LTC3406A
1.5MHz, 600mA
Synchronous Step-Down
Regulator in ThinSOT
DESCRIPTION
FEATURES
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High Efficiency: Up to 96%
Very Low Quiescent Current: Only 20µA
Low Output Ripple Voltage During Burst Mode®
Operation
600mA Output Current
2.5V to 5.5V Input Voltage Range
1.5MHz Constant Frequency Operation
No Schottky Diode Required
Low Dropout Operation: 100% Duty Cycle
±2% 0.6V Reference
Shutdown Mode Draw ≤1µA Supply Current
Internal Soft-Start Limits Inrush Current
Current Mode Operation for Excellent Line and
Load Transient Response
Overtemperature Protected
Low Profile (1mm) ThinSOTTM Package
APPLICATIONS
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The LTC®3406A is a high efficiency monolithic synchronous buck regulator using a constant frequency, current
mode architecture. Supply current during operation is only
20μA, dropping to ≤1μA in shutdown. The 2.5V to 5.5V
input voltage range makes the LTC3406A ideally suited
for single Li-Ion battery-powered applications. 100% duty
cycle provides low dropout operation, extending battery
runtime portable systems. Automatic Burst Mode operation increases efficiency at light loads, further extending
battery runtime.
Switching frequency is internally set at 1.5MHz, allowing
the use of small surface mount inductors and capacitors.
The internal synchronous switch increases efficiency
and eliminates the need for an external Schottky diode.
Low output voltages are easily supported with the 0.6V
feedback reference voltage. The LTC3406A is available in
a low profile (1mm) ThinSOT package.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
ThinSOT is a registered trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents including 5481178, 6580258.
Cellular Telephones
Wireless and DSL Modems
Digital Still Cameras
Media Players
Portable Instruments
Point of Load Regulation
TYPICAL APPLICATION
Efficiency vs Load Current
100
2.2μH
VIN
4.7μF
CER
SW
22pF
10μF
CER
LTC3406A
RUN
GND
VFB
619k
309k
3406A TA01
90
80
EFFICIENCY (%)
VIN
VOUT
1.8V
600mA
70
60
50
40
30
20
10
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
VOUT = 1.8V
0
0.1
10
100
1
OUTPUT CURRENT (mA)
1000
3406A TA01b
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LTC3406A
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
TOP VIEW
Input Supply Voltage ....................................– 0.3V to 6V
RUN, VFB Voltages .......................................–0.3V to VIN
SW Voltage (DC) ........................... – 0.3V to (VIN + 0.3V)
P-Channel Switch Source Current (DC)
(Note 7)................................................................800mA
N-Channel Switch Sink Current (DC) (Note 7) .....800mA
Peak SW Sink and Source Current (Note 7) .............1.3A
Operating Temperature Range (Note 2)
LTC3406AE ..............................................– 40°C to 85°C
LTC3406AI .............................................– 40°C to 125°C
Junction Temperature (Notes 3, 6)........................ 125°C
Storage Temperature Range...................– 65°C to 150°C
Lead Temperature (Soldering, 10 sec) .................. 300°C
RUN 1
5 VFB
GND 2
SW 3
4 VIN
S5 PACKAGE
5-LEAD PLASTIC TSOT-23
TJMAX = 125°C, θJA = 250°C/W, θJC = 90°C/W
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3406AES5#PBF
LTC3406AES5#TRPBF
LTCWJ
5-Lead Plastic TSOT-23
–40°C to 85°C
LTC3406AIS5#PBF
LTC3406AIS5#TRPBF
LTCWJ
5-Lead Plastic TSOT-23
–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 unless otherwise specified.
SYMBOL
PARAMETER
CONDITIONS
MIN
IVFB
Feedback Current
VFB
Regulated Feedback Voltage
(Note 4) LTC3406AE
(Note 4) LTC3406AI
●
●
∆VFB
Reference Voltage Line Regulation
VIN = 2.5V to 5.5V (Note 4) LTC3406AE
VIN = 2.5V to 5.5V (Note 4) LTC3406AI
●
●
IPK
Peak Inductor Current
VIN = 3V, VFB = 0.5V
Duty Cycle < 35%
VLOADREG
Output Voltage Load Regulation
VIN
Input Voltage Range
IS
Input DC Bias Current
Active Mode
Sleep Mode
Shutdown
TYP
MAX
UNITS
±30
nA
0.6
0.6
0.6120
0.615
V
V
0.04
0.04
0.4
0.6
%/V
%/V
1
1.25
A
●
0.5880
0.585
0.75
0.5
●
2.5
(Note 5)
VFB = 0V
VFB = 0.63V
VRUN = 0V, VIN = 5.5V
fOSC
Oscillator Frequency
VFB = 0.6V
RPFET
RDS(ON) of P-Channel FET
ISW = 100mA
5.5
V
300
30
1
μA
μA
μA
1.5
1.8
MHz
0.23
0.35
Ω
200
16
0.1
●
1.2
%
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LTC3406A
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.6V unless otherwise specified.
SYMBOL
PARAMETER
CONDITIONS
TYP
MAX
UNITS
RNFET
RDS(ON) of N-Channel FET
ISW = –100mA
0.21
0.35
Ω
ILSW
SW Leakage
VRUN = 0V, VSW = 0V or 5V, VIN = 5V
±0.01
±1
μA
tSOFT-START
Soft-Start Time
VFB from 10% to 90% Full-Scale
0.6
0.9
1.2
ms
VRUN
RUN Threshold
●
0.3
1
1.5
V
RUN Leakage Current
●
±0.01
±1
μA
IRUN
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 LTC3406AE is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LTC3406AI is guaranteed to meet the
specified performance over the full –40°C to 125°C operating temperature
range.
Note 3: TJ is calculated from the ambient temperature TA and power
dissipation PD according to the following formula:
LTC3406A: TJ = TA + (PD)(250°C/W)
MIN
Note 4: The LTC3406A is tested in a proprietary test mode that connects
VFB to the output of the error amplifier.
Note 5: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 6: 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.
Note 7: Limited by long term current density considerations.
TYPICAL PERFORMANCE CHARACTERISTICS
(From Front Page Figure Except for the Resistive Divider Resistor Values)
Efficiency vs Load Current
Efficiency vs Load Current
100
90
90
90
80
80
80
70
70
70
60
50
40
EFFICIENCY (%)
100
EFFICIENCY (%)
EFFICIENCY (%)
Efficiency vs Input Voltage
100
60
50
40
60
50
40
30
30
30
20
20
20
10
VOUT = 1.8V
0
2
3
IL = 10mA
IL = 100mA
IL = 600mA
4
5
INPUT VOLTAGE (V)
10
6
3406A G01
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
VOUT = 1.2V
0
0.1
1
10
100
OUTPUT CURRENT (mA)
1000
3406A G02
10
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
VOUT = 2.5V
0
0.1
1
10
100
OUTPUT CURRENT (mA)
1000
3406A G03
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LTC3406A
TYPICAL PERFORMANCE CHARACTERISTICS
(From Front Page Figure Except for the Resistive Divider Resistor Values)
VOUT = 1.8V
1.812
0.615
0.610
80
EFFICIENCY (%)
1.808
1.804
1.800
1.796
70
60
50
40
1.792
30
1.788
20
1.784
10
1.780
2
600
400
200
OUTPUT CURRENT (mA)
IL = 10mA
IL = 100mA
IL = 600mA
VOUT = 2.5V
0
0
3
4
5
INPUT VOLTAGE (V)
Oscillator Frequency vs
Temperature
0.595
0.590
0.585
–50 –25
6
1.55
1.50
1.45
1.40
1.35
125
Burst Mode
1.55
SW
2V/DIV
1.50
1.45
VOUT
20mV/DIV
AC COUPLED
1.40
IL
200mA/DIV
1.35
1.30
50
25
75
0
TEMPERATURE (°C)
100
1.20
2.0
125
2.5
3.0
3.5 4.0 4.5 5.0
INPUT VOLTAGE (V)
3406A G07
4μs/DIV
VIN = 3.6V
VOUT = 1.8V
ILOAD = 10mA
Burst Mode OPERATION
6.0
5.5
RDS(ON) vs Input Voltage
RDS(ON) vs Temperature
Dynamic Supply Current
0.40
10
9
0.35
0.35
0.30
RDS(ON) (Ω)
0.30
MAIN SWITCH
0.25
SYNCHRONOUS
SWITCH
0.20
VIN = 2.7V
VIN = 3.6V
0.25
VIN = 4.2V
0.20
0.15
0.10
0.15
0.05
0.10
4
3
5
2
INPUT VOLTAGE (V)
6
7
3406A G10
3406A G09
3406A G08
0.40
1
100
3406A G06
1.25
0
50
25
75
0
TEMPERATURE (°C)
1.60
OSCILLATOR FREQUENCY (MHz)
OSCILLATOR FREQUENCY (MHz)
0.600
Oscillator Frequency vs
Input Voltage
VIN = 3.6V
1.30
–50 –25
RDS(0N) (Ω)
0.605
3406A G05
3406A G04
1.60
VIN = 3.6V
90
0
–50 –25
SYNCHRONOUS SWITCH
MAIN SWITCH
75
50
25
TEMPERATURE (°C)
0
100
125
3406A G11
DYNAMIC SUPPLY CURRENT (μA)
OUTPUT VOLTAGE (V)
100
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
1.816
REFERENCE VOLTAGE (V)
1.820
Reference Voltage vs
Temperature
Efficiency vs Input Voltage
Output vs Load Current
VOUT = 1.2V
ILOAD = 0A
8
7
6
5
4
3
2
1
0
2.0 2.5
3.0
3.5 4.0 4.5 5.0
INPUT VOLTAGE (V)
5.5
6.0
3406A G12
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LTC3406A
TYPICAL PERFORMANCE CHARACTERISTICS
(From Front Page Figure Except for the Resistive Divider Resistor Values)
Dynamic Supply Current vs
Temperature
Switch Leakage vs Temperature
VIN = 3.6V
VOUT = 1.2V
ILOAD = 0A
1000
150
100
50
100
80
MAIN SWITCH
60
40
SYNCHRONOUS
SWITCH
20
50
25
75
0
TEMPERATURE (°C)
100
125
700
600
500
300
0
50
25
75
0
TEMPERATURE (°C)
100
125
0
1
3
4
2
INPUT VOLTAGE (V)
RUN
2V/DIV
VOUT
200mV/DIV
VOUT
1V/DIV
IL
500mA/DIV
ILOAD
500mA/DIV
ILOAD
500mA/DIV
Load Step
6
Load Step
VOUT
200mV/DIV
IL
500mA/DIV
ILOAD
500mA/DIV
VIN = 3.6V
20μs/DIV
VOUT = 1.8V
ILOAD = 0mA TO 600mA
3406A G16
5
3406A G15
Load Step
3406A G17
VIN = 3.6V
20μs/DIV
VOUT = 1.8V
ILOAD = 50mA TO 600mA
Load Step
3406A G18
Discontinuous Operation
VOUT
200mV/DIV
VOUT
200mV/DIV
SW
2V/DIV
IL
500mA/DIV
IL
500mA/DIV
VOUT
20mV/DIV
AC COUPLED
ILOAD
500mA/DIV
ILOAD
500mA/DIV
3406A G19
SYNCHRONOUS
SWITCH
200
3406A G14
Start-Up from Shutdown
VIN = 3.6V
20μs/DIV
VOUT = 1.8V
ILOAD = 100mA TO 600mA
MAIN SWITCH
400
100
0
–50 –25
3406A G13
500μs/DIV
VIN = 3.6V
VOUT = 1.8V
ILOAD = 600mA (3Ω RES)
SWITCH LEAKAGE (pA)
800
200
0
–50 –25
RUN = 0V
900
120
SWITCH LEAKAGE (nA)
DYNAMIC SUPPLY CURRENT (μA)
250
Switch Leakage vs Input Voltage
140
300
IL
200mA/DIV
VIN = 3.6V
20μs/DIV
VOUT = 1.8V
ILOAD = 200mA TO 600mA
3406A G20
VIN = 3.6V
VOUT = 1.8V
ILOAD = 50mA
500ns/DIV
3406A G21
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LTC3406A
PIN FUNCTIONS
RUN (Pin 1): Run Control Input. Forcing this pin above 1.5V
enables the part. Forcing this pin below 0.3V shuts down
the device. In shutdown, all functions are disabled drawing
40%. However, the LTC3406A uses a patented
scheme that counteracts this compensating ramp, which
allows the maximum inductor peak current to remain
unaffected throughout all duty cycles.
In Burst Mode operation, the peak current of the inductor
is set to approximately 100mA regardless of the output
load. Each burst event can last from a few cycles at light
loads to almost continuously cycling with short sleep
intervals at moderate loads. In between these burst events,
the power MOSFETs and any unneeded circuitry are turned
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LTC3406A
APPLICATIONS INFORMATION
The basic LTC3406A application circuit is shown on the
front page. External component selection is driven by the
load requirement and begins with the selection of L followed by CIN and COUT.
Table 1. Representative Surface Mount Inductors
PART
NUMBER
VALUE
(μH)
DCR
(Ω MAX)
MAX DC
CURRENT (A)
SIZE
W × L × H (mm3)
Sumida
CDRH3D16
1.5
2.2
3.3
4.7
0.043
0.075
0.110
0.162
1.55
1.20
1.10
0.90
3.8 × 3.8 × 1.8
Sumida
CMD4D06
2.2
3.3
4.7
0.116
0.174
0.216
0.950
0.770
0.750
3.5 × 4.3 × 0.8
Panasonic
ELT5KT
3.3
4.7
0.17
0.20
1.00
0.95
4.5 × 5.4 × 1.2
Murata
LQH32CN
1.0
2.2
4.7
0.060
0.097
0.150
1.00
0.79
0.65
2.5 × 3.2 × 2.0
Inductor Selection
For most applications, the value of the inductor will fall in
the range of 1μH to 4.7μH. Its value is chosen based on the
desired ripple current. Large value inductors lower ripple
current and small value inductors result in higher ripple
currents. Higher VIN or VOUT also increases the ripple current as shown in Equation 1. A reasonable starting point for
setting ripple current is ∆IL = 240mA (40% of 600mA).
IL =
V
1
VOUT 1 OUT
VIN
( f )(L )
(1)
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 720mA
rated inductor should be enough for most applications
(600mA + 120mA). For better efficiency, choose a low
DC-resistance inductor.
The inductor value also has an effect on Burst Mode operation. The transition to low current operation begins when
the inductor current peaks fall to approximately 100mA.
Lower inductor values (higher ∆IL) will cause this to occur
at lower load currents, which can cause 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.
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 LTC3406A requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3406A applications.
CIN and COUT Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle VOUT/VIN. To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum RMS
capacitor current is given by:
CIN required IRMS IOMAX
VOUT ( VIN VOUT )
VIN
1/2
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is
commonly used for design because even significant deviations do not offer much relief. Note that the capacitor
manufacturer’s ripple current ratings are often based on
2000 hours of life. This makes it advisable to further derate
the capacitor, or choose a capacitor rated at a higher temperature than required. Always consult the manufacturer
if there is any question.
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LTC3406A
APPLICATIONS INFORMATION
The selection of COUT is driven by the required effective
series resistance (ESR).
Typically, once the ESR requirement for COUT has been
met, the RMS current rating generally far exceeds the
IRIPPLE(P-P) requirement. The output ripple ∆VOUT is
determined by:
1
VOUT IL ESR +
8fCOUT
where f = operating frequency, COUT = output capacitance
and ∆IL = ripple current in the inductor. For a fixed output
voltage, the output ripple is highest at maximum input
voltage since ∆IL increases with input voltage.
Aluminum electrolytic and dry tantalum capacitors are both
available in surface mount configurations. In the case of
tantalum, 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 tantalum. These are
specially constructed and tested for low ESR so they give
the lowest ESR for a given volume. Other capacitor types
include Sanyo POSCAP, Kemet T510 and T495 series, and
Sprague 593D and 595D series. Consult the manufacturer
for other specific recommendations.
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.
Output Voltage Programming
In the adjustable version, the output voltage is set by a
resistive divider according to the following formula:
R2
VOUT = 0.6V 1+
R1
(2)
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 1.
0.6V ≤ VOUT ≤ 5.5V
R2
VFB
LTC3406A
R1
GND
Using Ceramic Input and Output Capacitors
Higher values, 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
LTC3406A’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
3406A F01
Figure 1. Setting the LTC3406A Output Voltage
Efficiency Considerations
The 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. Efficiency can be expressed as:
Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percentage of input power.
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LTC3406A
APPLICATIONS INFORMATION
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of
the losses in LTC3406A circuits: VIN quiescent current
and I2R losses. The VIN quiescent current loss dominates
the efficiency loss at very low load currents whereas the
I2R loss dominates the efficiency loss at medium to high
load currents. In a typical efficiency plot, the efficiency
curve at very low load currents can be misleading since
the actual power lost is of no consequence as illustrated
in Figure 2.
1
0.1
POWER LOSS (W)
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, simply add RSW to
RL and multiply the result by the square of the average
output current.
VIN = 3.6V
0.01
Other losses including CIN and COUT ESR dissipative losses
and inductor core losses generally account for less than
2% total additional loss.
0.001
0.0001
0.1
2. I2R losses are calculated from the 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 main
switch and the synchronous switch. 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:
VOUT = 1.2V
VOUT = 1.8V
VOUT = 2.5V
10
100
1
OUTPUT CURRENT (mA)
1000
3406A F02
Figure 2. Power Lost vs Load Current
1. The VIN quiescent current is due to two components:
the DC bias current as given in the electrical characteristics and the internal main switch and synchronous
switch gate charge currents. The gate charge current
results from switching the gate capacitance of the
internal power MOSFET switches. Each time the gate
is switched from high to low to high again, a packet of
charge, dQ, moves from VIN to ground. The resulting
dQ/dt is the current out of VIN that is typically larger
than the DC bias current. In continuous mode, IGATECHG
= f(QT + QB) where QT and QB are the gate charges of
the internal top and bottom switches. Both the DC bias
and gate charge losses are proportional to VIN and thus
their effects will be more pronounced at higher supply
voltages.
Thermal Considerations
In most applications the LTC3406A does not dissipate
much heat due to its high efficiency. But, in applications
where the LTC3406A 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 LTC3406A 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:
TR = (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.
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LTC3406A
APPLICATIONS INFORMATION
The junction temperature, TJ, is given by:
TJ = TA + TR
where TA is the ambient temperature.
As an example, consider the LTC3406A in dropout at an
input voltage of 2.7V, a load current of 600mA and an
ambient temperature of 70°C. From the typical performance graph of switch resistance, the RDS(ON) of the
P-channel switch at 70°C is approximately 0.27Ω. Therefore, power dissipated by the part is:
PD = ILOAD2 • RDS(ON) = 97.2mW
For the SOT-23 package, the θJA is 250°C/ W. Thus, the
junction temperature of the regulator is:
TJ = 70°C + (0.0972)(250) = 94.3°C
which is below the maximum junction temperature of
125°C.
Note that at higher supply voltages, the junction temperature
is lower due to reduced switch resistance (RDS(ON)).
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. 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 (∆ILOAD • ESR), where ESR is the effective series
resistance of COUT. ∆ILOAD also begins to charge or discharge COUT, which generates a feedback error signal. The
regulator loop then acts to return VOUT to its steady-state
value. During this recovery time VOUT can be monitored
for overshoot or ringing that would indicate a stability
problem. For a detailed explanation of switching control
loop theory, see Application Note 76.
A second, more severe transient is caused by switching
in loads with large (>1μF) supply bypass capacitors. The
discharged bypass 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
load switch resistance is low and it is driven quickly. The
only solution is to limit the rise time of the switch drive
so that the load rise time is limited to approximately
(25 • CLOAD). Thus, a 10μF capacitor charging to 3.3V
would require a 250μs rise time, limiting the charging
current to about 130mA.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3406A. These items are also illustrated graphically in
Figures 3 and 4. Check the following in your layout:
1. The power traces, consisting of the GND trace, the SW
trace, the VOUT trace and the VIN trace should be kept
short, direct and wide.
2. Does the VFB pin connect directly to the feedback
resistors? The resistive divider R1/R2 must be
connected between the (+) plate of COUT and ground.
3. Does CIN connect to VIN as closely as possible? This
capacitor provides the AC current to the internal power
MOSFETs.
4. Keep the switching node, SW, away from the sensitive
VFB node.
5. Keep the (–) plates of CIN and COUT, and the IC ground,
as close as possible.
3406afa
11
LTC3406A
APPLICATIONS INFORMATION
1
RUN
VFB
5
LTC3406A
2
R2
–
COUT
VOUT
3
+
R1
GND
SW
L1
VIN
CFWD
4
CIN
VIN
BOLD LINES INDICATE HIGH CURRENT PATHS
3406A F03
Figure 3. LTC3406A Layout Diagram
VIA TO VIN
CFWD
LTC3406A
L1
VIA TO VOUT
R2
PIN 1
VOUT
VIN
R1
SW
COUT
CIN
GND
3406A F04
Figure 4. LTC3406A Suggested Layout
Design Example
As a design example, assume the LTC3406A is used
in a single lithium-ion battery-powered cellular phone
application. The VIN will be operating from a maximum of
4.2V down to about 2.7V. The load current requirement
is a maximum of 0.6A but most of the time it will be in
standby mode, requiring only 2mA. Efficiency at both
low and high load currents is important. Output voltage
is 2.5V. With this information we can calculate L using
Equation (1),
L=
V
1
VOUT 1 OUT
VIN
( f )( IL )
(3)
Substituting VOUT = 2.5V, VIN = 4.2V, ∆IL = 240mA and
f = 1.5MHz in Equation (3) gives:
L=
2.5V
2.5V
1
= 2.81μH
1.5MHz(240mA) 4.2V
A 2.2μH inductor works well for this application. For best
efficiency choose a 720mA or greater inductor with less
than 0.2Ω series resistance.
CIN will require an RMS current rating of at least 0.3A ≅
ILOAD(MAX)/2 at temperature and COUT will require an ESR
of less than 0.25Ω. In most cases, a ceramic capacitor
will satisfy this requirement.
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12
LTC3406A
APPLICATIONS INFORMATION
Figure 5 shows the complete circuit along with its efficiency curve.
For the feedback resistors, choose R1 = 316k. R2 can
then be calculated from Equation (2) to be:
V
R2 = OUT 1 R1= 1000k
0.6
(4)
4
VIN
2.7V TO 4.2V
†
CIN
4.7μF
CER
VIN
SW
3
2.2μH*
VOUT
2.5V
600mA
COUT**
10μF
CER
22pF
LTC3406A
1
VFB
RUN
5
GND
2
1M
316k
3406A F05a
* MURATA LQH32CN2R2M33
** TAIYO YUDEN JMK316BJ106ML
†
TAIYO YUDEN LMK212BJ475MG
100
90
EFFICIENCY (%)
80
VOUT
100mV/DIV
70
60
IL
500mA/DIV
50
40
ILOAD
500mA/DIV
30
20
10
0
0.1
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
1
10
100
OUTPUT CURRENT (mA)
20μs/DIV
VIN = 3.6V
VOUT = 2.5V
ILOAD = 300mA TO 600mA
1000
3406A F05d
3406A F05b
Figure 5.
3406afa
13
LTC3406A
TYPICAL APPLICATIONS
Single Li-Ion 1.2V/600mA Regulator for High Efficiency and Small Footprint
4
VIN
†
CIN
4.7μF
CER
VIN
SW
3
2.2μH*
22pF
LTC3406A
1
VFB
RUN
GND
2
5
VOUT
1.2V
600mA
COUT**
10μF
CER
301k
* MURATA LQH32CN2R2M33
** TAIYO YUDEN JMK316BJ106ML
†
TAIYO YUDEN JMK212BJ475MG
3406A TA02
301k
Efficiency vs Load Current
Load Step
100
90
VOUT
100mV/DIV
EFFICIENCY (%)
80
70
60
IL
500mA/DIV
50
ILOAD
500mA/DIV
40
30
20
10
VOUT = 1.2V
0
0.1
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
1
10
100
OUTPUT CURRENT (mA)
1000
VIN = 3.6V
20μs/DIV
VOUT = 1.2V
ILOAD = 300mA TO 600mA
3406A TA05
3406A TA03
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14
LTC3406A
PACKAGE DESCRIPTION
S5 Package
5-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1635)
0.62
MAX
0.95
REF
2.90 BSC
(NOTE 4)
1.22 REF
1.4 MIN
3.85 MAX 2.62 REF
2.80 BSC
1.50 – 1.75
(NOTE 4)
PIN ONE
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
0.30 – 0.45 TYP
5 PLCS (NOTE 3)
0.95 BSC
0.80 – 0.90
0.20 BSC
0.01 – 0.10
1.00 MAX
DATUM ‘A’
0.30 – 0.50 REF
0.09 – 0.20
(NOTE 3)
NOTE:
1. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
3. DIMENSIONS ARE INCLUSIVE OF PLATING
4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR
5. MOLD FLASH SHALL NOT EXCEED 0.254mm
6. JEDEC PACKAGE REFERENCE IS MO-193
1.90 BSC
S5 TSOT-23 0302 REV B
3406afa
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTC3406A
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTC3406/LTC3406B
600mA (IOUT), 1.5MHz, Synchronous
Step-Down DC/DC Converters
96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 20μA,
ISD
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