Burst Mode Operation
IOUT ≤ 100mA, 3.1V ≤ VIN ≤ 5.5V (Note 5)
IOUT ≤ 200mA, 3.2V ≤ VIN ≤ 5.5V (Note 5)
IOUT ≤ 300mA, 3.3V ≤ VIN ≤ 5.5V (Note 5)
IOUT ≤ 500mA, 3.5V ≤ VIN ≤ 5.5V (Note 5)
●
●
●
1.44
1.44
1.44
1.44
1.5
1.5
1.5
1.5
1.56
1.56
1.56
1.56
V
V
V
V
VOUT Regulation Voltage (LTC3251-1.5)
Super Burst Operation
IOUT ≤ 40mA
●
1.44
1.5
1.56
V
IOUT Continuous Output Current (LTC3251)
VMD0 = 0, VMD1 = VIN or VMD0 = VIN, VMD1 = 0
●
500
IOUT Super Burst Output Current (LTC3251)
VMD0 = VIN, VMD1 = VIN
●
40
Load Regulation (LTC3251)
0mA ≤ IOUT ≤ 500mA, Referred to FB Pin
2.7
V
5.5
V
mA
mA
0.045
mV/mA
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LTC3251/
LTC3251-1.2/LTC3251-1.5
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.6V, C1 = C2 = 1µF, CIN = 1µF, COUT = 10µF,
VMODE = 0V for LTC3251-1.2V or LTC3251-1.5, VOUT = 1.5V for LTC3251, all capacitors ceramic, unless otherwise noted.
PARAMETER
CONDITIONS
Line Regulation (LTC3251)
IOUT = 500mA, 2.7V ≤ VIN ≤ 5.5V
MIN
Spread Spectrum Frequency Range
fMIN Switching Frequency
fMAX Switching Frequency
●
●
0.7
MODE = VIN
●
MD0, MD1 Input High Voltage
2.7V ≤ VIN ≤ 5.5V
●
MD0, MD1 Input Low Voltage
2.7V ≤ VIN ≤ 5.5V
MD0, MD1 Input High Current
TYP
MAX
UNITS
0.2
%/V
1.0
1.6
2
MHz
MHZ
1.3
1.6
2
MHz
0.8
1.2
●
0.4
0.8
MD0 = VIN, MD1 = VIN
●
–1
MD0, MD1 Input Low Current
MD0 = 0V, MD1 = 0V
●
FB Input Current (LTC3251)
VFB = 0.85V
●
MODE Input High Voltage (LTC3251-1.2/LTC3251-1.5)
2.7V ≤ VIN ≤ 5.5V
●
MODE Input Low Voltage (LTC3251-1.2/LTC3251-1.5)
2.7V ≤ VIN ≤ 5.5V
●
30
MODE Input High Current (LTC3251-1.2/LTC3251-1.5)
MODE = VIN
●
–1
1
µA
MODE Input Low Current (LTC3251-1.2/LTC3251-1.5)
MODE = 0V
●
–1
1
µA
Spread Spectrum Disabled Frequency
Turn-On Time (Burst or Continuous Mode Operation)
ROL = 3Ω, (Note 5)
Open-Loop Output Impedance (LTC3251)
VIN = 3V, IOUT = 200mA (Note 6)
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: Based on long term current density limitations.
Note 3: The LTC3251E is guaranteed to meet specified performance from
0°C to 70°C. Specifications over the – 40°C to 85°C operating temperature
range are assured by design, characterization and correlation with
statistical process controls.
V
V
1
µA
–1
1
µA
–50
50
nA
70
%/VIN
50
50
%/VIN
1
●
ms
0.45
Ω
0.7
Note 4: Minimum operating voltage required for regulation is:
VIN ≥ 2 • (VOUT + ROL • IOUT)
Note 5: VMODE = 0V or VMODE = VIN for LTC3251-1.2/LTC3251-1.5.
Note 6: Output not in regulation; ROL = (VIN/2 – VOUT)/IOUT.
(VFB = 0.76V). Burst or continuous mode operation.
Note 7: This IC includes overtemperature protection that is intended to
protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
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TYPICAL PERFOR A CE CHARACTERISTICS
No Load Supply Current vs Supply
Voltage (Continuous Mode Spread
Spectrum Enabled)
6
10
–40°C
25°C
85°C
9
8
50
–40°C
25°C
85°C
45
85°C
7
ICC (mA)
IIN (mA)
5
4
3
40
6
5
4
2
1
25°C
35
–40°C
30
3
2
0
2.7
No Load Supply Current vs Supply
Voltage (Burst Mode Operation)
IIN (µA)
7
No Load Supply Current vs Supply
Voltage (Continuous Mode,
Spread Spectrum Disabled)
25
1
3.2
3.7
4.2
VIN (V)
4.7
5.2
3251 G01
0
2.7
3.2
3.7
4.2
VIN (V)
4.7
5.2
3251 G17
20
2.7
3.2
3.7
4.2
VIN (V)
4.7
5.2
3251 G02
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LTC3251/
LTC3251-1.2/LTC3251-1.5
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TYPICAL PERFOR A CE CHARACTERISTICS
1.5V Output Voltage vs Supply
Voltage (Burst Mode Operation/
Continuous Mode)
1.60
18
1.58
16
1.56
14
1.54
12
85°C
10
25°C
8
–40°C
1.300
TA = 25°C
1.260
1.240
IOUT = 0mA
1.52
IOUT = 250mA
1.50
1.48
IOUT = 500mA
1.44
1.140
2
1.42
1.120
4.7
1.40
5.2
3.5
3
4
4.5
VIN (V)
5
5.5
3251 G02
0mA
10mA
40mA
1.56
1.28
1.24
1.52
1.22
VOUT (V)
1.54
1.48
1.44
1.14
1.42
1.12
3.5
4
4.5
VIN (V)
5
0mA
10mA
40mA
3251 G05
0.805
TA = 25°C
VOUT = 1.5V
0.785
3.2
3.7
4.2
VIN (V)
4.7
0.780
5.2
90
100
90
VIN = 2.7V
40
70
VIN = 3.5V
60
50
VIN = 4.5V
40
70
50
30
20
20
20
10
10
10
0.1
1
10
IOUT (mA)
100
1000
3251 G08
MD0 = VIN, MD1 = 0V
0
10
0.1
1
IOUT (mA)
600
VIN = 3.6V
VIN = 3.3V
100
1000
3251 G19
VIN = 4V
VIN = 5V
40
30
MD0 = VIN, MD1 = 0V
500
60
30
0
300
400
IOUT (mA)
80
EFFICIENCY (%)
VIN = 5V
50
VIN = 3V
80
EFFICIENCY (%)
60
200
1.5V Output Efficiency vs Output
Current (Super Burst Mode
Operation)
1.2V Output Efficiency vs Output
Current (Burst Mode Operation)
VIN = 3.6V
VIN = 4V
100
3251 G07
100
70
0
3251 G18
100
80
5.2
0.790
1.10
2.7
5.5
1.5V Output Efficiency vs Output
Current (Burst Mode Operation)
VIN = 3.3V
4.7
0.795
3251 G06
90
4.2
VIN (V)
0.800
1.18
1.16
3
3.7
FB Voltage vs Output Current
(Burst Mode Operation/
Continuous Mode)
1.20
1.46
1.40
TA = 25°C
1.26
1.50
3.2
VFB (V)
TA = 25°C
1.58
1.100
2.7
1.2V Output Voltage
vs Supply Voltage
(Super Burst Mode Operation)
1.30
1.60
IOUT = 500mA
3251 G04
1.5V Output Voltage
vs Supply Voltage
(Super Burst Mode Operation)
VOUT (V)
1.180
4
4.2
VIN (V)
IOUT = 0mA
1.200
1.160
3.7
IOUT = 250mA
1.220
1.46
3.2
TA = 25°C
1.280
6
0
2.7
EFFICIENCY (%)
1.2V Output Voltage vs Supply
Voltage (Burst Mode Operation/
Continuous Mode)
VOUT (V)
20
VOUT (V)
ICC (µA)
No Load Supply Current
vs Supply Voltage
(Super Burst Mode Operation)
MD0 = MD1 = VIN
0
10
1
0.1
IOUT (mA)
100
3251 G09
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LTC3251/
LTC3251-1.2/LTC3251-1.5
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TYPICAL PERFOR A CE CHARACTERISTICS
MD0/MD1 Input Threshold Voltage
vs Supply Voltage
Max/Min Oscillator Frequency
vs Supply Voltage
1.2
2.0
1.9
1.8
1.0
–40°C MAX
1.7
FREQUENCY (MHz)
MD0/MD1 THRESHOLD (V)
1.1
–40°C
0.9
25°C
0.8
85°C
0.7
0.6
25°C MAX
1.6
85°C MAX
1.5
1.4
1.3
1.2
25°C MIN
1.1
–40°C MIN
1.0
0.5
85°C MIN
0.9
0.4
2.7
3.2
3.7
4.2
VIN (V)
4.7
0.8
5.2
2.7
3.2
3.7
4.2
VIN (V)
4.7
3251 G11
3251 G10
Output Transient Response
(Burst Mode Operation)
Output Transient Response
(Continuous Mode)
IOUT
450mA
IOUT
450mA
50mA
50mA
VOUT
20mV/DIV
(AC)
VOUT
20mV/DIV
(AC)
TA = 25°C
10µs/DIV
COUT = 10µF X5R 6.3V
VOUT = 1.5V
VIN
VOUT
20mV/DIV
(AC)
10µs/DIV
TA = 25°C
COUT = 10µF X5R 6.3V
VOUT = 1.5V
3251 G13
3251 G14
LTC3251-1.5 Output Voltage
Ripple
Supply Transient Response
(Continuous Mode)
VIN
5.2
4.5V
SPREAD
SPECT
ENABLED
10mV/DIV (AC)
3.5V
VOUT
20mV/DIV (AC)
SPREAD
SPECT
DISABLED
10mV/DIV (AC)
20µs/DIV
TA = 25°C
COUT = 10µF X5R 6.3V
IOUT = 250mA
VOUT = 1.5V
3251 G15
TA = 25°C
200ns/DIV
COUT = 10µF X5R 6.3V
IOUT = 500mA
VOUT = 1.5V
3251 G16
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LTC3251/
LTC3251-1.2/LTC3251-1.5
U
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PI FU CTIO S
MD0 (Pin 1)/MD1 (Pin 9): Switching Mode Input Pins. The
Mode input pins are used to set the operating mode of the
LTC3251. The modes of operation are:
MD1
MD0
OPERATING MODE
0
0
Shutdown
0
1
Spread Spectrum with Burst
1
0
Continuous Spread Spectrum
1
1
Super Burst
MD0 and MD1 are high impedance CMOS inputs and must
not be allowed to float.
VIN (Pin 2): Input Supply Voltage. Operating VIN may be
between 2.7V and 5.5V. Bypass VIN with a ≥ 1µF low ESR
ceramic capacitor to GND (COUT).
C1+ (Pin 3): Flying Capacitor 1 Positive Terminal (C1).
C1– (Pin 4): Flying Capacitor 1 Negative Terminal (C1).
GND (Pin 5, 11): Ground. Connect to a ground plane for
best performance.
C2 – (Pin 6): Flying Capacitor 2 Negative Terminal (C2).
VOUT (Pin 7): Regulated Output Voltage. VOUT is disconnected from VIN during shutdown. Bypass VOUT with a
low ESR ceramic capacitor to GND (CIN). See VOUT
Capacitor Selection for capacitor size requirements.
C2 + (Pin 8): Flying Capacitor 2 Positive Terminal (C2).
FB (Pin 10) (LTC3251): Feedback Input Pin. An output
divider should be connected from VOUT to FB to program
the output voltage.
MODE (Pin 10) (LTC3251-1.2/LTC3251-1.5): Spread
Spectrum Operation Mode Pin. A low voltage on MODE
enables spread spectrum operation. When MODE is high
spread spectrum operation is disabled and switching
occurs at the maximum operating frequency.
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LTC3251/
LTC3251-1.2/LTC3251-1.5
W
W
SI PLIFIED BLOCK DIAGRA
LTC3251-1.2/
LTC3251-1.5
ONLY
1
10
9
MD0
MODE
MD1
OVERTEMP
SWITCH CONTROL
AND SOFT-START
CHARGE
PUMP 1
VIN
C1+
C1–
VOUT
CHARGE
PUMP 2
C2+
C2–
FB
–
3
4
INTERNAL ON
LTC3251-1.2/
LTC3251-1.5
7
8
6
10
BURST DETECT
CIRCUIT
+
2
SPREAD SPECTRUM
OSCILLATOR
GND
5
11
3251 BD
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LTC3251/
LTC3251-1.2/LTC3251-1.5
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OPERATIO (Refer to Block Diagram)
The LTC3251 family of parts use a dual phase switched
capacitor charge pump to step down VIN to a regulated
output voltage. Regulation is achieved by sensing the
output voltage through an external resistor divider and
modulating the charge pump output current based on the
error signal. A 2-phase nonoverlapping clock activates the
two charge pumps. The two charge pumps work in parallel, but out of phase from each other. On the first phase of
the clock, current is transferred from VIN, through the
external flying capacitor 1, to VOUT via the switches of
Charge Pump 1. Not only is current being delivered to VOUT
on the first phase, but the flying capacitor is also being
charged. On the second phase of the clock, flying capacitor 1 is connected from VOUT to ground, transferring the
charge stored during the first phase of the clock to VOUT via
the switches of Charge Pump 1. Charge Pump 2 operates
in the same manner, but with the phases of the clock
reversed. This dual phase architecture achieves extremely
low output and input noise by providing constant charge
transfer from VIN to VOUT.
Using this method of switching, only half of the output
current is delivered from VIN, thus achieving twice the
efficiency over a conventional LDO. A spread spectrum
oscillator, which utilizes random switching frequencies
between 1MHz and 1.6MHz, sets the rate of charging and
discharging of the flying capacitors. The LTC3251-1.2/
LTC3251-1.5 MODE pin can be used to disable spread
spectrum operation which causes switching to occur at
1.6MHz. The part also has two types of low current Burst
Mode operation to improve efficiency even at light loads.
In shutdown mode, all circuitry is turned off and the
LTC3251 family draws only leakage current from the VIN
supply. Furthermore, VOUT is disconnected from VIN. The
MD0 and MD1 pins are CMOS inputs with threshold
voltages of approximately 0.8V to allow regulator control
with low voltage logic levels. The MODE pin is also CMOS,
but has a threshold of about 1/2 • VIN. The LTC3251 family
is in shutdown when a logic low is applied to both mode
pins. Since MD0, MD1 and MODE pins are high impedance
CMOS inputs, they should never be allowed to float.
Always drive MD0, MD1 and Mode with valid logic levels.
Short-Circuit/Thermal Protection
The LTC3251 family has built-in short-circuit current
limiting as well as overtemperature protection. During
short-circuit conditions, internal circuitry automatically
limits the output current to approximately 800mA. At
higher temperatures, or in cases where internal power
dissipation causes excessive self heating on chip (i.e.,
output short circuit), the thermal shutdown circuitry will
shut down the charge pumps when the junction temperature exceeds approximately 160°C. It will re-enable the
charge pumps once the junction temperature drops back
to approximately 150°C. The LTC3251 will cycle in and out
of thermal shutdown without latch-up or damage until the
overstress condition is removed. Long term overstress
(IOUT > 650mA and/or TJ > 125°C) should be avoided as it
can degrade the performance or shorten the life of the part.
Soft-Start
To prevent excessive current flow at VIN during start-up,
the LTC3251 family has built-in soft-start circuitry. Softstart is achieved by increasing the amount of current
available to the output charge storage capacitor linearly
over a period of approximately 500µs. Soft-start is enabled whenever the device is brought out of shutdown,
and is disabled shortly after regulation is achieved.
Spread Spectrum Operation
Switching regulators can be particularly troublesome where
electromagnetic interference (EMI) is concerned. Switching regulators operate on a cycle-by-cycle basis to transfer
power to an output. In most cases the frequency of
operation is either fixed or is a constant based on the
output load. This method of conversion creates large
components of noise at the frequency of operation (fundamental) and multiples of the operating frequency (harmonics). Figure 1a shows a conventional buck switching
converter. Figures 1b and 1c are the input and output noise
spectrums for the buck converter of Figure 1 with VIN =
3.6V, VOUT = 1.5V and IOUT = 500mA.
32511215fb
8
LTC3251/
LTC3251-1.2/LTC3251-1.5
U
OPERATIO
(Refer to Block Diagram)
4.7µH
IN
VIN
10nH*
10nH*
SW
VOUT
10µF
22µF
IN
VIN
1µF
1µF
FB
COMP
GND
*10nH = 1cm OF PCB TRACE
1µF
–40
–40
–50
–50
–60
–70
–80
C2 –
GND
1µF
*10nH = 1cm OF PCB TRACE
3251 F02a
–60
–70
–80
–90
–90
START FREQ: 100kHz RBW: 10kHz STOP FREQ: 30MHz
START FREQ: 100kHz RBW: 10kHz STOP FREQ: 30MHz
3251 F01b
3251 F02b
Figure 1b. Conventional Buck Converter Output Noise
Spectrum with 22µF Output Capacitor (IO = 500mA)
Figure 2b. LTC3251 Output Noise Spectrum
with 10µF Output Capacitor (IO = 500mA)
–40
–40
–50
–50
NOISE (dBm)
NOISE (dBm)
C1–
FB
C2 +
1µF
Figure 2a. LTC3251 Buck Converter
NOISE (dBm)
NOISE (dBm)
Figure 1a. Conventional Buck Switching Converter
VOUT
10µF
LTC3251
C1+
3251 F01a
OUT
–60
–70
–80
–60
–70
–80
–90
START FREQ: 100kHz RBW: 10kHz STOP FREQ: 30MHz
3251 F01c
–90
START FREQ: 100kHz RBW: 10kHz STOP FREQ: 30MHz
3251 F02c
Figure 1c. Conventional Buck Converter Input Noise
Spectrum with 10µF Input Capacitor (IO = 500mA)
Unlike conventional buck converters, the LTC3251’s internal oscillator is designed to produce a clock pulse whose
period is random on a cycle-by-cycle basis, but fixed
between 1MHz and 1.6MHz. This has the benefit of spreading the switching noise over a range of frequencies, thus
significantly reducing the peak noise. Figures 2b and 2c
are the input and output noise spectrums for the LTC3251
of Figure 2a with VIN = 3.6V, VOUT = 1.5V and IOUT =
500mA. Note the significant reduction in peak output
noise (>20dBm) with only 1/2 the output capacitance and
the virtual elimination of input harmonics with only 1/10
the input capacitance. Spread spectrum operation is used
exclusively in “continuous” mode and for output currents
greater than about 50mA in Burst Mode operation.
Figure 2c. LTC3251 Input Noise Spectrum
with 1µF Input Capacitor (IO = 500mA)
Low Current Burst Mode Operation
To improve efficiency at low output currents, a Burst Mode
function is included in the LTC3251 family of parts. An
output current sense is used to detect when the required
output current drops below an internally set threshold
(50mA typ). When this occurs, the part shuts down the
internal oscillator and goes into a low current operating
state. The part will remain in the low current operating
state until the output voltage has dropped enough to
require another burst of current. When the output current
exceeds 50mA, the part will operate in continuous mode.
Unlike traditional charge pumps, where the burst current
is dependant on many factors (i.e., supply, switch strength,
32511215fb
9
LTC3251/
LTC3251-1.2/LTC3251-1.5
U
OPERATIO (Refer to Block Diagram)
Ultralow Current Super Burst Mode Operation
To further optimize the supply current for low output
current requirements, a Super Burst mode operaton is
included in the LTC3251 family of parts. This mode is very
similar to Burst Mode operation, but much of the internal
circuitry and switch is shut down to further reduce supply
current. In Super Burst mode operation an internal hysteretic comparator is used to enable/disable charge transfer.
The hysteresis of the comparator and the amount of
current deliverable to the output are limited to keep output
ripple low. The VOUT ripple voltage in Super Burst mode
operation is typically 35mV with a 10µF output capacitor.
The LTC3251 family can deliver 40mA of current in Super
Burst mode operation but does not switch to continuous
mode. The MODE pin of the LTC3251-1.2 and LTC32511.5 has no effect on operation in super-burst mode.
Diagram, the LTC3251 family uses a control loop to adjust
the strength of the charge pump to match the current
required at the output. The error signal of this loop is
stored directly on the output charge storage capacitor.
Thus the charge storage capacitor also serves to form the
dominant pole for the control loop. The desired output
voltage also affects stability. As the divider ratio (RA/RB)
drops, the effective closed-loop gain increases, thus requiring a larger output capacitor for stability. Figure 3
shows the suggested output capacitor for optimal transient response. The value of the output capacitance should
not drop below the minimum capacitance line to prevent
excessive ringing or instability. (see Ceramic Capacitor
Selection Guidelines section).
16
15
14
OPTIMUM CAPACITANCE
13
12
COUT (µF)
capacitor selection, etc.), the part’s burst current is set by
the burst threshold and hysteresis. This means that the
VOUT ripple voltage in Burst Mode operation will be fixed
and is typically 15mV with a 10µF output capacitor.
11
10
9
8
VOUT Capacitor Selection
The style and value of capacitors used with the LTC3251
family determine several important parameters such as
regulator control loop stability, output ripple and charge
pump strength.
The dual phase nature of the LTC3251 family minimizes
output noise significantly but not completely. What small
ripple that does exist is controlled by the value of COUT
directly. Increasing the size of COUT will proportionately
reduce the output ripple. The ESR (equivalent series
resistance) of COUT plays the dominant role in output
noise. When a part switches between clock phases there
is a period where all switches are turned off. This “blanking
period” shows up as a spike at the output and is a direct
function of the output current times the ESR value. To
reduce output noise and ripple, it is suggested that a low
ESR (<0.08Ω) ceramic capacitor be used for COUT. Tantalum and aluminum capacitors are not recommended because of their high ESR.
Both the style and value of COUT can significantly affect the
stability of the LTC3251 family. As shown in the Block
MINIMUM CAPACITANCE
7
6
5
4
0.9
1.0
1.1
1.2 1.3
VOUT (V)
1.4
1.5
1.6
3251 F03
Figure 3
Likewise excessive ESR on the output capacitor will tend
to degrade the loop stability. The closed loop output
impedance of the LTC3251 is approximately:
RO ≅ 0.045Ω •
VOUT
0.8 V
For example, with the output programmed to 1.5V, the RO
is 0.085Ω, which produces a 40mV output change for a
500mA load current step. For stability and good load
transient response, it is important for the output capacitor
to have 0.08Ω or less of ESR. Ceramic capacitors typically
have exceptional ESR, and combined with a tight board
layout, should yield excellent stability and load transient
performance.
32511215fb
10
LTC3251/
LTC3251-1.2/LTC3251-1.5
U
OPERATIO
(Refer to Block Diagram)
Further output noise reduction can be achieved by filtering
the LTC3251 output through a very small series inductor
as shown in Figure 4. A 10nH inductor will reject the fast
output transients caused by the blanking period. The 10nH
inductor can be fabricated on the PC board with about 1cm
(0.4") of 1mm wide PC board trace.
10nH
(TRACE INDUCTANCE)
VOUT
LTC3251
GND
VOUT
10µF
1µF
3251 F04
Figure 4. 10nH Inductor Used for
Additional Output Noise Reduction
VIN Capacitor Selection
The dual phase architecture used by the LTC3251 family
makes input noise filtering much less demanding than
conventional charge pump regulators. The input current
should be continuous at about IOUT/2. The blanking period
described in the VOUT section also effects the input. For
this reason it is recommended that a low ESR, 1µF (0.4µF
min) or greater ceramic capacitor be used for CIN (see
Ceramic Capacitor Selection Guidelines section).
In cases where the supply impedance is high, heavy output
transients can cause significant input transients. These
input transients feed back to the output which slows the
output transient recovery and increases overshoot and
output impedance. This effect can generally be avoided by
using low impedance supplies and short supply connections. If this is not possible, a ≥4.7µF capacitor is recommended for the input capacitor. Aluminum and tantalum
capacitors are not recommended because of their high
ESR.
Further input noise reduction can be achieved by filtering
the input through a very small series inductor as shown in
Figure 5. A 10nH inductor will reject the fast input transients caused by the blanking period, thereby presenting
a nearly constant load to the input supply. For economy,
the 10nH inductor can be fabricated on the PC board with
about 1cm (0.4") of 1mm wide PC board trace.
VIN
SUPPLY
10nH
(TRACE INDUCTANCE)
VIN
1µF
LTC3251
GND
3251 F05
Figure 5. 10nH Inductor Used for
Additional Input Noise Reduction
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or aluminum should never be used for the flying capacitors since
their voltages can reverse upon start-up of the LTC3251.
Ceramic capacitors should always be used for the flying
capacitors.
The flying capacitors control the strength of the charge
pump. In order to achieve the rated output current, it is
necessary for the flying capacitor to have at least 0.4µF of
capacitance over operating temperature with a 2V bias
(see Ceramic Capacitor Selection Guidelines). If only
200mA or less of output current is required for the
application, the flying capacitor minimum can be reduced
to 0.15µF.
Ceramic Capacitor Selection Guidelines
Capacitors of different materials lose their capacitance
with higher temperature and voltage at different rates. For
example, a ceramic capacitor made of X5R or X7R material
will retain most of its capacitance from – 40°C to 85°C,
whereas a Z5U or Y5V style capacitor will lose considerable capacitance over that range (60% to 80% loss typ).
Z5U and Y5V capacitors may also have a very strong
voltage coefficient, causing them to lose an additional
60% or more of their capacitance when the rated voltage
is applied. Therefore, when comparing different capacitors, it is often more appropriate to compare the amount
of achievable capacitance for a given case size rather than
discussing the specified capacitance value. For example,
over rated voltage and temperature conditions, a 4.7µF,
10V, Y5V ceramic capacitor in an 0805 case may not
provide any more capacitance than a 1µF, 10V, X5R or X7R
available in the same 0805 case. In fact, over bias and
32511215fb
11
LTC3251/
LTC3251-1.2/LTC3251-1.5
U
OPERATIO (Refer to Block Diagram)
temperature range, the 1µF, 10V, X5R or X7R will provide
more capacitance than the 4.7µF, 10V, Y5V. The capacitor
manufacturer’s data sheet should be consulted to determine what value of capacitor is needed to ensure minimum capacitance values are met over operating temperature and bias voltage.
Below is a list of ceramic capacitor manufacturers and
how to contact them:
AVX
www.avxcorp.com
Kemet
www.kemet.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
TDK
www.tdk.com
Thermal Management
Layout Considerations
Due to the high switching frequency and transient currents
produced by the LTC3251, careful board layout is necessary for optimal performance. A true ground plane and
short connections to all capacitors will improve performance and ensure proper regulation under all conditions.
Figure 6 shows the recommended layout configuration.
CI
1µF
LTC3251 COMPONENTS NOT USED ON
THE LTC3251-1.2 OR LTC3251-1.5
RB
If the junction temperature increases above approximately
160°C, the thermal shutdown circuitry will automatically
deactivate the output. To reduce the maximum junction
temperature, a good thermal connection to the PC board
is recommended. Connecting the 10-pin MSE paddle
directly to a ground plane, and maintaining a solid ground
plane under the device on one or more layers of the PC
board, can reduce the thermal resistance of the package
and PC board considerably. Using this method a θJA of
40°C/W should be achieved. The actual power dissipated
by the LTC3251 (PD) can be calculated by the following
equation:
⎞
⎛V
PD = ⎜ IN – VOUT⎟ IOUT
⎠
⎝ 2
VIN
RA
C1
1µF
CA
5pF
VOUT
C2
1µF
GND
The flying capacitor pins C1+, C1–, C2+, C2– will have very
high edge rate wave forms. The large dv/dt on these pins
can couple energy capacitively to adjacent printed circuit
board runs. Magnetic fields can also be generated if the
flying capacitors are not close to the part (i.e., the loop area
is large). To decouple capacitive energy transfer, a Faraday
shield may be used. This is a grounded PC trace between
the sensitive node and the IC’s pins. For a high quality AC
ground, it should be returned to a solid ground plane that
extends all the way to the part. Keep the FB trace of the
LTC3251 away from or shielded from the flying capacitor
traces or degraded performance could result.
CO
10µF
Figure 6. Recommended Layout
3251 F06
Power Efficiency
The power efficiency (η) of the LTC3251 family is approximately double that of a conventional linear regulator. This
occurs because the input current for a 2-to-1 step-down
charge pump is approximately half the output current. For
an ideal 2-to-1 step-down charge pump the power efficiency is given by:
η≡
POUT VOUT • IOUT 2VOUT
=
=
1
PIN
VIN
VIN • IOUT
2
32511215fb
12
LTC3251/
LTC3251-1.2/LTC3251-1.5
U
OPERATIO
(Refer to Block Diagram)
At moderate to high output power the switching losses
and quiescent current of the LTC3251 family is negligible
and the expression above is valid. For example with VIN =
3.6V, IOUT = 200mA and VOUT regulating to 1.5V the
measured efficiency is 81% which is in close agreement
with the theoretical 83.3% calculation.
For a 1.5V output, RO is 0.085Ω, which produces a 40mV
output change for a 500mA load current step. Thus, the
user may want to target an unloaded output voltage
slightly higher than desired to compensate for the output
load conditions. The output may be programmed for
regulation voltages of 0.9V to 1.6V.
Programming the LTC3251 Output Voltage (FB Pin)
Since the LTC3251 employs a 2-to-1 charge pump architecture, it is not possible to achieve output voltages
greater than half the available input voltage. The minimum
VIN supply required for regulation can be determined by
the following equation:
The LTC3251 is programmed to an arbitrary output voltage via an external resistive divider. Figure 7 shows the
required voltage divider connection. The voltage divider
ratio is given by the expression:
RA VOUT
=
–1
RB
0.8V
VOUT
LTC3251
CA
RA
COUT
FB
VOUT
R
0.8V 1 + A
RB
( )
RB
GND
3251 F07
Figure 7. Programming the LTC3251
Typical values for total voltage divider resistance can
range from several kΩs up to 1MΩ.
The user may want to consider load regulation when
setting the desired output voltage. The closed loop output
impedance of the LTC3251 is approximately:
RO ≅ 0.045Ω •
VOUT
0.8 V
VIN(MIN) ≥ 2 • (VOUT(MIN) + IOUT • ROL)
The compensation capacitor (CA) is necessary to counteract the pole caused by the large valued resistors RA and RB,
and the input capacitance of the FB pin. For best results, CA
should be 5pF for all RA or RB greater than 10k and can be
omitted if both RA and RB are less than 10k.
Disabling Spread Spectrum Operation on the
LTC3251-1.2/LTC3251-1.5 (MODE Pin)
Spread spectrum operation can be disabled by driving
MODE high. When Mode is high, switching takes place at
the maximum operating frequency (typ 1.6MHz). The
advantage of spread spectrum operation is that it reduces
the peak noise at and above the operating frequency at the
expense of a slightly increased noise floor and slightly
increased low frequency ripple caused by the converter
compensating for the changing operating frequency. Users who do not need the peak noise reduction gained by
using spread spectrum may wish to disable spread spectrum, thus improving the low frequency input/output
ripple.
32511215fb
13
LTC3251/
LTC3251-1.2/LTC3251-1.5
U
TYPICAL APPLICATIO S
0.9V Output Continuous/Burst Mode Operation with Shutdown
OFF ON
1
9
MD0 MD1
LTC3251
7
VOUT
VIN
3
8
1µF
C1+
C2+
1µF
4
6
C1–
C2–
5,11
10
GND
FB
VOUT = 0.9V
500mA
2
1-CELL
Li-Ion
OR
3-CELL
NiMH
10µF
1µF
4.7µF
5pF
73.2k
536k
3251 TA05
3.3V to 1.4V Conversion, Continuous
Spread Spectrum Operation with Shutdown
OFF ON
1
9
MD0 MD1
VIN
3.3V
LTC3251
7
VOUT
VIN
8
3
C1+
C2+
4
6
C1–
C2–
5,11
10
GND
FB
VOUT = 1.4V
IOUT ≤ 350mA
2
1µF
1µF
10µF
1µF
4.12k
5.36k
3251 TA03
32511215fb
14
LTC3251/
LTC3251-1.2/LTC3251-1.5
U
PACKAGE DESCRIPTIO
MSE Package
10-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1663)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.794 ± 0.102
(.110 ± .004)
5.23
(.206)
MIN
0.889 ± 0.127
(.035 ± .005)
1
2.06 ± 0.102
(.081 ± .004)
1.83 ± 0.102
(.072 ± .004)
2.083 ± 0.102 3.20 – 3.45
(.082 ± .004) (.126 – .136)
10
0.50
0.305 ± 0.038
(.0197)
(.0120 ± .0015)
BSC
TYP
RECOMMENDED SOLDER PAD LAYOUT
3.00 ± 0.102
(.118 ± .004)
(NOTE 3)
3.00 ± 0.102
(.118 ± .004)
(NOTE 4)
4.90 ± 0.152
(.193 ± .006)
0.254
(.010)
DETAIL “A”
0° – 6° TYP
1 2 3 4 5
GAUGE PLANE
0.53 ± 0.152
(.021 ± .006)
DETAIL “A”
0.18
(.007)
0.497 ± 0.076
(.0196 ± .003)
REF
10 9 8 7 6
SEATING
PLANE
0.86
(.034)
REF
1.10
(.043)
MAX
0.17 – 0.27
(.007 – .011)
TYP
0.50
(.0197)
BSC
0.127 ± 0.076
(.005 ± .003)
MSOP (MSE) 0603
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
32511215fb
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
LTC3251/
LTC3251-1.2/LTC3251-1.5
U
TYPICAL APPLICATIO
1.2V Output with mProcessor Control of Operating Modes (Spread Spectrum Disabled)
µP
1
9
MD0 MD1
LTC3251-1.2
7
VOUT
VIN
8
3
C1+
C2+
1µF 4
6
C1–
C2–
10
5,11
GND
MODE
2
1-CELL Li-Ion
OR
3-CELL NiMH
1µF
1µF
VOUT = 1.2V
IOUT UP TO 300mA, VIN ≥ 2.8V
10µF I
OUT UP TO 500mA, VIN ≥ 3.0V
X5R
6.3V
3251 TA04
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
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LTC3250-1.5
250mA, 1.5MHz, High Efficiency
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Up to 90% Efficiency, VIN: 3.1V to 5.5V, VOUT: 1.5V, Regulated Output,
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250mA, Dual, Low Noise, Inductorless
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Up to 90% Efficiency, VIN: 2.7V to 5.5V, VOUT: 0.9V to 1.6V,
IQ: 60µA, DFN Package
LTC3404
600mA (IOUT), 1.4MHz, Synchronous
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95% Efficiency, VIN: 2.7V to 6V, VOUT(MIN): 0.8V,
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LTC3405/LTC3405A 300mA (IOUT), 1.5MHz, Synchronous
Step-Down DC/DC Converter
95% Efficiency, VIN: 2.7V to 6V, VOUT(MIN): 0.8V,
IQ: 20µA, ISD: <1µA, ThinSOT Package
LTC3406/LTC3406B 600mA (IOUT), 1.5MHz, Synchronous
Step-Down DC/DC Converter
95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN): 0.6V,
IQ: 20µA, ISD: <1µA, ThinSOT Package
LTC3411
1.25A (IOUT), 4MHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN): 0.8V,
IQ: 60µA, ISD: <1µA, MS Package
LTC3412
2.5A (IOUT), 4MHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN): 0.8V,
IQ: 60µA, ISD: <1µA, TSSOP-16E Package
LTC3440
600mA (IOUT), 2MHz, Synchronous
Buck-Boost DC/DC Converter
95% Efficiency, VIN: 2.5V to 5.5V, VOUT: 2.5V to 5.5V,
IQ: <25µA, ISD: 1µA, MS Package
LTC3441
1.2A (IOUT), 1MHz, Synchronous
Buck-Boost DC/DC Converter
95% Efficiency, VIN: 2.4V to 5.5V, VOUT: 2.4V to 5.25V,
IQ: <25µA, ISD: 1µA, DFN Package
16
Linear Technology Corporation
32511215fb
LT 0306 REV B • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
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© LINEAR TECHNOLOGY CORPORATION 2003