LINER LTC3411AEMS-TRPBF 1.25a, 4mhz, synchronous step-down dc/dc converter Datasheet

LTC3411A
1.25A, 4MHz, Synchronous
Step-Down DC/DC Converter
FEATURES
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DESCRIPTION
Uses Tiny Capacitors and Inductor
High Frequency Operation: Up to 4MHz
Low RDS(ON) Internal Switches: 0.15Ω
High Efficiency: Up to 96%
Selectable Low Ripple (25mVP-P) Burst Mode®
Operation: IQ = 40μA
Stable with Ceramic Capacitors
Current Mode Operation for Excellent Line
and Load Transient Response
Short-Circuit Protected
Low Dropout Operation: 100% Duty Cycle
Low Shutdown Current: IQ ≤ 1μA
Output Voltages from 0.8V to 5V
Synchronizable to External Clock
Supports Pre-Biased Outputs
Small 10-Lead 3mm × 3mm DFN or MSOP Package
APPLICATIONS
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The LTC®3411A is a constant frequency, synchronous
step-down DC/DC converter. Intended for medium power
applications, it operates from a 2.5V to 5.5V input voltage
range and has a user configurable operating frequency
up to 4MHz, allowing the use of tiny, low cost capacitors
and inductors 1mm or less in height. The output voltage is
adjustable from 0.8V to 5.5V. Internal synchronous power
switches provide high efficiency. The LTC3411A’s current
mode architecture and external compensation allow the
transient response to be optimized over a wide range of
loads and output capacitors.
The LTC3411A can be configured for automatic power
saving Burst Mode operation (IQ = 40μA) to reduce gate
charge losses when the load current drops below the level
required for continuous operation. For reduced noise and
RF interference, the SYNC/MODE pin can be configured to
skip pulses or provide forced continuous operation.
To further maximize battery life, the P-channel MOSFET
is turned on continuously in dropout (100% duty cycle).
In shutdown, the device draws <1μA.
Notebook Computers
Digital Cameras
Cellular Phones
Handheld Instruments
Board Mounted Power Supplies
L, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners. Protected by
U.S. Patents including 5481178, 6580258, 6498466, 6611131.
TYPICAL APPLICATION
Efficiency and Power Loss vs Output Current
Step-Down 2.5V/1.25A Regulator
100
VIN
2.5V TO 5.5V
90
80
PVIN
PGOOD
SVIN
LTC3411A
2.2μH
SW
22pF
VOUT
2.5V
1.25A
0.1
70
60
0.01
50
40
30
ITH
12.1k
680pF
887k
SHDN/RT
SGND
549k
VFB
22μF
10
PGND
412k
3411a TA01a
0.001
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
20
0
0.1
1
100
1000
10
OUTPUT CURRENT (mA)
fO = 1MHz
Burst Mode OPERATION
POWER LOSS (W)
SYNC/MODE
EFFICIENCY (%)
10μF
SYNC
1
0.0001
10000
3411A TA01b
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LTC3411A
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PVIN, SVIN Voltages ..................................... –0.3V to 6V
VFB, ITH, SHDN/RT Voltages ..........–0.3V to (VIN + 0.3V)
SYNC/MODE Voltage .....................–0.3V to (VIN + 0.3V)
SW Voltage ..................................–0.3V to (VIN + 0.3V)
PGOOD Voltage ........................................... –0.3V to 6V
Operating Junction Temperature Range
(Notes 2, 5, 8) ........................................ –40°C to 125°C
Storage Temperature Range................... –65°C to 125°C
Lead Temperatu re (Soldering, 10 sec) ............... 300°C
PIN CONFIGURATION
TOP VIEW
SHDN/RT
TOP VIEW
10 ITH
1
SHDN/RT
SYNC/MODE
SGND
SW
PGND
9 VFB
SYNC/MODE
2
SGND
3
SW
4
7 SVIN
PGND
5
6 PVIN
11
8 PGOOD
10
9
8
7
6
1
2
3
4
5
ITH
VFB
PGOOD
SVIN
PVIN
MS PACKAGE
10-LEAD PLASTIC MSOP
TJMAX = 125°C, θJA = 120°C/W, θJC = 10°C/W
DD PACKAGE
10-LEAD (3mm × 3mm) PLASTIC DFN
TJMAX = 125°C, θJA = 43°C/W, θJC = 3°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3411AEDD#PBF
LTC3411AEDD#TRPBF
LAJM
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LTC3411AIDD#PBF
LTC3411AIDD#TRPBF
LAJM
10-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LTC3411AEMS#PBF
LTC3411AEMS#TRPBF
LTAJK
10-Lead Plastic MSOP
–40°C to 125°C
LTC3411AIMS#PBF
LTC3411AIMS#TRPBF
LTAJK
10-Lead Plastic MSOP
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *Temperature grades are identified by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C, VIN = 3.6V, RT = 125k unless otherwise specified. (Note 2)
SYMBOL
PARAMETER
CONDITIONS
VIN
Operating Voltage Range
IFB
Feedback Pin Input Current
(Note 3)
VFB
Feedback Voltage
(Note 3)
ΔVLINEREG
Reference Voltage Line Regulation
VIN = 2.5V to 5.5V
ΔVLOADREG
Output Voltage Load Regulation
ITH = 0.55V to 0.9V
gm(EA)
Error Amplifier Transconductance
ITH Pin Load = ±5μA (Note 3)
MIN
●
●
●
TYP
2.5
0.784
MAX
UNITS
5.5
V
±0.1
μA
0.8
0.816
V
0.04
0.2
%/V
0.02
0.2
%
300
μS
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LTC3411A
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C, VIN = 3.6V, RT = 125k unless otherwise specified. (Note 2)
SYMBOL
PARAMETER
IS
fOSC
Input DC Supply Current (Note 4)
Active Mode
Sleep Mode
Shutdown
Shutdown Threshold High Active
Oscillator Resistor
Oscillator Frequency
MIN
TYP
MAX
2.25
Synchronization Frequency
RT = 125k
(Note 7)
(Note 7)
330
40
0.1
VIN – 0.6
125k
2.5
450
60
1
VIN – 0.4
1M
2.8
4
4
fSYNC
ILIM
Peak Switch Current Limit
VFB = 0.5V
1.6
2.1
2.6
A
RDS(ON)
Top Switch On-Resistance
MS Package
DD Package (Note 6)
MS Package
DD Package (Note 6)
VIN = 5.5V, VSHDN/RT = 5.5V, VSW = 0V
or 5.5V
VIN Ramping Down
0.15
0.15
0.13
0.13
0.01
0.18
0.16
1
Ω
Ω
Ω
Ω
μA
1.8
2.1
2.4
V
–5
5
–7
7
–10
10
15
–12
12
30
%
%
%
%
Ω
VSHDN/RT
Bottom Switch On-Resistance
ISW(LKG)
Switch Leakage Current
VUVLO
Undervoltage Lockout Threshold
PGOOD
Power Good Threshold
Power Bad Threshold
RPGOOD
tSOFT-START
VSYNC/MODE = 3.6V, VFB = 0.75V
VSYNC/MODE = 3.6V, VFB = 0.84V
VSHDN/RT = 3.6V
0.4
V FB Ramping Up from 0.68V to 0.8V
VFB Ramping Down from 0.92V to 0.8V
VFB Ramping Down from 0.8V to 0.68V
VFB Ramping Up from 0.8V to 0.9V
Power Good Pull-Down On-Resistance
VFB Step from 0V to 0.8V
VFB Step from 0.8V to 0V
PGOOD Blanking
VSYNC-MODE
CONDITIONS
Pulse Skip
Force Continous
Burst
10% to 90% of Regulation
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LTC3411AE is guaranteed to meet performance specifications
from 0°C to 85°C junction temperature. Specifications over the –40°C
to 125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LTC3411AI is guaranteed over the full –40°C to 125°C operating junction
temperature range.
Note 3: The LTC3411A is tested in a feedback loop which servos VFB to
the midpoint for the error amplifier (VITH = 0.7V).
Note 4: Dynamic supply current is higher due to the internal gate charge
being delivered at the switching frequency.
40
105
0.93
VIN – 0.75
0.5
0.63
VIN – 1.05
0.8
1.0
UNITS
μA
μA
μA
V
Ω
MHz
MHz
MHz
μs
μs
V
V
V
ms
Note 5: TJ is calculated from the ambient TA and power dissipation PD
according to the following formulas:
LTC3411AEDD: TJ = TA + (PD • 43°C/W)
LTC3411AEMS: TJ = TA + (PD • 120°C/W)
Note 6: For the DD package, switch on-resistance is sampled at wafer
level measurements and assured by design, characterization and
correlation with statistical process controls.
Note 7: 4MHz operation is guaranteed by design but not production
tested and is subject to duty cycle limitations (see Applications
Information).
Note 8: 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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LTC3411A
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, VIN = 3.6V, fO = 1MHz, unless
otherwise noted.
Efficiency vs Input Voltage
Efficiency vs Output Current
IOUT = 100mA
IOUT = 10mA
90
IOUT = 1.25A
70
EFFICIENCY (%)
EFFICIENCY (%)
80
IOUT = 1mA
60
50
100
100
90
90
80
80
70
70
EFFICIENCY (%)
100
60
50
40
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
20
40
10
VOUT = 1.8V
4.5
4.0
3.5
INPUT VOLTAGE(V)
3.0
VOUT = 1.8V
0
0.1
5.5
5.0
1
100
1000
10
OUTPUT CURRENT (mA)
Efficiency vs Output Current
10
0
0.1
10000
Efficiency vs Frequency
FORCED CONTINUOUS
40
30
20
2.2μH
92
91
1μH
0
0.1
1
10
100
1000
OUTPUT CURRENT (mA)
Burst Mode OPERATION
0.25
PULSE SKIP
0.00
–0.25
VOUT = 1.8V
ILOAD = 400mA
VOUT = 1.8V
88
0
10000
0.50
FORCED CONTINUOUS
89
VOUT = 1.8V
3
2
FREQUENCY (MHz)
1
3411A G04
–0.50
4
0
5
200
400 600 800 1000 1200 1400
OUTPUT CURRENT(mA)
3411A G06
3411A G05
Reference Voltage vs
Temperature
Line Regulation
Frequency Variation vs
Temperature
815
0.6
10000
0.75
90
10
10
100
1000
OUTPUT CURRENT (mA)
4.7μH
VOUT ERROR (%)
EFFICIENCY (%)
EFFICIENCY (%)
50
1
Load Regulation
93
60
VOUT = 1.5V
1.00
94
PULSE
SKIP
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
3411A G03
80
70
40
20
95
Burst Mode
OPERATION
90
50
3411A G02
3411A G01
100
60
30
30
IOUT = 0.1mA
30
2.5
Efficiency vs Output Current
6
VIN = 3.6V
0.2
0.0
–0.2
–0.4
–0.6
2.5
4.5
4.0
3.5
INPUT VOLTAGE(V)
5.0
805
800
795
790
VOUT = 1.8V
ILOAD = 400mA
3.0
810
FREQUENCY VARIATION (%)
REFERENCE VOLTAGE (mV)
VOUT ERROR (%)
0.4
5.5
3411A G07
785
–50
4
2
0
–2
–4
–25
50
25
75
0
TEMPERATURE(°C)
100
125
3411A G08
–6
–50
–25
50
25
75
0
TEMPERATURE(°C)
100
125
3411A G09
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LTC3411A
TYPICAL PERFORMANCE CHARACTERISTICS
otherwise noted.
Frequency Variation vs VIN
RDS(ON) vs Input Voltage
6
4
FREQUENCY VARIATION (%)
TA = 25°C, VIN = 3.6V, fO = 1MHz, unless
RDS(ON) vs Temperature
0.25
0.30
0.20
0.25
0
–2
RDS(ON) (Ω)
RDS(ON) (Ω)
2
0.15
0.10
0.20
0.15
0.10
–4
0.05
–6
–8
2.5
3.0
3.5
4.0
VIN (V)
4.5
5.0
0.0
2.5
5.5
3.0
4.5
4.0
3.5
INPUT VOLTAGE (V)
5.0
Dynamic Supply Current vs Input
Voltage
0.0
–50
5.5
PULSE SKIP
Burst Mode
OPERATION
0.1
0.01
VOUT = 1.8V
ILOAD = 0A
0.001
2.5
3.0
3.5
4.0
VIN (V)
4.5
5.0
5.5
100
FORCED CONTINUOUS
10
2000
1
PULSE SKIP
Burst Mode
OPERATION
0.1
0.01
MAIN SWITCH
1500
1000
SYNCHRONOUS SWITCH
500
VOUT = 1.8V
ILOAD = 0A
0.001
–50
0
–25
0
25
50
75
TEMPERATURE (°C)
3411A G13
100
125
0
1
4
3
2
INPUT VOLTAGE(V)
3411A G14
5
6
3411A G15
Burst Mode Operation
Switch Leakage vs Temperature
125
Switch Leakage vs Input Voltage
SWITCH LEAKAGE (pA)
DYNAMIC SUPPLY CURRENT (mA)
1
50
25
75
0
TEMPERATURE (°C)
2500
100
FORCED CONTINUOUS
–25
3411A G12
Dynamic Supply Current vs
Temperature
100
10
MAIN SWITCH
SYNCHRONOUS SWITCH
3411A G11
3411A G10
DYNAMIC SUPPLY CURRENT (mA)
0.05
MAIN SWITCH
SYNCHRONOUS SWITCH
Pulse Skippng Mode
600
SW
2V/DIV
SW
2V/DIV
300
VOUT
50mV/DIV
AC COUPLED
VOUT
50mV/DIV
AC COUPLED
200
IL
200mA/DIV
IL
200mA/DIV
SWITCH LEAKAGE (nA)
500
400
MAIN SWITCH
SYNCHRONOUS SWITCH
100
0
–50
4μs/DIV
–25
50
25
75
0
TEMPERATURE (°C)
100
125
VIN = 3.6V
VOUT = 1.8V
ILOAD = 50mA
3411A G17
4μs/DIV
3411A G18
VIN = 3.6V
VOUT = 1.8V
ILOAD = 5mA
3411A G16
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LTC3411A
TYPICAL PERFORMANCE CHARACTERISTICS
otherwise noted.
Forced Continuous Mode
TA = 25°C, VIN = 3.6V fO = 1MHz, unless
Start-Up from Shutdown
SW
2V/DIV
VOUT
50mV/DIV
AC COUPLED
IL
200mA/DIV
SHDN/RT
2V/DIV
SHDN/RT
2V/DIV
VOUT
1V/DIV
VOUT
1V/DIV
IL
1A/DIV
IL
1A/DIV
3411A G19
2μs/DIV
Start-Up from Shutdown
200μs/DIV
VIN = 3.6V
VOUT = 1.8V
ILOAD = 80mA
Start-Up from Shutdown with
a Prebiased Output (Forced
Continuous Mode)
3411A G24
Load Step
VOUT
100mV/DIV
AC COUPLED
IL
500mA/DIV
IL
1A/DIV
IL
1A/DIV
ILOAD
1A/DIV
ILOAD
1A/DIV
40μs/DIV
VIN = 3.6V
VOUT = 1.8V
ILOAD = 0A to 1.25A
Burst Mode OPERATION
3411A G23
VOUT Short to VIN (Forced
Continuous Mode)
VOUT Short to Ground
Load Step
40μs/DIV
VIN = 3.6V
VOUT = 1.8V
ILOAD = 50mA to 1.25A
Burst Mode OPERATION
Load Step
VOUT
100mV/DIV
AC COUPLED
3411A G22
3411A G21
VIN = 3.6V
VOUT = 1.8V
ILOAD = 1.25A
VOUT
1V/DIV
200μs/DIV
VIN = 3.6V
PREBIASED VOUT = 3V, VOUT = 1.8V
ILOAD = 0A
200μs/DIV
3411A G20
VIN = 3.6V
VOUT = 1.8V
ILOAD = 0A
VOUT
1V/DIV
VOUT
100mV/DIV
AC COUPLED
VOUT
1V/DIV
IL
1A/DIV
IL
2A/DIV
IL
500mA/DIV
ILOAD
1A/DIV
40μs/DIV
VIN = 3.6V
VOUT = 1.8V
ILOAD = 250mA to 1.25A
Burst Mode OPERATION
40μs/DIV
3411A G25
VIN = 3.6V
VOUT = 1.8V
ILOAD = 0A
3411A G26
40μs/DIV
3411A G27
VIN = 3.6V
VOUT = 1.8V
ILOAD = 0A
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LTC3411A
PIN FUNCTIONS
SHDN/RT (Pin 1): Combination Shutdown and Timing
Resistor Pin. The oscillator frequency is programmed by
connecting a resistor from this pin to ground. Forcing
this pin to SVIN causes the device to be shut down. In
shutdown all functions are disabled.
PGND (Pin 5): Main Power Ground Pin. Connect to the
(–) terminal of COUT, and (–) terminal of CIN.
PVIN (Pin 6): Main Supply Pin. Must be closely decoupled
to PGND.
SVIN (Pin 7): The Signal Power Pin. All active circuitry
is powered from this pin. Must be closely decoupled to
SGND. SVIN must be greater than or equal to PVIN.
SYNC/MODE (Pin 2): Combination Mode Selection and
Oscillator Synchronization Pin. This pin controls the operation of the device. When tied to SVIN or SGND, Burst
Mode operation or pulse skipping mode is selected,
respectively. If this pin is held at half of SVIN, the forced
continuous mode is selected. The oscillation frequency
can be synchronized to an external oscillator applied to
this pin. When synchronized to an external clock pulse
skip mode is selected.
PGOOD (Pin 8): The Power Good Pin. This common drain
logic output is pulled to SGND when the output voltage is
not within ±7% of regulation.
VFB (Pin 9): Receives the feedback voltage from the external resistive divider across the output. Nominal voltage
for this pin is 0.8V.
SGND (Pin 3): The Signal Ground Pin. All small-signal components and compensation components should be connected to this ground (see Board Layout Considerations).
ITH (Pin 10): Error Amplifier Compensation Point. The
current comparator threshold increases with this control
voltage. Nominal voltage range for this pin is 0.4V to
1.4V.
SW (Pin 4): The Switch Node Connection to the Inductor.
This pin swings from PVIN to PGND.
PIN
NAME
1
SHDN/RT
2
SYNC/MODE
3
SGND
Exposed Pad (Pin 11): Power Ground. Must be soldered
to electrical ground on PCB.
DESCRIPTION
MIN
NOMINAL (V)
TYP
MAX
MIN
MAX
Shutdown/Timing Resistor
–0.3
0.8
SVIN
–0.3
SVIN + 0.3
SVIN
–0.3
SVIN + 0.3
PVIN
–0.3
PVIN + 0.3
5.5
–0.3
SVIN + 0.3
Mode Select/Sychronization Pin
0
Signal Ground
Switch Node
ABSOLUTE MAX (V)
0
4
SW
5
PGND
Main Power Ground
0
6
PVIN
Main Power Supply
–0.3
Signal Power Supply
0
2.5
5.5
–0.3
6
Power Good Pin
0
SVIN
–0.3
6
VFB
Output Feedback Pin
0
1.0
–0.3
SVIN + 0.3
ITH
Error Amplifier Compensation and Run Pin
0
1.5
–0.3
SVIN + 0.3
7
SVIN
8
PGOOD
9
10
0.8
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LTC3411A
BLOCK DIAGRAM
SVIN
SGND
ITH
PVIN
7
3
10
6
0.8V
PMOS CURRENT
COMPARATOR
ITH
LIMIT
VOLTAGE
REFERENCE
+
BCLAMP
+
–
–
VFB 9
ERROR
AMPLIFIER
VB
0.74V
+
–
–
+
BURST
COMPARATOR
SLOPE
COMPENSATION
OSCILLATOR
4 SW
+
0.86V
LOGIC
–
+
PGOOD 8
NMOS
COMPARATOR
–
–
REVERSE
COMPARATOR
1
2
SHDN/RT
SYNC/MODE
+
5 PGND
3411A BD
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LTC3411A
OPERATION
The LTC3411A uses a constant frequency, current mode
architecture. The operating frequency is determined by the
value of the RT resistor or can be synchronized to an external
oscillator. To suit a variety of applications, the selectable
MODE pin allows the user to trade-off noise for efficiency.
The output voltage is set by an external divider returned
to the VFB pin. An error amplifier compares the divided
output voltage with the reference voltage of 0.8V and adjusts the peak inductor current accordingly. Overvoltage
and undervoltage comparators will pull the PGOOD output
low if the output voltage is not within ±7% of its regulated
value. A tripping delay of 40μs and untripping delay of
105μs ensures PGOOD will not glitch due to transient
spikes on VOUT.
Main Control Loop
During normal operation, the top power switch (P-channel
MOSFET) is turned on at the beginning of a clock cycle.
Current flows through this switch into the inductor and the
load, increasing until the peak inductor current reaches
the limit set by the voltage on the ITH pin. Then the top
switch is turned off, the bottom switch is turned on, and
the energy stored in the inductor forces the current to flow
through the bottom switch and the inductor, out into the
load until the next clock cycle.
The peak inductor current is controlled by the voltage
on the ITH pin, which is the output of the error amplifier.
The output is developed by the error amplifier comparing
the feedback voltage, VFB, to the 0.8V reference voltage.
When the load current increases, the output voltage and
VFB decrease slightly. This decrease in VFB causes the error amplifier to increase the ITH voltage until the average
inductor current matches the new load current.
The main control loop is shut down by pulling the SHDN/RT
pin to SVIN, resetting the internal soft-start. Re-enabling the
main control loop by releasing the SHDN/RT pin activates the
internal soft-start, which slowly ramps the output voltage
over approximately 0.8ms until it reaches regulation.
switch from continuous operation to the selected mode
when the load current is low.
To optimize efficiency, the Burst Mode operation can be
selected. When the load is relatively light, the LTC3411A
automatically switches into Burst Mode operation in which
the PMOS switch operates intermittently based on load
demand. By running cycles periodically, the switching
losses which are dominated by the gate charge losses
of the power MOSFETs are minimized. The main control
loop is interrupted when the output voltage reaches the
desired regulated value. The burst comparator trips when
ITH is below approximately 0.5V, shutting off the switch
and reducing the power. The output capacitor and the
inductor supply the power to the load until ITH rises above
approximately 0.5V, turning on the switch and the main
control loop which starts another cycle.
For lower output voltage ripple at low currents, pulse
skipping mode can be used. In this mode, the LTC3411A
continues to switch at a constant frequency down to
very low currents, where it will eventually begin skipping
pulses.
Finally, in forced continuous mode, the inductor current is constantly cycled which creates a fixed output
voltage ripple at all output current levels. This feature is
desirable in telecommunications since the noise is at a
constant frequency and is thus, easy to filter out. Another
advantage of this mode is that the regulator is capable
of both sourcing current into a load and sinking current
from the output.
Dropout Operation
When the input supply voltage decreases toward the output
voltage, the duty cycle increases to 100% which is the
dropout condition. In dropout, the PMOS switch is turned
on continuously with the output voltage being equal to the
input voltage minus the voltage drop across the internal
P-channel MOSFET and the inductor.
Low Supply Operation
Low Current Operation
Three modes are available to control the operation of the
LTC3411A at low currents. All three modes automatically
The LTC3411A incorporates an undervoltage lockout circuit
which shuts down the part when the input voltage drops
below about 2.1V to prevent unstable operation.
3411afa
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LTC3411A
APPLICATIONS INFORMATION
A general LTC3411A application circuit is shown in
Figure 4. External component selection is driven by the load
requirement, and begins with the selection of the inductor
L1. Once L1 is chosen, CIN and COUT can be selected.
Operating Frequency
Selection of the operating frequency is a trade-off between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves efficiency by
reducing internal gate charge losses but requires larger
inductance values and/or capacitance to maintain low
output ripple voltage.
The operating frequency, fO, of the LTC3411A is determined
by an external resistor that is connected between the RT
pin and ground. The value of the resistor sets the ramp
current that is used to charge and discharge an internal
timing capacitor within the oscillator and can be calculated
by using the following equation:
A reasonable starting point for setting ripple current is
ΔIL = 0.4 • IOUT(MAX), where IOUT(MAX) is 1.25A. The largest
ripple current ΔIL occurs at the maximum input voltage. To
guarantee that the ripple current stays below a specified
maximum, the inductor value should be chosen according
to the following equation:
V
VOUT L = OUT • 1
fO • IL V IN(MAX) The inductor value will also have an effect on Burst Mode
operation. The transition from low current operation
begins when the peak inductor current falls below a level
set by the burst clamp. Lower inductor values result in
higher ripple current which causes this to occur at lower
load currents. This causes a dip in efficiency in the upper
range of low current operation. In Burst Mode operation,
lower inductance values will cause the burst frequency
to increase.
5000
RT
= 5 × 107 (f
–1.6508 (kΩ),
O)
TA = 25°C
4500
The maximum usable operating frequency is limited by
the minimum on-time and the duty cycle. This can be
calculated as:
V
fO(MAX) ≈ 6.67 • OUT (MHz)
VIN(MAX)
The minimum frequency is internally set at around 200kHz.
FREQUENCY (kHz)
4000
where fO is in kHz, or can be selected using Figure 1.
3500
3000
2500
2000
1500
1000
500
0
0
400
800
1200
RT (kΩ)
1600
3411A F01
Inductor Selection
The operating frequency, fO, has a direct effect on the
inductor value, which in turn influences the inductor ripple
current ΔIL:
V
IL = OUT
fO • L
V • 1 OUT V IN The inductor ripple current decreases with larger inductance or frequency, and increases with higher VIN or VOUT.
Accepting larger values of ΔIL allows the use of lower
inductances, but results in higher output ripple voltage,
greater core loss and lower output capability.
Figure 1. Frequency vs RT
Inductor Core Selection
Different core materials and shapes will change the
size/current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy
materials are small and don’t radiate much energy, but
generally cost more than powdered iron core inductors
with similar electrical characteristics. The choice of which
style inductor to use often depends more on the price vs
size requirements and any radiated field/EMI requirements
than on what the LTC3411A requires to operate. Table 1
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LTC3411A
APPLICATIONS INFORMATION
shows some typical surface mount inductors that work
well in LTC3411A applications.
Table 1. Representative Surface Mount Inductors
MANUFACTURER PART NUMBER
MAX DC
VALUE CURRENT DCR HEIGHT
Toko
A914BYW-1R2M=P3:
D52LC
1.2μH
2.15A
44mΩ
A960AW-1R2M=P3:
D518LC
1.2μH
1.8A
46mΩ 1.8mm
DB3015C-1068AS-1R0N 1.0μH
2.1A
43mΩ 1.5mm
DB3018C-1069AS-1R0N 1.0μH
2.1A
45mΩ 1.8mm
DB3020C-1070AS-1R0N 1.0μH
2.1A
47mΩ
2mm
A914BYW-2R2M-D52LC 2.2μH
2.05A
49mΩ
2mm
A915AY-2ROM-D53LC
3.3A
22mΩ
3mm
Coilcraft
Sumida
2.0μH
2mm
LPO1704-122ML
1.2μH
2.1A
80mΩ
1mm
D01608C-222
2.2μH
2.3A
70mΩ
3mm
LP01704-222M
2.2μH
2.4A
120mΩ 1mm
CR32-1R0
1.0μH
2.1A
72mΩ
CR5D11-1R0
1.0μH
2.2A
40mΩ 1.2mm
CDRH3D14-1R2
1.2μH
2.2A
36mΩ 1.5mm
CDRH4D18C/LD-1R1
1.1μH
2.1A
24mΩ
CDRH4D28C/LD-1R0
1.0μH
3.0A
17.5mΩ 3mm
2mm
CDRH4D28C-1R1
1.1μH
3.8A
CDRH4D28-1R2
1.2μH
2.56A
23.6mΩ 3mm
CDRH6D12-1R0
1.0μH
2.80A
37.5mΩ 1.5mm
CDRH4D282R2
2.2μH
2.04A
23mΩ
CDC5D232R2
2.2μH
2.16A
30mΩ 2.5mm
Taiyo Yuden NPO3SB1ROM
22mΩ
3mm
3mm
3mm
1.0μH
2.6A
27mΩ 1.8mm
N06DB2R2M
2.2μH
3.2A
29mΩ 3.2mm
N05DB2R2M
2.2μH
2.9A
32mΩ 2.8mm
Murata
LQN6C2R2M04
2.2μH
3.2A
24mΩ
5mm
FDK
MIPW3226DORGM
0.9μH
1.4A
80mΩ
1mm
Catch Diode Selection
Although unnecessary in most applications, a small
improvement in efficiency can be obtained in a few applications by including the optional diode D1 shown in
Figure 4, which conducts when the synchronous switch
is off. When using Burst Mode operation or pulse skip
mode, the synchronous switch is turned off at a low
current and the remaining current will be carried by the
optional diode. It is important to adequately specify the
diode peak current and average power dissipation so as
not to exceed the diode ratings. The main problem with
Schottky diodes is that their parasitic capacitance reduces
the efficiency, usually negating the possible benefits for
LTC3411A circuits. Another problem that a Schottky diode
can introduce is higher leakage current at high temperatures, which could reduce the low current efficiency.
Remember to keep lead lengths short and observe proper
grounding (see Board Layout Considerations) to avoid ringing and increased dissipation when using a catch diode.
Input Capacitor (CIN) Selection
In continuous mode, the input current of the converter is a
square wave with a duty cycle of approximately VOUT/VIN.
To prevent large voltage transients, a low equivalent series
resistance (ESR) input capacitor sized for the maximum
RMS current must be used. The maximum RMS capacitor
current is given by:
IRMS ≈ IMAX
VOUT (VIN − VOUT )
VIN
where the maximum average output current IMAX equals
the peak current minus half the peak-to-peak ripple current, IMAX = ILIM – ΔIL /2.
This formula has a maximum at VIN = 2VOUT, where
IRMS ≅ IOUT/2. This simple worst case is commonly used
to design because even significant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple current ratings are often based on only 2000 hours lifetime.
This makes it advisable to further derate the capacitor,
or choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to meet
the size or height requirements of the design. An additional
0.1μF to 1μF ceramic capacitor is also recommended on
VIN for high frequency decoupling, when not using an all
ceramic capacitor solution.
Output Capacitor (COUT) Selection
The selection of COUT is driven by the required ESR to
minimize voltage ripple and load step transients. Typically,
once the ESR requirement is satisfied, the capacitance
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LTC3411A
APPLICATIONS INFORMATION
is adequate for filtering. The output ripple (ΔVOUT) is
determined by:
1 VOUT IL ESR +
8fO COUT where fO = operating frequency, COUT = output capacitance
and ΔIL = ripple current in the inductor. The output ripple
is highest at maximum input voltage since ΔIL increases
with input voltage. With ΔIL = 0.4 • IOUT(MAX) the output ripple
will be less than 100mV at maximum VIN, a minimum COUT
value of 10μF and fO = 1MHz with:
ESRCOUT < 150mΩ
Once the ESR requirements for COUT have been met, the
RMS current rating generally far exceeds the IRIPPLE(P-P)
requirement, except for an all ceramic solution.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or RMS
current handling requirement of the application. Aluminum
electrolytic, special polymer, ceramic and dry tantalum
capacitors are all available in surface mount packages.
The OS-CON semiconductor dielectric capacitor available from Sanyo has the lowest ESR(size) product of any
aluminum electrolytic at a somewhat higher price. Special
polymer capacitors, such as Sanyo POSCAP, offer very
low ESR, but have a lower capacitance density than other
types. Tantalum capacitors have the highest capacitance
density, but it has a larger ESR and it is critical that the
capacitors are surge tested for use in switching power
supplies. An excellent choice is the AVX TPS series of
surface mount tantalums, available in case heights ranging
from 2mm to 4mm. Aluminum electrolytic capacitors have
a significantly larger ESR, and is often used in extremely
cost-sensitive applications provided that consideration is
given to ripple current ratings and long term reliability.
Ceramic capacitors have the lowest ESR and cost but also
have the lowest capacitance density, a high voltage and
temperature coefficient and exhibit audible piezoelectric
effects. In addition, the high Q of ceramic capacitors along
with trace inductance can lead to significant ringing. Other
capacitor types include the Panasonic specialty polymer
(SP) capacitors.
In most cases, 0.1μF to 1μF of ceramic capacitors should
also be placed close to the LTC3411A in parallel with the
main capacitors for high frequency decoupling.
Ceramic Input and Output Capacitors
Higher value, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them
ideal for switching regulator applications. Because the
LTC3411A’s control loop does not depend on the output
capacitor’s ESR for stable operation, ceramic capacitors
can be used freely to achieve very low output ripple and
small circuit size.
However, care must be taken when ceramic capacitors
are used at the input and the output. When a ceramic
capacitor is used at the input and the power is supplied
by a wall adapter through long wires, a load step at the
output can induce ringing at the input, VIN. At best, this
ringing can couple to the output and be mistaken as loop
instability. At worst, a sudden inrush of current through
the long wires can potentially cause a voltage spike at VIN,
large enough to damage the part.
When choosing the input and output ceramic capacitors,
choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage characteristics of all the ceramics for a given value and size.
Since the ESR of a ceramic capacitor is so low, the input
and output capacitor must instead fulfill a charge storage
requirement. During a load step, the output capacitor must
instantaneously supply the current to support the load
until the feedback loop raises the switch current enough
to support the load. The time required for the feedback
loop to respond is dependent on the compensation components and the output capacitor size. Typically, 3 to 4
cycles are required to respond to a load step, but only in
the first cycle does the output drop linearly. The output
droop, VDROOP, is usually about 2 to 3 times the linear
drop of the first cycle. Thus, a good place to start is with
the output capacitor value of approximately:
COUT ≈ 2.5
ΔIOUT
fO • VDROOP
More capacitance may be required depending on the duty
cycle and load step requirements.
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LTC3411A
APPLICATIONS INFORMATION
In most applications, the input capacitor is merely required
to supply high frequency bypassing, since the impedance
to the supply is very low. A 10μF ceramic capacitor is
usually enough for these conditions.
Setting the Output Voltage
The LTC3411A develops a 0.8V reference voltage between
the feedback pin, VFB, and the signal ground as shown in
Figure 4. The output voltage is set by a resistive divider
according to the following formula:
R2
VOUT 0.8V 1+ R1
Keeping the current small (<5μA) in these resistors maximizes efficiency, but making them too small may allow
stray capacitance to cause noise problems and reduce
the phase margin of the error amp loop.
To improve the frequency response, a feed-forward capacitor CF may also be used. Great care should be taken
to route the VFB line away from noise sources, such as
the inductor or the SW line.
Shutdown and Soft-Start
The SHDN/RT pin is a dual purpose pin that sets the oscillator frequency and provides a means to shut down the
LTC3411A. This pin can be interfaced with control logic in
several ways, as shown in Figure 2 and Figure 3. In both
configurations, Run = “0” shuts down the LTC3411A and
Run = “1” activates the LTC3411A.
of how to switch from force continuous mode to pulse
skipping mode when RUN goes low. The parasitic drain
capacitance of a large transistor coupled with a large pull
up resistor results in large RC constants. As RUN goes
low, the transistor drain charges up slowly, gradually decreasing the oscillator frequency of the part. This leads to
large inductor current ripples translating into large output
voltage ripples. In some cases, the output voltage could
rise up to dangerous levels.
When activating the LTC3411A, an internal soft-start slowly
ramps the output voltage up until regulation. Soft-start
prevents surge currents from VIN by gradually ramping the
output voltage up during start-up. The output will ramp
from zero to full scale over a time period of approximately
0.7ms. This prevents the LTC3411A from having to quickly
charge the output capacitor and thus supplying an excessive amount of instantaneous current.
The LTC3411A can start into a back-biased output in forced
continuous operation. When the output is pre-biased at
either a higher or lower value than the regulated output
voltage, the LTC3411A will sink or source current as needed
to bring the output back into regulation. However, during
soft-start the regulator will always start in pulse skipping
mode ignoring the mode selected with the SYNC/MODE
SHDN/RT
SVIN
RT
100k
RUN
3411A F03
Care must be taken when using Figure 3 to shut down
the part in force continuous mode. The pull up resistor
should be as small as the application would allow and
the pull down transistor should be as small as possible
to minimize its parasitic drain capacitance. If possible,
always shut down the part while in pulse skipping mode
or Burst Mode operation. Figure 4 shows an example
SHDN/RT
Figure 3. SHDN/RT Pin Activated with a Switch
SHDN/RT
RT
0V
OFF ON
SVIN
1M
3V
100k
RT
SYNC/MODE
100k
RUN
3411A F02
3411A F04
Figure 2. SHDN/RT Pin Activated with a Logic Input
Figure 4. Automatic Mode Change Circuit
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LTC3411A
APPLICATIONS INFORMATION
pin. This prevents the output from discharging to below
the regulation point when soft-starting.
Mode Selection and Frequency Synchronization
The SYNC/MODE pin is a multipurpose pin which provides
mode selection and frequency synchronization. Connecting this pin to VIN enables Burst Mode operation, which
provides the best low current efficiency at the cost of a
higher output voltage ripple. When this pin is connected to
ground, pulse skipping operation is selected which provides
the lowest output voltage and current ripple at the cost
of low current efficiency. Applying a voltage that is half
the value of the input voltage results in forced continuous
mode, which creates a fixed output ripple and is capable of
sinking up to 0.4A. Since the switching noise is constant
in this mode, it is also the easiest to filter out.
The LTC3411A can also be synchronized to an external
clock signal by the SYNC/MODE pin. The internal oscillator frequency should be set to ±20% of the external clock
frequency to ensure adequate slope compensation, since
slope compensation is derived from the internal oscillator.
During synchronization, the mode is set to pulse skipping
and the top switch turn on is synchronized to the falling
edge of the external clock.
Checking Transient Response
The OPTI-LOOP® compensation allows the transient response to be optimized for a wide range of loads and output
capacitors. The availability of the ITH pin not only allows
VIN
+
C6
The ITH external components shown in the circuit on the
front page of this data sheet will provide an adequate starting point for most applications. The series R-C filter sets
the dominant pole-zero loop compensation. The values can
be modified slightly (from 0.5 to 2 times their suggested
values) to optimize transient response once the final PC
layout is done and the particular output capacitor type
and value have been determined. The output capacitors
need to be selected because the various types and values
determine the loop feedback factor gain and phase. An
output current pulse of 20% to 100% of full load current
having a rise time of 1μs to 10μs will produce output voltage
and ITH pin waveforms that will give a sense of the overall
loop stability without breaking the feedback loop.
Switching regulators take several cycles to respond to a
step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to ΔILOAD • ESR, where
ESR is the effective series resistance of COUT. ΔILOAD also
begins to charge or discharge COUT generating a feedback
error signal used by the regulator to return VOUT to its
OPTI-LOOP is a registered trademark of Linear Technology Corporation.
R5
R6
CIN
SVIN
PVIN
PGOOD
C8
PGND
optimization of the control loop behavior but also provides
a DC coupled and AC filtered closed loop response test
point. The DC step, rise time and settling time at this test
point truly reflects the closed loop response. Assuming a
predominantly second order system, phase margin and/or
damping factor can be estimated using the percentage
of overshoot seen at this pin. The bandwidth can also be
estimated by examining the rise time at the pin.
L1
PGOOD
SW
PGND
SGND
SYNC/MODE
ITH
SGND
RC
CITH
+
D1
OPTIONAL
LTC3411A
VOUT
COUT
C5
CF
VFB
SGND PGND
R2
SHDN/RT
RT
CC
PGND
PGND
R1
3411A F05
SGND
SGND
GND
SGND SGND
Figure 5. LTC3411A General Schematic
3411afa
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LTC3411A
APPLICATIONS INFORMATION
steady-state value. During this recovery time, VOUT can
be monitored for overshoot or ringing that would indicate
a stability problem.
The initial output voltage step may not be within the
bandwidth of the feedback loop, so the standard second
order overshoot/DC ratio cannot be used to determine
phase margin. The gain of the loop increases with R and
the bandwidth of the loop increases with decreasing C.
If R is increased by the same factor that C is decreased,
the zero frequency will be kept the same, thereby keeping
the phase the same in the most critical frequency range
of the feedback loop. In addition, a feedforward capacitor
CF can be added to improve the high frequency response,
as shown in Figure 5. Capacitor CF provides phase lead by
creating a high frequency zero with R2 which improves
the phase margin.
The output voltage settling behavior is related to the stability
of the closed-loop system and will demonstrate the actual
overall supply performance. For a detailed explanation of
optimizing the compensation components, including a
review of control loop theory, refer to Linear Technology
Application Note 76.
Although a buck regulator is capable of providing the full
output current in dropout, it should be noted that as the
input voltage VIN drops toward VOUT, the load step capability
does decrease due to the decreasing voltage across the
inductor. Applications that require large load step capability near dropout should use a different topology such as
SEPIC, Zeta or single inductor, positive buck/boost.
1
VIN = 3.6V
fO = 1MHz
POWER LOSS (W)
0.1
0.01
0.001
0.0001
0.1
VOUT = 1.2V
VOUT = 1.5V
= 1.8V
VOUTVOUT
= 1.2V
- 1.8V
1
10
100
1000
LOAD CURRENT (mA)
10000
3411A F06
Figure 6. Power Loss vs Load Currrent
In some applications, a more severe transient can be
caused by switching in loads with large (>1μF) input capacitors. The discharged input capacitors are effectively
put in parallel with COUT, causing a rapid drop in VOUT. No
regulator can deliver enough current to prevent this problem, if the switch connecting the load has low resistance
and is driven quickly. The solution is to limit the turn-on
speed of the load switch driver. A Hot Swap™ controller
is designed specifically for this purpose and usually incorporates current limiting, short-circuit protection, and
soft-starting.
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Percent efficiency can
be expressed as:
%Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percentage of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC3411A circuits: 1) VIN current, 2) switching
losses, 3) I2R losses, 4) other losses.
1) The VIN current is the DC supply current given in the
electrical characteristics which excludes MOSFET driver
and control currents. VIN current results in a small (<0.1%)
loss that increases with VIN, even at no load.
2) The switching current is the sum of the MOSFET driver
and control currents. The MOSFET driver current results
from switching the gate capacitance of the power MOSFETs.
Each time a MOSFET gate is switched from low to high
to low again, a packet of charge dQ moves from VIN to
ground. The resulting dQ/dt is a current out of VIN that is
typically much larger than the DC bias current. In continuous mode, IGATECHG = fO(QT + QB), where QT and QB are
the gate charges of the internal top and bottom MOSFET
switches. The gate charge losses are proportional to VIN
and thus their effects will be more pronounced at higher
supply voltages.
Hot Swap is a trademark of Linear Technology Corporation.
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LTC3411A
APPLICATIONS INFORMATION
3) I2R Losses are calculated from the DC resistances of
the internal switches, RSW, and external inductor, RL. In
continuous mode, the average output current flowing
through inductor L is “chopped” between the internal top
and bottom switches. Thus, the series resistance looking into the SW pin is a function of both top and bottom
MOSFET RDS(ON) and the duty cycle (DC) as follows:
RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC)
The RDS(ON) for both the top and bottom MOSFETs can
be obtained from the Typical Performance Characteristics
curves. Thus, to obtain I2R losses:
I2R losses = IOUT2(RSW + RL)
4) Other “hidden” losses such as copper trace and internal
battery resistances can account for additional efficiency
degradations in portable systems. It is very important
to include these “system” level losses in the design of a
system. The internal battery and fuse resistance losses
can be minimized by making sure that CIN has adequate
charge storage and very low ESR at the switching frequency. Other losses including diode conduction losses
during dead-time and inductor core losses which generally
account for less than 2% total additional loss.
Thermal Considerations
In a majority of applications, the LTC3411A does not
dissipate much heat due to its high efficiency. However,
in applications where the LTC3411A is running at high
ambient temperature with low supply voltage and high
duty cycles, such as in dropout, the heat dissipated may
exceed the maximum junction temperature of the part. If
the junction temperature reaches approximately 150°C,
both power switches will be turned off and the SW node
will become high impedance.
To avoid the LTC3411A from exceeding the maximum junction temperature, the user will need to do some thermal
analysis. The goal of the thermal analysis is to determine
whether the power dissipated exceeds the maximum junction
temperature of the part. The temperature rise is given by:
TRISE = PD • θJA
where PD is the power dissipated by the regulator and θJA
is the thermal resistance from the junction of the die to
the ambient temperature.
The junction temperature, TJ, is given by:
TJ = TRISE + TAMBIENT
As an example, consider the case when the LTC3411A is
in dropout at an input voltage of 3.3V with a load current
of 1A. From the Typical Performance Characteristics
graph of Switch Resistance, the RDS(ON) resistance of the
P-channel switch is 0.15Ω. Therefore, power dissipated
by the part is:
PD = I2 • RDS(ON) = 150mW
The MS10 package junction-to-ambient thermal resistance,
θJA, will be in the range of 100°C/W to 120°C/W. Therefore,
the junction temperature of the regulator operating in a
70°C ambient temperature is approximately:
TJ = 0.15 • 120 + 70 = 88°C
Remembering that the above junction temperature is
obtained from an RDS(ON) at 25°C, we might recalculate
the junction temperature based on a higher RDS(ON) since
it increases with temperature. However, we can safely assume that the actual junction temperature will not exceed
the absolute maximum junction temperature of 125°C.
Design Example
As a design example, consider using the LTC3411A in a
portable application with a Li-Ion battery. The battery provides a VIN = 2.5V to 4.2V. The load requires a maximum
of 1.25A in active mode and 10mA in standby mode. The
output voltage is VOUT = 2.5V. Since the load still needs
power in standby, Burst Mode operation is selected for
good low load efficiency.
First, calculate the timing resistor for 1MHz operation:
RT = 5 × 107 (103)–1.6508 = 557.9k
Use a standard value of 549k. Next, calculate the inductor
value for about 40% ripple current at maximum VIN:
L=
2.5V 2.5V
• 1
= 2μH
1MHz • 500mA 4.2V Choosing the closest standard inductor value from a vendor
of 2.2μH, results in a maximum ripple current of:
IL =
2.5V
1MHz • 2.2μ
2.5V • 1
= 460mA
4.2V 3411afa
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LTC3411A
APPLICATIONS INFORMATION
For cost reasons, a ceramic capacitor will be used. COUT
selection is then based on load step droop instead of ESR
requirements. For a 5% output droop:
the LTC3411A. These items are also illustrated graphically
in the layout diagram of Figure 7. Check the following in
your layout:
1.25A
= 25μF
1MHz • (5% • 2.5V)
1. Does the capacitor CIN connect to the power VIN (Pin 6)
and power GND (Pin 5) as close as possible? This capacitor
provides the AC current to the internal power MOSFETs
and their drivers.
COUT ≈ 2.5
The closest standard value is 22μF. Since the output
impedance of a Li-Ion battery is very low, CIN is typically
10μF. In noisy environments, decoupling SVIN from PVIN
with an R6/C8 filter of 1Ω/0.1μF may help, but is typically
not needed.
2. Are the COUT and L1 closely connected? The (–) plate of
COUT returns current to PGND and the (–) plate of CIN.
3. The resistor divider, R1 and R2, must be connected
between the (+) plate of COUT and a ground line terminated
near SGND (Pin 3). The feedback signal VFB should be
routed away from noisy components and traces, such as
the SW line (Pin 4), and its trace should be minimized.
The output voltage can now be programmed by choosing
the values of R1 and R2. To maintain high efficiency, the
current in these resistors should be kept small. Choosing
2μA with the 0.8V feedback voltage makes R1~400k. A close
standard 1% resistor value is 412k. Then R2 is 887k.
4. Keep sensitive components away from the SW pin. The
input capacitor CIN, the compensation capacitor CC and
CITH and all the resistors R1, R2, RT, and RC should be
routed away from the SW trace and the inductor L1. The
SW pin pad should be kept as small as possible.
The compensation should be optimized for these components by examining the load step response but a good place
to start for the LTC3411A is with a 12.1kΩ and 680pF filter.
The output capacitor may need to be increased depending
on the actual undershoot during a load step.
5. A ground plane is preferred, but if not available, keep
the signal and power grounds segregated with small signal
components returning to the SGND pin at one point which
is then connected to the PGND pin.
The PGOOD pin is a common drain output and requires a pullup resistor. A 100k resistor is used for adequate speed.
The circuit on page 1 of this data sheet shows the complete
schematic for this design example.
6. Flood all unused areas on all layers with copper. Flooding with copper will reduce the temperature rise of power
components. These copper areas should be connected to
one of the input supplies: PVIN, PGND, SVIN or SGND.
Board Layout Considerations
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
CIN
VIN
PVIN
PGND
SVIN
SW
LTC3411A SGND
R5
PGOOD
C4
R2
R1
COUT
L1
VOUT
VIN
PGOOD
VFB
SYNC/MODE
ITH
SHDN/RT
PS
RC
CC
CITH
BM
RT
3411A F07
BOLD LINES INDICATE HIGH CURRENT PATHS
Figure 7. LTC3411A Layout Diagram (See Board Layout Checklist)
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17
LTC3411A
TYPICAL APPLICATIONS
General Purpose Buck Regulator Using Ceramic Capacitors
VIN
2.5V TO
5.5V
C1
10μF
PGND
PVIN
SVIN
RS1
1M
BM
PGOOD
LTC3411A
SYNC/MODE
FC
PS RS2
1M
ITH
R5
100k
PGOOD
L1
2.2μH
VOUT
1.2V/1.5V/1.8V
AT 1.25A
SW
R2 442k
VFB
SHDN/RT
1.8V
SGND
1.5V
1.2V
PGND
R3
12.1k
R4
549k
C3
680pF
C2
22μF
C4 22pF
R1A
357k
R1B
511k
R1C
887k
3411A TA02a
SGND
SGND
GND
SGND
PGND
NOTE: IN DROPOUT, THE OUTPUT TRACKS THE INPUT VOLTAGE
C1, C2: TAIYO YUDEN JMK325BJ226MM
L1: TOKO A914BYW-2R2M (D52LC SERIES)
Efficiency vs Output Current
100
90
Burst Mode
OPERATION
VOUT
100mV/DIV
AC COUPLED
EFFICIENCY (%)
80
70
PULSE SKIP
IL
1A/DIV
60
50
40
ILOAD
1A/DIV
FORCED CONTINUOUS
30
20
VIN = 3.6V
VOUT = 1.2V
fO = 1MHz
10
0
0.1
1
10
100
1000
OUTPUT CURRENT (mA)
40μs/DIV
VIN = 3.6V
VOUT = 1.2V
ILOAD = 100mA TO 1.25A
Burst Mode OPERATION
10000
3411A TA02c
3411A TA02b
VOUT
100mV/DIV
AC COUPLED
IL
1A/DIV
ILOAD
1A/DIV
40μs/DIV
VIN = 3.6V
VOUT = 1.8V
ILOAD = 100mA TO 1.25A
PULSE SKIPPING MODE
3411A TA02d
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18
LTC3411A
PACKAGE DESCRIPTION
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1699)
R = 0.115
TYP
6
0.38 ± 0.10
10
0.675 ±0.05
3.50 ±0.05
1.65 ±0.05
2.15 ±0.05 (2 SIDES)
3.00 ±0.10
(4 SIDES)
PACKAGE
OUTLINE
1.65 ± 0.10
(2 SIDES)
PIN 1
TOP MARK
(SEE NOTE 6)
(DD) DFN 1103
5
0.200 REF
0.25 ± 0.05
1
0.25 ± 0.05
0.50 BSC
0.75 ±0.05
0.50
BSC
2.38 ±0.05
(2 SIDES)
2.38 ±0.10
(2 SIDES)
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
RECOMMENDED SOLDER PAD
PITCH AND DIMENSIONS
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2).
CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
MS Package
10-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1661 Rev E)
0.889 ± 0.127
(.035 ± .005)
5.23
(.206)
MIN
3.20 – 3.45
(.126 – .136)
3.00 ± 0.102
(.118 ± .004)
(NOTE 3)
10 9 8 7 6
0.497 ± 0.076
(.0196 ± .003)
REF
3.00 ± 0.102
(.118 ± .004)
(NOTE 4)
4.90 ± 0.152
(.193 ± .006)
0.254
(.010)
DETAIL “A”
0° – 6° TYP
GAUGE PLANE
0.50
0.305 ± 0.038
(.0197)
(.0120 ± .0015)
BSC
TYP
RECOMMENDED SOLDER PAD LAYOUT
1 2 3 4 5
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
0.50
(.0197)
BSC
0.1016 ± 0.0508
(.004 ± .002)
MSOP (MS) 0307 REV E
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
3411afa
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.
19
LTC3411A
TYPICAL APPLICATION
1mm Height, 2MHz, Li-Ion to 1.8V Converter
VIN
2.5V
TO 4.2V
R5
100k
C1
10μF
PVIN
PGOOD
SVIN
SW
LTC3411A
PGOOD
L1
0.9μH
C4 22pF
C2
10μF
×2
SYNC/MODE
ITH
R3
13.3k SGND PGND
C3
470pF
VFB
SHDN/RT
R4
178k
R1
698k
VOUT
1.8V
AT 1.25A
R2
887k
C1, C2: TAIYO YUDEN JMK107BJ106MA
L1: FDK MIPW3226DORGM
3411A TA04a
Efficiency vs Output Current
100
90
EFFICIENCY (%)
80
VOUT
100mV/DIV
AC COUPLED
VOUT
100mV/DIV
AC COUPLED
IL
1A/DIV
IL
1A/DIV
ILOAD
1A/DIV
ILOAD
1A/DIV
70
60
50
40
30
20
10
0
0.1
VOUT = 1.8V
fO = 2MHz
1
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
10
100
1000
OUTPUT CURRENT (mA)
40μs/DIV
VIN = 3.6V
VOUT = 1.8V
ILOAD = 50mA TO 1.25A
10000
3411A TA04c
40μs/DIV
VIN = 3.6V
VOUT = 1.8V
ILOAD = 250mA TO 1.25A
3411A TA04d
3411A TA04b
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3411afa
20 Linear Technology Corporation
LT 0908 REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
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