LTC1911EMS8-1.8#TRPBF [Linear]
LTC1911 - Low Noise, High Efficiency, Inductorless Step-Down DC/DC Converter; Package: MSOP; Pins: 8; Temperature Range: -40°C to 85°C;
| 型号： | LTC1911EMS8-1.8#TRPBF |
| 厂家： | 
Linear |
| 描述： | LTC1911 - Low Noise, High Efficiency, Inductorless Step-Down DC/DC Converter; Package: MSOP; Pins: 8; Temperature Range: -40°C to 85°C 光电二极管 |
| 文件： | 总12页 (文件大小：223K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC1911-1.5/LTC1911-1.8
Low Noise, High Efficiency,
Inductorless Step-Down
DC/DC Converter
U
FEATURES
DESCRIPTIO
■
Low Noise Constant Frequency Operation
The LTC®1911 is a switched capacitor step-down DC/DC
converter that produces a 1.5V or 1.8V regulated output
from a 2.7V to 5.5V input. The part uses switched capaci-
torfractionalconversiontoachievehighefficiencyoverthe
entire input range. No inductors are required. Internal cir-
cuitry controls the step-down conversion ratio to optimize
efficiency as the input voltage and load conditions vary.*
Typical efficiency is over 25% higher than that of a linear
regulator.
■
2.7V to 5.5V Input Voltage Range
■
No Inductors
■
Typical Efficiency 25% Higher Than LDOs
■
Shutdown Disconnects Load from VIN
■
Output Voltage: 1.8V ±4% or 1.5V ±4%
■
Output Current: 250mA
■
Low Operating Current: IIN = 180µA Typ
■
Low Shutdown Current: IIN = 10µA Typ
■
Oscillator Frequency: 1.5MHz
A unique constant frequency architecture provides a low
noise regulated output as well as lower input noise than
conventional charge pump regulators. High frequency
operation (fOSC = 1.5MHz) simplifies output filtering to
further reduce conducted noise. To optimize efficiency,
the part enters Burst Mode® operation under light load
conditions.
■
Soft-Start Limits Inrush Current at Turn On
■
Short-Circuit and Overtemperature Protected
■
Available in an 8-Pin MSOP Package
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APPLICATIO S
■
Handheld Computers
■
Cellular Phones
Low operating current (180µA with no load, 10µA in
shutdown) and low external parts count (two 1µF flying
capacitors and two 10µF bypass capacitors) make the
LTC1911 ideally suited for space constrained battery-
powered applications. The part is short-circuit and
overtemperature protected, and is available in an 8-pin
MSOP package.
■
Smart Card Readers
■
Portable Electronic Equipment
■
Handheld Medical Instruments
■
Low Power DSP Supplies
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
*U.S. Patent #6,438,005
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TYPICAL APPLICATIO
Efficiency
90
Single Cell Li-Ion to 1.8V DC/DC Converter
80
100mA
250mA
70
60
LTC1911-1.8
2.7V TO 5.5V INPUT
1
2
3
4
8
6
7
5
V
SS/SHDN
IN
1-CELL Li-Ion
OR
V
I
= 1.8V
OUT
OUT
+
10µF*
1µF*
C2
C2
V
OUT
= 250mA
3-CELL NiMH
50
40
30
+
10µF*
–
C1
IDEAL LDO
1µF*
–
GND
C1
1911 TA01
*CERAMIC CAPACITOR
V
OUT
= 1.8V
2
3
4
5
6
INPUT VOLTAGE (V)
1911 G05
1911f
1
LTC1911-1.5/LTC1911-1.8
W W U W
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ABSOLUTE AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
(Note 1)
ORDER PART
VIN to GND...................................................–0.3V to 6V
SS/SHDN to GND........................ – 0.3V to (VIN + 0.3V)
VOUT Short-Circuit Duration............................ Indefinite
Operating Temperature Range (Note 2) .. –40°C to 85°C
Storage Temperature Range ................. –40°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
NUMBER
TOP VIEW
V
C2
C2
1
2
3
4
8 SS/SHDN
7 C1
IN
+
+
LTC1911EMS8-1.5
LTC1911EMS8-1.8
–
6 V
5 C1
OUT
–
GND
MS8 PACKAGE
8-LEAD PLASTIC MSOP
MS8 PART MARKING
TJMAX = 125°C, θJA = 160°C/ W
LTMY
LTNU
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are TA = 25°C. VIN = 3.6V, C1 = 1µF, C2 = 1µF, CIN = 10µF, COUT = 10µF unless
otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
V
Operating Voltage
IN
OUT
●
2.7
5.5
V
LTC1911-1.5, 0mA ≤ I
LTC1911-1.8, 0mA ≤ I
≤ 250mA, V = 2.7V to 5.5V
●
●
1.44
1.73
1.5
1.8
1.56
1.87
V
V
OUT
OUT
IN
≤ 250mA, V = 2.7V to 5.5V
IN
V
V
Operating Current
I
= 0mA, V = 2.7V to 5.5V
●
●
180
10
350
20
µA
µA
IN
IN
OUT
IN
Shutdown Current
SS/SHDN = 0V, V = 2.7V to 5.5V
IN
Output Ripple
I
I
= 10mA
= 250mA
5
12
mV
mV
OUT
OUT
P-P
P-P
V
Short-Circuit Current
V
= 0V
OUT
600
1.5
0.6
mA
MHz
V
OUT
Switching Frequency
Oscillator Free Running
1.2
0.3
–5
1.8
1
SS/SHDN Input Threshold
SS/SHDN Soft-Start Current
●
●
V
V
= 0V (Note 3)
–2
0.01
–1
µA
µA
SS/SHDN
SS/SHDN
= V
IN
Turn-On Time
C
C
= 0pF, V = 3.3V
0.03
10
ms
ms
SS
SS
IN
= 10nF, V = 3.3V
IN
Load Regulation
Line Regulation
0V ≤ I
0V ≤ I
≤ 250mA
≤ 250mA
0.13
0.3
mV/mA
%/V
OUT
OUT
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 3: Currents flowing into the device are positive polarity. Currents
flowing out of the device are negative polarity.
Note 2: The LTC1911E 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.
1911f
2
LTC1911-1.5/LTC1911-1.8
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TYPICAL PERFOR A CE CHARACTERISTICS
Input Operating Current
vs Input Voltage
Input Shutdown Current
vs Input Voltage
LTC1911-1.8
Output Voltage vs Input Voltage
210
200
190
180
170
160
150
1.90
15
13
11
9
V
V
= 0V
I
= 250mA
OUT
(SS/SHDN)
OUT
= 0V
T
T
T
= –40°C
= 25°C
= 85°C
A
A
A
T
= 85°C
= 25°C
1.85
1.80
1.75
1.70
A
T
T
= 85°C
= 25°C
A
A
T
A
T
= –40°C
A
T
= –40°C
A
7
5
4
3
4
5
2
4
5
6
2
3
5
6
2
3
6
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
1911 G01
1911 G02
1911 G03
LTC1911-1.8
Efficiency vs Output Current
LTC1911-1.5
Output Voltage vs Input Voltage
LTC1911-1.5 Efficiency vs Input
Voltage (Falling Input Voltage)
90
80
70
60
50
40
30
1.55
1.53
1.51
1.49
1.47
1.45
100
90
80
70
60
50
40
30
20
I
= 250mA
OUT
T
T
T
= –40°C
= 25°C
= 85°C
A
A
A
100mA
250mA
V
:
IN
2.7V
3.2V
3.7V
4.2V
5.1V
5.5V
IDEAL LDO
1
10
100
OUTPUT CURRENT (mA)
1000
4
5
4
2
3
2
3
5
6
6
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
1911 G06
LTXXXX • TPCXX
1911 G05
LTC1911-1.5
Efficiency vs Output Current
LTC1911-1.8
Output Voltage vs Output Current
LTC1911-1.5
Output Voltage vs Output Current
90
80
70
60
50
40
30
1.54
1.52
1.50
1.48
1.46
1.44
1.84
1.82
1.80
1.78
1.76
1.74
V
= 3.6V
IN
V
= 3.6V
T
T
T
= –40°C
= 25°C
= 85°C
IN
A
A
A
T
A
= 85°C
T
= 25°C
A
T
= –40°C
A
V
:
IN
2.8V
3.3V
3.7V
4.3V
5.1V
5.5V
1
10
100
1000
1
10
100
0.1
1
10
100
1000
0.1
1000
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
1911 G07
1911 G09
1911 G08
1911f
3
LTC1911-1.5/LTC1911-1.8
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TYPICAL PERFOR A CE CHARACTERISTICS
Start-Up Time
vs Soft-Start Capacitor
Output Ripple
vs Output Load Current
Oscillator Frequency
vs Input Supply Voltage
100
10
1
30
25
20
15
10
5
1.60
1.55
1.50
1.45
V
IN
= 3.6V
T
A
T
A
T
A
= –40°C
= 25°C
= 85°C
T
= –40°C
A
C
= 4.7µF
OUT
T
= 25°C
= 85°C
A
C
C
= 10µF
= 22µF
OUT
OUT
T
A
1.40
0
0.1
0
100
150
200
250
300
2.5
3.0
3.5
4.0
4.5
5.0
5.5
50
0.1
1
10
100
V
(V)
OUTPUT LOAD CURRENT (mA)
SOFT-START CAPACITOR (nF)
IN
1911 G11
1911 G15
1911 G10
LTC1911-1.8 Output Voltage Ripple
Output Current Transient Response
Line Transient Response
VOUT
250mA
4V
50mV/DIV
IOUT
VIN
2-TO-1 MODE
25mA
500mV/DIV
V
IN = 5V
VOUT
3V
50mV/DIV
3-TO-2 MODE
VOUT
20mV/DIV
V
IN = 3.6V
VOUT
20mV/DIV
VOUT
50mV/DIV
1-TO-1 MODE
VIN = 2.7V
I
OUT = 250mA
100ns/DIV
1911 G12
VIN = 3.6V
10µs/DIV
1911 G13
I
OUT = 225mA
20µs/DIV
1911 G14
ALL WAVEFORMS AC COUPLED
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PI FU CTIO S
VIN (Pin 1): Input Supply Voltage. VIN may be between
2.7V and 5.5V. Suggested bypass for VIN is a 10µF (1µF
min) ceramic low ESR capacitor.
C2+ (Pin 2): Flying Capacitor Two Positive Terminal.
C2– (Pin 3): Flying Capacitor Two Negative Terminal.
SS/SHDN (Pin 8): Soft-Start/Shutdown Control Pin. This
pin is designed to be driven with an external open-drain
output. Holding the SS/SHDN pin below 0.3V will force
the LTC1911-X into shutdown mode. An internal pull-up
current of 2µA will force the SS/SHDN voltage to climb to
VIN once the device driving the pin is forced into a Hi-Z
state. To limit inrush current on start-up, connect a
capacitor between the SS/SHDN pin and GND. Capaci-
tance on the SS/SHDN pin will limit the dV/dt of the pin
GND (Pin 4): Ground. Connect to a ground plane for best
performance.
C1– (Pin 5): Flying Capacitor One Negative Terminal.
during turn on which, in turn, will limit the dV/dt of VOUT
.
By selecting an appropriate soft-start capacitor, the user
can control the inrush current for a known output capaci-
tor during turn-on (see Application Information). If nei-
ther of the two functions are desired, the pin may be left
floating or tied to VIN.
VOUT (Pin 6): Regulated Output Voltage. VOUT is discon-
nected from VIN during shutdown. Bypass VOUT with a
≥10µF ceramic low ESR capacitor (4µF min, ESR < 0.1Ω
max).
C1+ (Pin 7): Flying Capacitor One Positive Terminal.
1911f
4
LTC1911-1.5/LTC1911-1.8
W
W
SI PLIFIED BLOCK DIAGRA
R
A
V
IN
1
C
IN
300k
50k
+
C1
7
+
–
C1
–
C1
5
STEP-DOWN
MODE
CONTROL
CHARGE
+
–
PUMP
C2
C2
2
+
–
150k
C2
3
6
SHDN
R
SENSE
V
OUT
C
OUT
+
–
+
–
ADJ
OFFSET
AMP1
COMP1
COMP2
V
REF
–
+
BURST
THRESHOLD
60k
+
–
OVERTEMP
DETECT
SHORT-CIRCUIT
THRESHOLD
–
AMP2
+
1.5MHz
OSCILLATOR
V
IN
140k
–
V
RAMP
REF
SOFT-START
2µA
600mV
+
+
1.26V
SS/SHDN
V
+
–
REF
GND
8
4
SHDN
+
600mV
1911 BD
1911f
5
LTC1911-1.5/LTC1911-1.8
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APPLICATIO S I FOR ATIO
General Operation
Step-Down Charge Transfer Operation
The LTC1911 uses a switch capacitor-based DC/DC con-
version to provide the efficiency advantages associated
with inductor-based circuits as well as the cost and
simplicityadvantagesofalinearregulator. TheLTC1911’s
unique constant frequency architecture provides a low
noise regulated output as well as lower input noise than
conventional switch-capacitor charge pump regulators.
Figure 1a shows the switch configuration that is used for
2-to-1 step down mode. In this mode, a 2-phase clock
generates the switch control signals. On phase one of the
clock, the top plate of C1 is connected to VIN through RA
and S4, the bottom plate is connected to VOUT through S3.
The amount of charge transferred to C1 (and VOUT) is set
by the value of RA.
The LTC1911 uses an internal switch network and frac-
tional conversion ratios to achieve high efficiency over
widely varying VIN and output load conditions. Internal
control circuitry selects the appropriate step-down con-
versionratiobasedonVIN andloadconditionstooptimize
efficiency. The part has three possible step-down modes:
2-to-1, 3-to-2 or 1-to-1 step-down mode. Only two exter-
nal flying caps are needed to operate in all three modes.
2-to-1 mode is chosen when VIN is greater than two times
the desired VOUT. 3-to-2 mode is chosen when VIN is
greater than 1.5 times VOUT but less than 2 times VOUT. 1-
On phase two, flying capacitor C1 is connected to VOUT
through S1 and to GND through S2. The charge that was
transferred onto C1 from the previous cycle is now trans-
ferred to the output. Thus, in 2-to-1 mode, charge is
transferred to VOUT on both phases of the clock. Since
charge current is sourced from GND on the second phase
of the clock, current multiplication is realized with respect
to VIN, i.e., IOUT equals approximately 2 • IIN. This results
in significant efficiency improvement relative to a linear
regulator. The value of RA is set by the control loop of the
regulator.
to-1 mode is chosen when VIN falls below 1.5 times VOUT
.
S4
φ1
S1
φ2
+
R
A
C1
An internal load current sense circuit controls the switch
point of the step-down ratio as needed to maintain output
regulation over all load conditions.
V
V
OUT
IN
C1
S3
φ1
–
C1
Regulation is achieved by sensing the output voltage and
regulating the amount of charge transferred per cycle.
This method of regulation provides much lower input and
output ripple than that of conventional switched capacitor
charge pumps. The constant frequency charge transfer
also makes additional output or input filtering much less
demanding than conventional switched capacitor charge
pumps.
1911 F01a
S2
φ2
Figure 1a. Step-Down Charge Transfer in 2-to-1 Mode
The 3-to-2 conversion mode also uses a nonoverlapping
clock for switch control but requires two flying capacitors
andatotalofsevenswitches(seeFigure1b).Onphaseone
of the clock, the two capacitors are connected in parallel to
VIN through RA by switches S5 and S7, and to VOUT
through S4 and S6. The amount of charge transferred to
C1||C2 (and VOUT) is set by the regulator control loop
which determines the value of RA. On phase two, C1 and
C2 are connected in series from VOUT to GND through
switches S1, S2 and S3. On phase two, half of the charge
The LTC1911 also has a Burst Mode function that delivers
a minimum amount of charge for one cycle then goes into
alowcurrentstateuntiltheoutputdropsenoughtorequire
another burst of charge. Burst Mode operation allows the
LTC1911toachievehighefficiencyevenatlightloads. The
part has shutdown capability as well as user-controlled
inrush current limiting. In addition, the part has short-
circuit and overtemperature protection.
1911f
6
LTC1911-1.5/LTC1911-1.8
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APPLICATIO S I FOR ATIO
transferred to the parallel combination of C1 and C2 is
transferred to the VOUT. In this manner, charge is again
transferred from the flying capacitors to the output on
both phases of the clock. As in 2-to-1 mode, charge
current is sourced from GND on phase two of the clock
resulting in increased power efficiency. IOUT in 3-to-2
mode equals approximately (3/2)IIN.
Mode Selection
The optimal step-down conversion mode is chosen based
on VIN and output load conditions. Two internal compara-
tors are used to select the default step-down mode based
on the input voltage. Each comparator has an adjustable
offset built in that increases (decreases) in proportion to
the increasing (decreasing) output load current. In this
manner, the mode switch point is optimized to provide
peak efficiency over all supply and load conditions. Each
comparatoralsohasbuilt-inhysteresisofabout300mVto
ensurethattheLTC1911doesnotoscillatebetweenmodes
when a transition point is reached.
In 1-to-1 mode (see Figure 1c), switch S1 is always closed
connecting the top plate of C1 to VOUT. Switch S2 remains
closed for almost the entire clock period, opening only
briefly at the end of clock phase one. In this manner, VOUT
is connected to VIN through RA. The value of RA is set by
theregulatorcontrolloopwhichdeterminestheamountof
currenttransferredtoVOUT duringtheonperiodofS2. The
LTC1911 acts much like a linear regulator in this mode.
Since all of the VOUT current is sourced from VIN, the
efficiency in 1-to-1 mode is approximately equal to that of
a linear regulator.
Soft-Start/Shutdown Operation
The SS/SHDN pin is used to implement both low current
shutdown and soft-start. The soft-start feature limits
inrush currents when the regulator is initially powered up
or taken out of shutdown. Forcing a voltage lower than
0.6V (typ) on the SS/SHDN pin will put the LTC1911 into
shutdown mode. Shutdown mode disables all control
circuitry and forces VOUT into a high impedance state. A
2µA pull-up current on the SS/SHDN pin will force the part
into active mode if the pin is left floating or is driven with
an open-drain output that is in a high impedance state. If
the pin is not driven with an open-drain device, it must be
forced to a logic high voltage of 2.2V (min) to ensure
proper VOUT regulation. The SS/SHDN pin should not be
driven to a voltage higher than VIN. To implement soft-
start, the SS/SHDN pin must be driven with an open-drain
device and a capacitor must be connected from the SS/
SHDN pin to GND. Once the open-drain device is turned
off,the2µApull-upcurrentwillbeginchargingtheexternal
soft-start capacitor and force the voltage on the pin to
ramp towards VIN. As soon as the shutdown threshold is
reached (0.6V typ), the internal reference voltage that
controls the VOUT regulation point will follow the ramp
voltage on the SS/SHDN pin (minus a 0.6V offset to
account for the shutdown threshold) until the reference
reaches its final band gap voltage. This occurs when the
voltage on the SS/SHDN pin reaches approximately 1.9V.
SincetheramprateontheSS/SHDNpincontrolstheramp
rate on VOUT, the average inrush current can be controlled
through the selection of CSS and COUT. For example, a
S5
φ1
S1
φ2
+
R
C1
A
V
IN
V
OUT
C1
S4
φ1
–
C1
S2
φ2
S7
φ1
+
–
C2
C2
C2
S6
φ1
1911 F01b
S3
φ2
GND
Figure 1b. Step-Down Charge Transfer in 3-to-2 Mode
+
R
A
C1
S2
S1
V
V
OUT
IN
C1
–
1911 F01c
C1
Figure 1c. Step-Down Charge Transfer in 1-to-1 Mode
1911f
7
LTC1911-1.5/LTC1911-1.8
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APPLICATIO S I FOR ATIO
4.7nF capacitor on SS/SHDN results in a 3ms ramp time
from 0.6V to 1.9V on the pin. If COUT is 10µF, the 3ms VREF
ramp time results in an average COUT charge current of
only 6mA (see Figure 2).
Low Current Burst Mode Operation
Toimproveefficiencyatlowoutputcurrents,aBurstMode
function was included in the design of the LTC1911. An
output current sense circuit is used to detect when the
required output current drops below 30mA typ. When this
occurs, the oscillator shuts down and the part goes into a
low current operating state. The LTC1911 will remain in
the low current operating state until VOUT has dropped
enough to require another burst of current. Unlike tradi-
tional charge pumps who’s burst current is dependant on
many factors (i.e., supply, switch strength, capacitor
selection, etc.), the LTC1911 burst current is set by the
burst threshold. This means that the output ripple voltage
during Burst Mode operaton will be fixed and is typically
5mV for COUT = 10µF.
6
V
OUT
R
LOAD
C
OUT
LTC1911
8
SS/SHDN
ON OFF V
CTRL
C
SS
(2a)
VCTRL
2V/DIV
Short-Circuit/Thermal Protection
The LTC1911 has built-in short-circuit current limiting as
well as overtemperature protection. During short-circuit
conditions it will automatically limit its output current to
approximately 600mA. The LTC1911 will shut down if the
junction temperature exceeds approximately 160°C. Un-
dernormaloperatingconditions, theLTC1911shouldnot
go into thermal shutdown but it is included to protect the
IC in cases of excessively high ambient temperatures, or
in cases of excessive power dissipation inside the IC (i.e.,
overcurrent or short circuit). The charge transfer will
reactivate once the junction temperature drops back to
approximately 150°C. The LTC1911 can cycle in and out
of thermal shutdown indefinitely without latch-up or
damage until the fault condition is removed.
VOUT
1V/DIV
C
SS = 0nF
2ms/DIV
1911 F02b
COUT = 10µF
RLOAD = 10Ω
(2b)
VCTRL
2V/DIV
VOUT Ripple and Capacitor Selection
VOUT
1V/DIV
The type and value of capacitors used with the
LTC1911 determine several important parameters such
as regulator control loop stability, output ripple and
charge pump strength.
C
SS = 4.7nF
2ms/DIV
1911 F02c
COUT = 10µF
ROUT = 10Ω
The value of COUT directly controls the amount of output
ripple for a given load current. Increasing the size of COUT
will reduce the output ripple.
(2c)
Figure 2. Shutdown/Soft-Start Operation
1911f
8
LTC1911-1.5/LTC1911-1.8
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APPLICATIO S I FOR ATIO
To reduce output noise and ripple, it is suggested that a
low ESR (≤0.1Ω) ceramic capacitor (10µF or greater) be
used for COUT. Tantalum and Aluminum capacitors are not
recommended because of their high ESR (equivalent
series resistance).
less than 1µF but the increasing input noise will feed
through to the output causing degraded performance.
For best performance a 1µF or greater capacitor is sug-
gested for CIN. Aluminum capacitors are not recom-
mended because of their high ESR.
BoththestyleandvalueofCOUT cansignificantlyaffectthe
stability of the LTC1911. As shown in the Block Diagram,
the part uses a control loop to adjust the strength of the
charge pump to match the current required at the output.
Theerrorsignalofthisloopisstoreddirectlyontheoutput
charge storage capacitor. The charge storage capacitor
alsoservestoformthedominantpoleforthecontrolloop.
To prevent ringing or instability it is important for the
output capacitor to maintain at least 4µF of capacitance
over all conditions (See Ceramic Capacitor Selection
Guidelines).
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or
aluminumshouldneverbeusedfortheflyingcapacitors
since their voltage can reverse upon start-up of the
LTC1911. Ceramiccapacitorsshouldalwaysbeusedfor
the flying capacitor.
The flying capacitor controls 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
100mA or less of output current is required the flying
capacitor minimum can be reduced to 0.15µF.
Likewise excessive ESR on the output capacitor will tend
to degrade the loop stability of the LTC1911. The closed-
loopoutputresistanceofthepartisdesignedtobe0.13Ω.
For a 250mA load current change, the output voltage will
change by about 33mV. If the output capacitor has 0.13Ω
or more of ESR, the closed-loop frequency response will
cease to roll-off in a simple 1-pole fashion and poor load
transient response or instability could result. Ceramic
capacitors typically have exceptional ESR performance,
and combined with a tight board layout, should yield
excellent stability and load transient performance.
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 X7R material will
retainmostofitscapacitancefrom–40°Cto85°Cwhereas
a Z5U or Y5V style capacitor will lose considerable capaci-
tance 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 capaci-
tance for a given case size rather than discussing the
specified capacitance value. For example, over rated volt-
age and temperature conditions, a 4.7µF, 10V, Y5V ce-
ramic capacitor in a 0805 case may not provide any more
capacitance than a 1µF, 10V, X7R available in the same
0805 case. In fact, over bias and temperature range, the
1µF, 10V, 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
VIN Capacitor Selection
The constant frequency architecture used by the
LTC1911 makes input noise filtering much less demand-
ing than with conventional regulated charge pumps. De-
pendingonthemodeofoperationtheinputcurrentofthe
LTC1911 can vary from IOUT to 0mA on a cycle-by-cycle
basis. Lower ESR will reduce the voltage steps caused by
changinginputcurrent,whiletheabsolutecapacitorvalue
will determine the level of ripple. For optimal input noise
and ripple reduction, it is recommended that a low ESR
ceramic capacitor be used for CIN. A tantalum capacitor
may be used for CIN but the higher ESR will lead to more
input noise. The LTC1911 will operate with capacitors
1911f
9
LTC1911-1.5/LTC1911-1.8
W U U
U
APPLICATIO S I FOR ATIO
is needed to ensure that minimum capacitance values are
Additional output filtering can be achieved by placing a
second output capacitor, connected to the ground plane,
about 2cm or more from the LTC1911 output capacitor
(C4). The inductance of the trace running to the second
outputcapacitorwillsignificantlyattenuatethehighspeed
switching transients of the LTC1911. Even small capaci-
tors as low as 0.1µF will provide excellent results.
met over operating temperature and bias voltage.
Table 1 is a list of ceramic capacitor manufacturers and
how to contact them.
Table 1. Ceramic Capacitor Manufacturers
AVX
1-(803)-448-1943
1-(864) 963-6300
1-(800) 831-9172
1-(800) 348-2496
1-(800) 487-9437
www.avxcorp.com
www.kemet.com
www.murata.com
www.t-yuden.com
www.vishay.com
Kemet
Murata
Taiyo Yuden
Vishay
Thermal Management
The power dissipation in the LTC1911 can cause the
junction temperature to rise at rates of up to 160°C/W. If
the specified operating conditions are followed, the junc-
tion temperature should never exceed the 160°C thermal
shutdown temperature. The junction temperature can
come very close and possibly exceed the specified 125°C
operating junction temperature. To reduce the maximum
junctiontemperature,agoodthermalconnectiontothePC
boardisrecommended. ConnectingtheGNDpin(Pin4)to
a ground plane, and maintaining a solid ground plane
underthedeviceontwolayersofthePCboard, canreduce
the thermal resistance of the package and PC board
considerably.
Layout Considerations
Due to the high switching frequency and transient cur-
rents produced by the LTC1911, careful board layout is
necessary for optimal performance. A true ground plane
and short connections to all capacitors will optimize
performance, reduce noise and ensure proper regulation
over all conditions. Figure 3 shows the recommended
layout configuration.
C3
V
IN
SS/SHDN
U1
C4
C2
C1
GND
OUT
1911 F03
: CONNECT TO GND PLANE ON BACK OF BOARD
Figure 3. Recommended Component Placement and Grounding
1911f
10
LTC1911-1.5/LTC1911-1.8
U
PACKAGE DESCRIPTIO
MS8 Package
8-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1660)
0.889 ± 0.127
(.035 ± .005)
5.23
(.206)
MIN
3.2 – 3.45
(.126 – .136)
3.00 ± 0.102
(.118 ± .004)
(NOTE 3)
0.52
(.206)
REF
0.65
(.0256)
BSC
0.42 ± 0.04
(.0165 ± .0015)
TYP
8
7 6 5
RECOMMENDED SOLDER PAD LAYOUT
3.00 ± 0.102
(.118 ± .004)
NOTE 4
4.88 ± 0.1
(.192 ± .004)
DETAIL “A”
0.254
(.010)
0° – 6° TYP
GAUGE PLANE
1
2
3
4
0.53 ± 0.015
(.021 ± .006)
1.10
(.043)
MAX
0.86
(.034)
REF
DETAIL “A”
0.18
(.077)
SEATING
PLANE
0.22 – 0.38
(.009 – .015)
0.13 ± 0.05
(.005 ± .002)
0.65
(.0256)
BCS
MSOP (MS8) 0102
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
1911f
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-
tation that the interconnection of its circuits as described herein will notinfringe onexisting patent rights.
11
LTC1911-1.5/LTC1911-1.8
U
TYPICAL APPLICATIO
DC/DC Converter with Shutdown and Soft-Start
LTC1911-1.5
2.7V TO 5.5V INPUT
1
2
3
4
8
7
6
5
V
SS/SHDN
IN
1-CELL Li-Ion
OR
3-CELL NiMH
+
+
10µF*
1µF*
10nF
C2
C2
C1
1µF*
2N7002
ON OFF
V
= 1.5V
= 250mA
OUT
OUT
–
V
OUT
I
–
10µF*
GND
C1
1911 TA03
*CERAMIC CAPACITOR
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
= 2.7V to 10V, V
S8 Package
LTC1514
50mA, 650kHz, Step-Up/Down Charge Pump
with Low Battery Comparator
V
IN
= 3V to 5V, I = 60µA, I = 10µA,
Q SD
OUT
LTC1515
LT1776
50mA, 650kHz, Step-Up/Down Charge Pump
with Power On Reset
V
= 2.7V to 10V, V
= 3.3V or 5V, I = 60µA, I = <1µA,
IN
OUT
Q
SD
S8 Package
500mA (I ), 200kHz, High Efficiency Step-Down
90% Efficiency, V = 7.4V to 40V, V
= 1.24V, I = 3.2mA,
OUT Q
OUT
IN
DC/DC Converter
I
= 30µA, N8,S8 Packages
SD
LTC3250-1.5
LTC3251
LTC3404
LTC3405A
LTC3406B
LTC3411
LTC3412
LTC3440
250mA, 1.5MHz, High Efficiency, Step-Down Charge Pump
85% Efficiency, V = 3.1V to 5.5V, V
= 1.5V, I = 35µA,
IN
OUT
Q
I
= <1µA, ThinSOT Package
SD
500mA, 1MHz to 1.6MHz, Spread Spectrum,
Step-Down Charge Pump
85% Efficiency, V = 3.1V to 5.5V, V
= 0.9V to 1.6V,
IN
OUT
I = 9µA, I = <1µA, MS Package
Q
SD
600mA (I ), 1.4MHz, Synchronous Step-Down
95% Efficiency, V = 2.7V to 6V, V
= 0.8V, I = 10µA,
Q
OUT
IN
OUT
OUT
DC/DC Converter
I
= <1µA, MS8 Package
SD
300mA (I ), 1.5MHz, Synchronous Step-Down
95% Efficiency, V = 2.7V to 6V, V
= 0.8V, I = 20µA,
Q
OUT
IN
DC/DC Converter
I
= <1µA, ThinSOT Package
SD
600mA (I ), 1.5MHz, Synchronous Step-Down
95% Efficiency, V = 2.5V to 5.5V, V
= 0.6V, I = 20µA,
Q
OUT
IN
OUT
OUT
OUT
OUT
DC/DC Converter
I
= <1µA, ThinSOT Package
SD
1.25A (I ), 4MHz, Synchronous Step-Down
95% Efficiency, V = 2.5V to 5.5V, V
= 0.8V, I = 60µA,
Q
OUT
IN
DC/DC Converter
I
= <1µA, MS Package
SD
2.5A (I ), 4MHz, Synchronous Step-Down
95% Efficiency, V = 2.5V to 5.5V, V
= 0.8V, I = 60µA,
Q
OUT
IN
DC/DC Converter
I
= <1µA, TSSOP16E Package
SD
600mA (I ), 2MHz, Synchronous Buck-Boost
95% Efficiency, V = 2.5V to 5.5V, V
= 2.5V, I = 25µA,
Q
OUT
IN
DC/DC Converter
I
= <1µA, MS Package
SD
ThinSOT is a trademark of Linear Technology Corporation.
1911f
LT/TP 1102 2K • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
12
●
●
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
LINEAR TECHNOLOGY CORPORATION 2001
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