LTC3251_15 [Linear]
500mA High Efficiency, Low Noise, Inductorless Step-Down DC/DC Converter;
| 型号： | LTC3251_15 |
| 厂家： | 
Linear |
| 描述： | 500mA High Efficiency, Low Noise, Inductorless Step-Down DC/DC Converter |
| 文件： | 总16页 (文件大小：379K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC3251/
LTC3251-1.2/LTC3251-1.5
500mA High Efficiency,
Low Noise, Inductorless
Step-Down DC/DC Converter
U
FEATURES
DESCRIPTIO
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 isresistorprogrammablefrom0.9Vto1.6V
or fixed at 1.2V or 1.5V, with up to 500mA of load current
available.
■
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
A unique 2-phase spread spectrum architecture provides
averylownoiseregulatedoutputaswellaslownoiseatthe
input.* The parts have four operating modes: Continuous
Spread Spectrum, Spread Spectrum with Burst Mode
operation, Super BurstTM mode operation and shutdown.
■
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
Low operating current (35µA in Burst Mode operation,
10µA in Super Burst mode operation) and low external
partscountmaketheLTC3251/LTC3251-1.2/LTC3251-1.5
ideally suited for space-constrained battery-powered
applications. The parts are short-circuit and overtempera-
ture 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.
■
Soft-Start Limits Inrush Current at Turn-On
■
Short-Circuit and Overtemperature Protected
■
Available in a Thermally Enhanced
10-Pin MSOP Package
U
APPLICATIO S
■
Handheld Devices
■
Cellular Phones
■
Portable Electronic Equipment
DSP Power Supplies
■
U
1.5V Efficiency vs Input Voltage
(Burst Mode Operation)
TYPICAL APPLICATIO
100
I
= 200mA
OUT
90
80
70
60
50
40
30
20
10
0
Spread Spectrum Step-Down Converter
LTC3251-1.5
OFF ON
1
9
MD0 MD1
LTC3251-1.5
2
3
4
V
= 1.5V
7
OUT
LDO
V
V
OUT
1-CELL Li-Ion
OR
3-CELL NiMH
IN
500mA
1µF
8
+
+
10µF
C1
C1
C2
1µF
1µF
6
–
–
C2
5, 11
10
GND
MODE
3251 TA01
3
3.5
4
4.5
5
5.5
INPUT VOLTAGE (V)
3251 TA02
32511215fb
1
LTC3251/
LTC3251-1.2/LTC3251-1.5
W W
U W
ABSOLUTE AXI U 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)
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
I
OUT (Note 2) ...................................................... 650mA
U W
U
PACKAGE/ORDER I FOR ATIO
TOP VIEW
TOP VIEW
ORDER PART
NUMBER
ORDER PART
MD0
1
2
3
4
5
10 MODE
MD0
V
1
2
3
4
5
10 MODE
NUMBER
V
9
8
7
6
MD1
9
8
7
6
MD1
IN
IN
+
–
+
+
+
C1
11
C2
C1
11
C2
–
LTC3251EMSE-1.2
LTC3251EMSE-1.5
C1
V
C1
V
OUT
OUT
LTC3251EMSE
–
–
GND
C2
GND
C2
MSE PACKAGE
10-LEAD PLASTIC MSOP
MSE PACKAGE
10-LEAD PLASTIC MSOP
MSE PART MARKING
LTB4
MSE PART MARKING
EXPOSED PAD IS GND (PIN 11),
MUST BE SOLDERED TO PCB
TJMAX = 125°C, θJA = 40°C/W, θJC = 10°C/W
EXPOSED PAD IS GND (PIN 11),
MUST BE SOLDERED TO PCB
TJMAX = 125°C, θJA = 40°C/W, θJC = 10°C/W
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 T = 25°C. V = 3.6V, C1 = C2 = 1µF, C = 1µF, C = 10µF,
OUT
A
IN
IN
V
= 0V for LTC3251-1.2V or LTC3251-1.5, V
= 1.5V for LTC3251, all capacitors ceramic, unless otherwise noted.
OUT
MODE
PARAMETER
CONDITIONS
(Notes 4,5)
(Note 5)
MIN
TYP
MAX
UNITS
V
V
V
Minimum Operating Voltage
●
●
2.7
V
V
IN
IN
IN
Maximum Operating Voltage
Continuous Mode Operating Current
5.5
I
= 0mA, V
= 0, V
= V
IN
●
●
3
3.75
5
6
mA
mA
OUT
MD0
MD1
Spread Spectrum Disabled MODE = V
IN
IN
IN
V
V
Burst Mode Operating Current
I
= 0mA, V
= V , V = 0
IN MD1
●
●
35
35
60
60
µA
µA
IN
IN
OUT
MD0
Spread Spectrum Disabled MODE = V
Super Burst Mode Operating Current
I
= 0mA, V = V , V = V
●
●
10
10
15
15
µA
µA
OUT
MD0
IN MD1
IN
Spread Spectrum Disabled MODE = V
V
V
V
Shutdown Current
V
= 0V, V = 0V (Note 5)
●
●
0.01
0.8
1
µA
IN
MD0
MD1
Regulation Voltage (LTC3251)
I
= 0mA, 2.7V ≤ V ≤ 5.5V
0.78
0.82
V
FB
OUT
IN
Regulation Voltage (LTC3251-1.2)
I
I
I
≤ 200mA, 2.7V ≤ V ≤ 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
OUT
OUT
OUT
OUT
IN
Continuous Mode or Burst Mode Operation
≤ 300mA, 2.8V ≤ V ≤ 5.5V (Note 5)
IN
≤ 500mA, 3V ≤ V ≤ 5.5V (Note 5)
IN
V
Regulation Voltage (LTC3251-1.2)
I
≤ 40mA
●
1.15
1.2
1.25
V
OUT
OUT
Super Burst Operation
V
Regulation Voltage (LTC3251-1.5)
I
I
I
I
≤ 100mA, 3.1V ≤ V ≤ 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
OUT
OUT
OUT
OUT
OUT
IN
Continuous Mode or Burst Mode Operation
≤ 200mA, 3.2V ≤ V ≤ 5.5V (Note 5)
IN
≤ 300mA, 3.3V ≤ V ≤ 5.5V (Note 5)
IN
≤ 500mA, 3.5V ≤ V ≤ 5.5V (Note 5)
IN
V
Regulation Voltage (LTC3251-1.5)
I
≤ 40mA
●
1.44
1.5
1.56
V
OUT
OUT
Super Burst Operation
I
I
Continuous Output Current (LTC3251)
Super Burst Output Current (LTC3251)
V
V
= 0, V
= V or V
= V , V = 0
IN MD1
●
●
500
40
mA
mA
OUT
OUT
MD0
MD0
MD1
IN
MD0
= V , V
= V
IN
IN MD1
Load Regulation (LTC3251)
0mA ≤ I
≤ 500mA, Referred to FB Pin
0.045
mV/mA
32511215fb
OUT
2
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 T = 25°C. V = 3.6V, C1 = C2 = 1µF, C = 1µF, C = 10µF,
OUT
A
IN
IN
V
MODE
= 0V for LTC3251-1.2V or LTC3251-1.5, V
= 1.5V for LTC3251, all capacitors ceramic, unless otherwise noted.
OUT
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Line Regulation (LTC3251)
Spread Spectrum Frequency Range
I
= 500mA, 2.7V ≤ V ≤ 5.5V
0.2
%/V
OUT
IN
f
f
Switching Frequency
Switching Frequency
●
●
0.7
1.3
1.0
1.6
MHz
MHZ
MIN
2
2
MAX
Spread Spectrum Disabled Frequency
MODE = V
●
●
●
●
●
●
●
●
●
●
1.6
0.8
0.8
MHz
V
IN
MD0, MD1 Input High Voltage
2.7V ≤ V ≤ 5.5V
1.2
IN
MD0, MD1 Input Low Voltage
2.7V ≤ V ≤ 5.5V
0.4
–1
V
IN
MD0, MD1 Input High Current
MD0 = V , MD1 = V
1
1
µA
µA
nA
IN
IN
MD0, MD1 Input Low Current
MD0 = 0V, MD1 = 0V
= 0.85V
–1
FB Input Current (LTC3251)
V
–50
50
70
FB
MODE Input High Voltage (LTC3251-1.2/LTC3251-1.5)
MODE Input Low Voltage (LTC3251-1.2/LTC3251-1.5)
MODE Input High Current (LTC3251-1.2/LTC3251-1.5)
MODE Input Low Current (LTC3251-1.2/LTC3251-1.5)
Turn-On Time (Burst or Continuous Mode Operation)
Open-Loop Output Impedance (LTC3251)
2.7V ≤ V ≤ 5.5V
50
50
%/V
%/V
IN
IN
IN
2.7V ≤ V ≤ 5.5V
30
–1
–1
IN
MODE = V
1
1
µA
µA
ms
Ω
IN
MODE = 0V
R
= 3Ω, (Note 5)
1
OL
IN
V
= 3V, I
= 200mA (Note 6)
●
0.45
0.7
OUT
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 4: Minimum operating voltage required for regulation is:
≥ 2 • (V + R • I
V
)
OL OUT
IN
OUT
Note 5: V
= 0V or V = V for LTC3251-1.2/LTC3251-1.5.
MODE IN
MODE
Note 6: Output not in regulation; R = (V /2 – V )/I .
OUT OUT
OL
IN
Note 2: Based on long term current density limitations.
(V = 0.76V). Burst or continuous mode operation.
FB
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.
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.
U W
TYPICAL PERFOR A CE CHARACTERISTICS
No Load Supply Current vs Supply
Voltage (Continuous Mode Spread
Spectrum Enabled)
No Load Supply Current vs Supply
Voltage (Continuous Mode,
Spread Spectrum Disabled)
No Load Supply Current vs Supply
Voltage (Burst Mode Operation)
10
9
8
7
6
5
4
3
2
1
0
50
45
40
35
30
25
20
7
6
5
4
3
2
1
0
–40°C
25°C
85°C
–40°C
25°C
85°C
85°C
25°C
–40°C
4.7
2.7
3.2
3.7
4.2
(V)
5.2
4.7
2.7
3.2
3.7
4.2
(V)
4.7
5.2
2.7
3.2
3.7
4.2
(V)
5.2
V
IN
V
V
IN
IN
3251 G17
3251 G01
3251 G02
32511215fb
3
LTC3251/
LTC3251-1.2/LTC3251-1.5
U W
TYPICAL PERFOR A CE CHARACTERISTICS
No Load Supply Current
1.5V Output Voltage vs Supply
Voltage (Burst Mode Operation/
Continuous Mode)
1.2V Output Voltage vs Supply
Voltage (Burst Mode Operation/
Continuous Mode)
vs Supply Voltage
(Super Burst Mode Operation)
1.300
1.280
1.260
1.240
1.220
1.200
1.180
1.160
1.140
1.120
1.100
20
18
16
14
12
10
8
1.60
1.58
1.56
1.54
1.52
1.50
1.48
1.46
1.44
1.42
1.40
T
= 25°C
T
= 25°C
A
A
I
= 0mA
OUT
I
= 250mA
I
= 0mA
OUT
OUT
85°C
25°C
I
I
= 250mA
OUT
–40°C
I
= 500mA
= 500mA
OUT
OUT
6
4
2
0
4.7
2.7
3.2
3.7
4.2
(V)
5.2
4.7
2.7
3.2
3.7
4.2
(V)
5.2
3
3.5
4
4.5
5
5.5
V
V
IN
V
(V)
IN
IN
3251 G05
3251 G02
3251 G04
1.2V Output Voltage
vs Supply Voltage
(Super Burst Mode Operation)
1.5V Output Voltage
vs Supply Voltage
(Super Burst Mode Operation)
FB Voltage vs Output Current
(Burst Mode Operation/
Continuous Mode)
1.30
1.28
1.26
1.24
1.22
1.20
1.18
1.16
1.14
1.12
1.10
1.60
1.58
1.56
1.54
1.52
1.50
1.48
1.46
1.44
1.42
1.40
0.805
0.800
0.795
0.790
T = 25°C
A
0mA
10mA
40mA
T
= 25°C
0mA
10mA
40mA
A
T
OUT
= 25°C
A
V
= 1.5V
0.785
0.780
4.7
2.7
3.2
3.7
4.2
(V)
5.2
3
3.5
4
4.5
(V)
5
5.5
0
200
300
(mA)
400
500
600
100
V
I
V
IN
IN
OUT
3251 G18
3251 G06
3251 G07
1.5V Output Efficiency vs Output
Current (Super Burst Mode
Operation)
1.2V Output Efficiency vs Output
Current (Burst Mode Operation)
1.5V Output Efficiency vs Output
Current (Burst Mode Operation)
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
V
= 3.6V
IN
V
= 3V
IN
V
= 3.6V
V
= 2.7V
IN
V
= 3.3V
IN
IN
V
= 3.3V
IN
V
V
= 3.5V
V
V
= 4V
= 5V
IN
IN
V
= 4V
IN
IN
= 4.5V
V
= 5V
IN
IN
MD0 = V , MD1 = 0V
MD0 = MD1 = V
IN
MD0 = V , MD1 = 0V
IN
IN
0.1
1
10
(mA)
100
1000
0.1
1
10
(mA)
100
1000
0.1
1
10
100
I
I
(mA)
OUT
I
OUT
OUT
3251 G08
3251 G19
3251 G09
32511215fb
4
LTC3251/
LTC3251-1.2/LTC3251-1.5
U W
TYPICAL PERFOR A CE CHARACTERISTICS
MD0/MD1 Input Threshold Voltage
vs Supply Voltage
Max/Min Oscillator Frequency
vs Supply Voltage
1.2
1.1
1.0
2.0
1.9
1.8
25°C MAX
–40°C MAX
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
–40°C
0.9
85°C MAX
25°C
0.8
85°C
0.7
25°C MIN
85°C MIN
0.6
0.5
0.4
–40°C MIN
4.7
2.7
3.7
4.2
(V)
4.7
5.2
2.7
3.2
3.7
4.2
(V)
5.2
3.2
V
V
IN
IN
3251 G10
3251 G11
Output Transient Response
(Burst Mode Operation)
Output Transient Response
(Continuous Mode)
450mA
50mA
450mA
50mA
IOUT
IOUT
VIN
VOUT
20mV/DIV
(AC)
VOUT
20mV/DIV
(AC)
VOUT
20mV/DIV
(AC)
TA = 25°C
COUT = 10µF X5R 6.3V
OUT = 1.5V
10µs/DIV
3251 G14
T
A = 25°C
10µs/DIV
3251 G13
COUT = 10µF X5R 6.3V
V
VOUT = 1.5V
Supply Transient Response
(Continuous Mode)
LTC3251-1.5 Output Voltage
Ripple
4.5V
VIN
SPREAD
SPECT
3.5V
ENABLED
10mV/DIV (AC)
VOUT
20mV/DIV (AC)
SPREAD
SPECT
DISABLED
10mV/DIV (AC)
TA = 25°C
20µs/DIV
3251 G15
T
A = 25°C
200ns/DIV
3251 G16
COUT = 10µF X5R 6.3V
IOUT = 250mA
COUT = 10µF X5R 6.3V
IOUT = 500mA
VOUT = 1.5V
VOUT = 1.5V
32511215fb
5
LTC3251/
LTC3251-1.2/LTC3251-1.5
U
U
U
PI FU CTIO S
MD0(Pin1)/MD1(Pin9):SwitchingModeInputPins.The
Mode input pins are used to set the operating mode of the
LTC3251. The modes of operation are:
GND (Pin 5, 11): Ground. Connect to a ground plane for
best performance.
C2– (Pin 6): Flying Capacitor 2 Negative Terminal (C2).
MD1
MD0
OPERATING MODE
Shutdown
VOUT (Pin 7): Regulated Output Voltage. VOUT is discon-
nected from VIN during shutdown. Bypass VOUT with a
low ESR ceramic capacitor to GND (CIN). See VOUT
Capacitor Selection for capacitor size requirements.
0
0
1
1
0
1
0
1
Spread Spectrum with Burst
Continuous Spread Spectrum
Super Burst
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.
MD0andMD1arehighimpedanceCMOSinputsandmust
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).
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.
32511215fb
6
LTC3251/
LTC3251-1.2/LTC3251-1.5
W
W
SI PLIFIED BLOCK DIAGRA
LTC3251-1.2/
LTC3251-1.5
ONLY
1
9
10
MD0
MD1
MODE
OVERTEMP
SWITCH CONTROL
AND SOFT-START
SPREAD SPECTRUM
OSCILLATOR
CHARGE
PUMP 1
V
IN
2
+
–
C1
C1
3
4
7
INTERNAL ON
LTC3251-1.2/
LTC3251-1.5
V
OUT
CHARGE
PUMP 2
+
C2
8
–
C2
6
–
+
FB
10
BURST DETECT
CIRCUIT
GND
5
11
3251 BD
32511215fb
7
LTC3251/
LTC3251-1.2/LTC3251-1.5
U
(Refer to Block Diagram)
OPERATIO
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
errorsignal. A2-phasenonoverlappingclockactivatesthe
two charge pumps. The two charge pumps work in paral-
lel, 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
ChargePump1.NotonlyiscurrentbeingdeliveredtoVOUT
on the first phase, but the flying capacitor is also being
charged. On the second phase of the clock, flying capaci-
tor 1 is connected from VOUT to ground, transferring the
chargestoredduringthefirstphaseoftheclocktoVOUT 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
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 tempera-
ture exceeds approximately 160°C. It will re-enable the
charge pumps once the junction temperature drops back
toapproximately150°C. TheLTC3251willcycleinandout
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
candegradetheperformanceorshortenthelifeofthepart.
Soft-Start
To prevent excessive current flow at VIN during start-up,
the LTC3251 family has built-in soft-start circuitry. Soft-
start 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 en-
abled whenever the device is brought out of shutdown,
and is disabled shortly after regulation is achieved.
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.
Spread Spectrum Operation
Switchingregulatorscanbeparticularlytroublesomewhere
electromagnetic interference (EMI) is concerned. Switch-
ingregulatorsoperateonacycle-by-cyclebasistotransfer
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
componentsofnoiseatthefrequencyofoperation(funda-
mental) and multiples of the operating frequency (har-
monics). Figure 1a shows a conventional buck switching
converter.Figures1band1caretheinputandoutputnoise
spectrums for the buck converter of Figure 1 with VIN =
3.6V, VOUT = 1.5V and IOUT = 500mA.
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.SinceMD0,MD1andMODEpinsarehighimpedance
CMOS inputs, they should never be allowed to float.
Always drive MD0, MD1 and Mode with valid logic levels.
32511215fb
8
LTC3251/
LTC3251-1.2/LTC3251-1.5
U
(Refer to Block Diagram)
OPERATIO
4.7µH
10nH*
22µF
10nH*
IN
SW
FB
V
IN
OUT
V
OUT
V
V
IN
OUT
IN
10µF
1µF
1µF
1µF
10µF
1µF
LTC3251
FB
+
+
–
COMP
C1
C1
C2
*10nH = 1cm OF PCB TRACE
*10nH = 1cm OF PCB TRACE
1µF
GND
–
C2
GND
3251 F01a
3251 F02a
Figure 1a. Conventional Buck Switching Converter
Figure 2a. LTC3251 Buck Converter
–40
–50
–60
–70
–80
–90
–40
–50
–60
–70
–80
–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
Figure 2b. LTC3251 Output Noise Spectrum
Spectrum with 22µF Output Capacitor (I = 500mA)
with 10µF Output Capacitor (I = 500mA)
O
O
–40
–50
–60
–70
–80
–90
–40
–50
–60
–70
–80
–90
START FREQ: 100kHz RBW: 10kHz STOP FREQ: 30MHz
START FREQ: 100kHz RBW: 10kHz STOP FREQ: 30MHz
3251 F01c
3251 F02c
Figure 1c. Conventional Buck Converter Input Noise
Figure 2c. LTC3251 Input Noise Spectrum
with 1µF Input Capacitor (I = 500mA)
O
Spectrum with 10µF Input Capacitor (I = 500mA)
O
Low Current Burst Mode Operation
Unlikeconventionalbuckconverters,theLTC3251’sinter-
nal oscillator is designed to produce a clock pulse whose
period is random on a cycle-by-cycle basis, but fixed
between1MHzand1.6MHz.Thishasthebenefitofspread-
ing 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
Toimproveefficiencyatlowoutputcurrents,aBurstMode
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
isdependantonmanyfactors(i.e.,supply,switchstrength,
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.
32511215fb
9
LTC3251/
LTC3251-1.2/LTC3251-1.5
U
(Refer to Block Diagram)
OPERATIO
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 re-
quiring a larger output capacitor for stability. Figure 3
shows the suggested output capacitor for optimal tran-
sientresponse.Thevalueoftheoutputcapacitanceshould
not drop below the minimum capacitance line to prevent
excessive ringing or instability. (see Ceramic Capacitor
Selection Guidelines section).
capacitor selection, etc.), the part’s burst current is set by
the burst threshold and hysteresis. This means that the
V
OUT ripple voltage in Burst Mode operation will be fixed
and is typically 15mV with a 10µF output capacitor.
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 hyster-
etic 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 LTC3251-
1.5 has no effect on operation in super-burst mode.
16
15
14
OPTIMUM CAPACITANCE
13
12
11
10
9
8
MINIMUM CAPACITANCE
7
V
OUT Capacitor Selection
6
5
4
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.
0.9
1.1 1.2 1.3
(V)
1.4 1.5 1.6
1.0
V
OUT
3251 F03
Figure 3
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
isaperiodwhereallswitchesareturnedoff.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. Tanta-
lum and aluminum capacitors are not recommended be-
cause of their high ESR.
Likewise excessive ESR on the output capacitor will tend
to degrade the loop stability. The closed loop output
impedance of the LTC3251 is approximately:
VOUT
RO ≅ 0.045Ω •
0.8V
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.
Both the style and value of COUT can significantly affect the
stability of the LTC3251 family. As shown in the Block
32511215fb
10
LTC3251/
LTC3251-1.2/LTC3251-1.5
U
(Refer to Block Diagram)
OPERATIO
10nH
Furtheroutputnoisereductioncanbeachievedbyfiltering
the LTC3251 output through a very small series inductor
as shown in Figure 4. A 10nH inductor will reject the fast
outputtransientscausedbytheblankingperiod.The10nH
inductorcanbefabricatedonthePCboardwithabout1cm
(0.4") of 1mm wide PC board trace.
(TRACE INDUCTANCE)
V
IN
V
IN
SUPPLY
1µF
LTC3251
GND
3251 F05
Figure 5. 10nH Inductor Used for
Additional Input Noise Reduction
10nH
(TRACE INDUCTANCE)
Flying Capacitor Selection
V
V
OUT
OUT
10µF
1µF
LTC3251
GND
Warning: A polarized capacitor such as tantalum or alumi-
num 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.
3251 F04
Figure 4. 10nH Inductor Used for
Additional Output Noise Reduction
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.
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).
Ceramic Capacitor Selection Guidelines
Incaseswherethesupplyimpedanceishigh,heavyoutput
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 connec-
tions. If this is not possible, a ≥4.7µF capacitor is recom-
mended for the input capacitor. Aluminum and tantalum
capacitors are not recommended because of their high
ESR.
Capacitors of different materials lose their capacitance
with higher temperature and voltage at different rates. For
example,aceramiccapacitormadeofX5RorX7Rmaterial
will retain most of its capacitance from –40°C to 85°C,
whereas a Z5U or Y5V style capacitor will lose consider-
able 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 capaci-
tors, 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
provideanymorecapacitancethana1µF,10V,X5RorX7R
available in the same 0805 case. In fact, over bias and
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 tran-
sients 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.
32511215fb
11
LTC3251/
LTC3251-1.2/LTC3251-1.5
U
(Refer to Block Diagram)
OPERATIO
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 deter-
mine what value of capacitor is needed to ensure mini-
mum capacitance values are met over operating tempera-
ture and bias voltage.
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
flyingcapacitorsarenotclosetothepart(i.e.,thelooparea
islarge).Todecouplecapacitiveenergytransfer,aFaraday
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.
Below is a list of ceramic capacitor manufacturers and
how to contact them:
AVX
Kemet
www.avxcorp.com
www.kemet.com
www.murata.com
www.t-yuden.com
www.tdk.com
Murata
Taiyo Yuden
TDK
Thermal Management
Ifthejunctiontemperatureincreasesaboveapproximately
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:
Layout Considerations
Duetothehighswitchingfrequencyandtransientcurrents
produced by the LTC3251, careful board layout is neces-
sary for optimal performance. A true ground plane and
short connections to all capacitors will improve perfor-
mance and ensure proper regulation under all conditions.
Figure 6 shows the recommended layout configuration.
LTC3251 COMPONENTS NOT USED ON
C
THE LTC3251-1.2 OR LTC3251-1.5
I
1µF
R
B
⎛
⎞
V
2
IN
PD =
– VOUT IOUT
⎜
⎟
⎝
⎠
V
C
IN
A
R
A
5pF
C1
1µF
Power Efficiency
V
OUT
C2
1µF
GND
Thepowerefficiency(η)oftheLTC3251familyisapproxi-
mately 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 effi-
ciency is given by:
3251 F06
C
O
10µF
Figure 6. Recommended Layout
POUT VOUT •IOUT 2VOUT
η ≡
=
=
P
1
V
IN
IN
V • IOUT
IN
2
32511215fb
12
LTC3251/
LTC3251-1.2/LTC3251-1.5
U
(Refer to Block Diagram)
OPERATIO
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.
Since the LTC3251 employs a 2-to-1 charge pump archi-
tecture, 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:
Programming the LTC3251 Output Voltage (FB Pin)
The LTC3251 is programmed to an arbitrary output volt-
age via an external resistive divider. Figure 7 shows the
required voltage divider connection. The voltage divider
ratio is given by the expression:
VIN(MIN) ≥ 2 • (VOUT(MIN) + IOUT • ROL)
RA VOUT
RB 0.8V
The compensation capacitor (CA) is necessary to counter-
actthepolecausedbythelargevaluedresistorsRA andRB,
andtheinputcapacitanceoftheFBpin.Forbestresults,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.
=
– 1
V
V
OUT
OUT
R
R
A
B
C
R
R
LTC3251
FB
A
A
B
0.8V 1 +
(
)
C
OUT
Disabling Spread Spectrum Operation on the
LTC3251-1.2/LTC3251-1.5 (MODE Pin)
GND
3251 F07
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. Us-
ers who do not need the peak noise reduction gained by
using spread spectrum may wish to disable spread spec-
trum, thus improving the low frequency input/output
ripple.
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:
VOUT
RO ≅ 0.045Ω •
0.8V
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
V
OUT
= 0.9V
500mA
2
3
7
V
V
OUT
IN
1-CELL
Li-Ion
OR
3-CELL
NiMH
8
+
+
1µF
1µF
10µF
4.7µF
C1
C1
C2
1µF
73.2k
4
6
–
–
5pF
C2
5,11
10
GND
FB
536k
3251 TA05
3.3V to 1.4V Conversion, Continuous
Spread Spectrum Operation with Shutdown
OFF ON
1
9
MD0 MD1
LTC3251
V
V
I
= 1.4V
≤ 350mA
OUT
OUT
2
3
7
V
IN
V
IN
OUT
3.3V
8
1µF
1µF
+
+
10µF
C1
C1
C2
C2
1µF
4
6
–
–
4.12k
5,11
10
GND
FB
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
2.06 ± 0.102
(.081 ± .004)
1.83 ± 0.102
(.072 ± .004)
2.794 ± 0.102
(.110 ± .004)
0.889 ± 0.127
(.035 ± .005)
1
5.23
(.206)
MIN
2.083 ± 0.102 3.20 – 3.45
(.082 ± .004) (.126 – .136)
10
0.50
(.0197)
BSC
0.305 ± 0.038
(.0120 ± .0015)
TYP
3.00 ± 0.102
(.118 ± .004)
(NOTE 3)
0.497 ± 0.076
(.0196 ± .003)
REF
10 9
8
7 6
RECOMMENDED SOLDER PAD LAYOUT
3.00 ± 0.102
(.118 ± .004)
(NOTE 4)
4.90 ± 0.152
(.193 ± .006)
DETAIL “A”
0.254
(.010)
0° – 6° TYP
1
2
3
4 5
GAUGE PLANE
0.53 ± 0.152
(.021 ± .006)
0.86
(.034)
REF
1.10
(.043)
MAX
DETAIL “A”
0.18
(.007)
SEATING
PLANE
0.17 – 0.27
(.007 – .011)
TYP
0.127 ± 0.076
(.005 ± .003)
MSOP (MSE) 0603
0.50
(.0197)
BSC
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 represen-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
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
V
I
I
= 1.2V
OUT
OUT
OUT
2
3
4
7
V
IN
V
OUT
UP TO 300mA, V ≥ 2.8V
1-CELL Li-Ion
OR
3-CELL NiMH
IN
10µF
X5R
6.3V
8
UP TO 500mA, V ≥ 3.0V
IN
1µF
+
+
C1
C1
C2
1µF
1µF
6
–
–
C2
5,11
10
GND
MODE
3251 TA04
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PART NUMBER
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LTC3441
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2.5A (I ), 4MHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, V : 2.5V to 5.5V, V
IN
I : 60µA, I : <1µA, TSSOP-16E Package
Q SD
OUT
OUT(MIN)
600mA (I ), 2MHz, Synchronous
Buck-Boost DC/DC Converter
95% Efficiency, V : 2.5V to 5.5V, V : 2.5V to 5.5V,
IN OUT
I : <25µA, I : 1µA, MS Package
Q SD
OUT
1.2A (I ), 1MHz, Synchronous
Buck-Boost DC/DC Converter
95% Efficiency, V : 2.4V to 5.5V, V : 2.4V to 5.25V,
IN OUT
I : <25µA, I : 1µA, DFN Package
Q SD
OUT
32511215fb
LT 0306 REV B • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
16
●
●
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2003
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