LTC3251/
LTC3251-1.2/LTC3251-1.5
500mA High Efficiency,
Low Noise, Inductorless
Step-Down DC/DC Converter
DESCRIPTIO
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FEATURES
■
■
■
■
■
■
■
■
■
■
■
■
■
■
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
32511215fb
1
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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6
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
U
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 (
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