LINER LTC3410

LTC3410-1.875
2.25MHz, 300mA
Synchronous Step-Down
Regulator in SC70
U
FEATURES
DESCRIPTIO
■
The LTC ®3410-1.875 is a high efficiency monolithic synchronous buck regulator using a constant frequency,
current mode architecture. Supply current during operation is only 26µA, dropping to <1µA in shutdown. The 2.5V
to 5.5V input voltage range makes the LTC3410-1.875
ideally suited for single Li-Ion battery-powered applications. 100% duty cycle provides low dropout operation,
extending battery life in portable systems.
■
■
■
■
■
■
■
■
■
■
■
■
■
High Efficiency: Up to 93%
Very Low Quiescent Current: Only 26µA
Low Output Voltage Ripple
300mA Output Current at VIN = 3V
380mA Minimum Peak Switch Current
2.5V to 5.5V Input Voltage Range
2.25MHz Constant Frequency Operation
No Schottky Diode Required
Stable with Ceramic Capacitors
Shutdown Mode Draws < 1µA Supply Current
±2% Output Voltage Accuracy
Current Mode Operation for Excellent Line and
Load Transient Response
Overtemperature Protected
Available in Low Profile SC70 Package
Switching frequency is internally set at 2.25MHz, allowing
the use of small surface mount inductors and capacitors.
The LTC3410-1.875 is specifically designed to work well
with ceramic output capacitors, achieving very low output
voltage ripple and a small PCB footprint.
The internal synchronous switch increases efficiency and
eliminates the need for an external Schottky diode. The
LTC3410-1.875 is available in a tiny, low profile SC70
package.
U
APPLICATIO S
■
■
■
■
Cellular Telephones
Wireless and DSL Modems
Digital Cameras
MP3 Players
Portable Instruments
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners. Protected by U.S. Patents,
including 5481178, 5994885, 6127815, 6304066, 6498466, 6580258, 6611131.
U
■
TYPICAL APPLICATIO
Efficiency and Power Loss vs
Output Current
100
4.7µH
VOUT
1.875V
SW
COUT
4.7µF
CER
LTC3410-1.875
RUN
VOUT
GND
80
4.2V
70
60
3.6V
0.01
50
40
20
0.1
3.6V
2.7V
30
34101875 TA01
1
4.2V
POWER LOSS
VIN
POWER LOSS (W)
CIN
4.7µF
CER
VIN
EFFICIENCY (%)
VIN
2.7V
TO 5.5V
2.7V
EFFICIENCY
VIN
90
0.001
10
0
0.1
1
10
100
OUTPUT CURRENT (mA)
0.0001
1000
34101875 TA02
34101875f
1
LTC3410-1.875
W W
W
AXI U
U
ABSOLUTE
RATI GS
U
U
W
PACKAGE/ORDER I FOR ATIO
(Note 1)
Input Supply Voltage .................................. – 0.3V to 6V
RUN, VOUT Voltages................................... – 0.3V to VIN
SW Voltage (DC) ......................... – 0.3V to (VIN + 0.3V)
P-Channel Switch Source Current (DC) ............. 500mA
N-Channel Switch Sink Current (DC) ................. 500mA
Peak SW Sink and Source Current .................... 630mA
Operating Temperature Range (Note 2) .. – 40°C to 85°C
Junction Temperature (Notes 3, 5) ...................... 125°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
ORDER PART
NUMBER
TOP VIEW
RUN 1
6 VOUT
GND 2
5 GND
SW 3
4 VIN
LTC3410ESC6-1.875
SC6 PACKAGE
6-LEAD PLASTIC SC70
SC6 PART MARKING
TJMAX = 125°C, θJA = 250°C/ W
LCFQ
Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking: http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.
VIN = 3.6V unless otherwise specified.
SYMBOL
PARAMETER
CONDITIONS
IVOUT
Output Voltage Feedback Current
IPK
Peak Inductor Current
VOUT
Regulated Output Voltage
∆VOUT
Output Voltage Line Regulation
VIN = 2.5V to 5.5V
VLOADREG
Output Voltage Load Regulation
ILOAD = 50mA to 250mA
VIN
Input Voltage Range
VUVLO
Undervoltage Lockout Threshold
VIN Rising
VIN Falling
IS
Input DC Bias Current
Burst Mode® Operation
Shutdown
(Note 4)
VOUT = 1.945V, ILOAD = 0A
VRUN = 0V
fOSC
Oscillator Frequency
VOUT = 1.875V
VOUT = 0V
RPFET
RDS(ON) of P-Channel FET
ISW = 100mA
RNFET
RDS(ON) of N-Channel FET
ISW = –100mA
ILSW
SW Leakage
VRUN = 0V, VSW = 0V or 5V, VIN = 5V
VRUN
RUN Threshold
●
IRUN
RUN Leakage Current
●
MIN
TYP
MAX
3.3
6
380
500
1.837
1.875
1.913
0.04
0.4
●
VIN = 3V, VOUT = 1.64V, Duty Cycle < 35%
●
●
●
2.5
1.8
0.3
µA
mA
0.5
●
UNITS
V
%/V
%
5.5
V
2
1.94
2.3
V
V
26
0.1
35
1
µA
µA
2.25
310
2.7
MHz
kHz
0.75
0.9
Ω
0.55
0.7
Ω
±0.01
±1
µA
1
1.5
V
±0.01
±1
µA
Burst Mode is a registered trademark of Linear Technology Corporation.
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 LTC3410E-1.875 is guaranteed to meet performance
specifications from 0°C to 70°C. Specifications over the –40°C to 85°C
operating temperature range are assured by design, characterization and
correlation with statistical process controls.
Note 3: TJ is calculated from the ambient temperature TA and power
dissipation PD according to the following formula:
LTC3410-1.875: TJ = TA + (PD)(250°C/W)
Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 5: 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.
34101875f
2
LTC3410-1.875
U W
TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure 1)
Efficiency vs Input Voltage
Efficiency vs Output Current
100
100
IOUT = 100mA
90
90
IOUT = 250mA
70
EFFICIENCY (%)
EFFFICIENCY (%)
80
IOUT = 10mA
80
IOUT = 1mA
60
IOUT = 0.1mA
50
70
60
50
40
30
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
20
40
10
30
2.5
4.5
4
3.5
INPUT VOLTAGE (V)
3
5
0
0.1
5.5
1
10
100
OUTPUT CURRENT (mA)
34101875 G02
34101875 G01
Output Voltage
vs Temperature
Oscillator Frequency
vs Temperature
1.911
2.7
VIN = 3.6V
VIN = 3.6V
2.6
OSCILLATOR FREQUENCY (MHz)
1.899
OUTPUT VOLTAGE (V)
1000
1.887
1.875
1.863
1.851
2.5
2.4
2.3
2.2
2.1
2.0
1.9
1.839
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
1.8
–50
125
–25
0
25
50
75
TEMPERATURE (°C)
34101875 G03
125
34101875 G04
Oscillator Frequency
vs Supply Voltage
Output Voltage vs Load Current
2.7
1.900
2.6
1.895
1.890
2.5
OUTPUT VOLTAGE (V)
OSCILLATOR FREQUENCY (MHz)
100
2.4
2.3
2.2
2.1
2.0
1.885
1.880
1.875
1.870
1.865
1.860
1.9
1.855
1.8
1.850
2
3
5
4
SUPPLY VOLTAGE (V)
6
34101875 G05
0
200
100
300
LOAD CURRENT (mA)
400
34101875 G06
34101875f
3
LTC3410-1.875
U W
TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure 1)
RDS(ON) vs Input Voltage
RDS(ON) vs Temperature
1.2
1.2
1.1
VIN = 4.2V
1.0
1.0
VIN = 2.7V
MAIN SWITCH
0.8
0.8
RDS (ON) (Ω)
RDS (ON) (Ω)
0.9
0.7
0.6
0.5
SYNCHRONOUS SWITCH
0.6
VIN = 4.2V
0.4
0.4
VIN = 2.7V
0.3
0.2
0.2
1
5
4
3
INPUT VOLTAGE (V)
2
6
VIN = 3.6V
MAIN SWITCH
SYNCHRONOUS SWITCH
0.1
0
VIN = 3.6V
0
–50 –30 –10 10 30 50 70 90 110 130
TEMPERATURE (°C)
7
34101875 G07
34101875 G08
Dynamic Supply Current
vs Temperature
Dynamic Supply Current vs VIN
50
50
DYNAMIC SUPPLY CURRENT (µA)
DYNAMIC SUPPLY CURRENT (µA)
ILOAD = 0A
40
30
20
10
0
2
1
4
3
5
40
30
20
10
0
–50 –25
6
VIN (V)
50
25
0
75
TEMPERATURE (°C)
34101875 G09
Switch Leakage vs Input Voltage
Switch Leakage vs Temperature
600
VIN = 5.5V
RUN = 0V
550
90
500
80
450
70
LEAKAGE CURRENT (pA)
SWITCH LEAKAGE (nA)
125
34101875 G10
110
100
100
SYNCHRONOUS
SWITCH
60
50
40
30
MAIN
SWITCH
20
350
MAIN
SWITCH
300
250
200
150
SYNCHRONOUS
SWITCH
100
50
10
0
–50
400
0
–25
50
25
0
75
TEMPERATURE (°C)
100
125
34101875 G11
0
1
4
3
2
INPUT VOLTAGE (V)
5
6
34101875 G12
34101875f
4
LTC3410-1.875
U W
TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure 1)
Burst Mode Operation
Start-Up from Shutdown
SW
5V/DIV
RUN
2V/DIV
VOUT
50mV/DIV
AC COUPLED
VOUT
1V/DIV
IL
100mA/DIV
IL
200mA/DIV
VIN = 3.6V
ILOAD = 10mA
2µs/DIV
VIN = 3.6V
ILOAD = 300mA
34101875 G13
Start-Up from Shutdown
200µs/DIV
34101875 G14
Load Step
VOUT
RUN
2V/DIV
VOUT
1V/DIV
100mV/DIV
AC-COUPLED
IL
200mA/DIV
IL
200mA/DIV
ILOAD
200mA/DIV
VIN = 3.6V
ILOAD = 0A
200µs/DIV
10µs/DIV
VIN = 3.6V
ILOAD = 0mA TO 300mA
34101875 G15
34101875 G16
Load Step
VOUT
100mV/DIV
AC-COUPLED
IL
200mA/DIV
ILOAD
200mA/DIV
10µs/DIV
VIN = 3.6V
ILOAD = 15mA TO 300mA
34101875 G17
34101875f
5
LTC3410-1.875
U
U
U
PI FU CTIO S
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 <1µA supply current. Do not leave RUN floating.
GND (Pins 2, 5): Ground Pin.
VIN (Pin 4): Main Supply Pin. Must be closely decoupled
to GND, Pin 2, with a 2.2µF or greater ceramic capacitor.
VOUT (Pin 6): Output Voltage Feedback. An internal resistive divider divides the output voltage down for comparison to the internal 0.8V reference voltage.
SW (Pin 3): Switch Node Connection to Inductor. This pin
connects to the drains of the internal main and synchronous power MOSFET switches.
W
FU CTIO AL DIAGRA
U
U
SLOPE
COMP
0.65V
OSC
OSC
4 VIN
FREQ
SHIFT
–
6
R1
322.5k
+
0.8V
VFB
R2
240k
0.4V
EN
SLEEP
–
+
– EA
S
Q
R
Q
RUN
0.8V REF
SHUTDOWN
5Ω
+
ICOMP
BURST
RS LATCH
VIN
1
+
–
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
ANTISHOOTTHRU
3 SW
+
VOUT
IRCMP
5
2 GND
–
34101875 BD
34101875f
6
LTC3410-1.875
U
OPERATIO (Refer to Functional Diagram)
Main Control Loop
Short-Circuit Protection
The LTC3410-1.875 uses a constant frequency, current
mode step-down architecture. Both the main (P-channel
MOSFET) and synchronous (N-channel MOSFET) switches
are internal. During normal operation, the internal top
power MOSFET is turned on each cycle when the oscillator
sets the RS latch, and turned off when the current comparator, ICOMP, resets the RS latch. The peak inductor
current at which ICOMP resets the RS latch, is controlled by
the output of error amplifier EA. The VOUT pin, described in
the Pin Functions section, allows EA to receive an output
feedback voltage from the internal resistive divider. When
the load current increases, it causes a slight decrease in
the feedback voltage relative to the 0.8V reference, which
in turn, causes the EA amplifier’s output voltage to increase until the average inductor current matches the new
load current. While the top MOSFET is off, the bottom
MOSFET is turned on until either the inductor current
starts to reverse, as indicated by the current reversal
comparator IRCMP, or the beginning of the next clock cycle.
When the output is shorted to ground, the frequency of the
oscillator is reduced to about 310kHz, 1/7 the nominal
frequency. This frequency foldback ensures that the inductor current has more time to decay, thereby preventing
runaway. The oscillator’s frequency will progressively
increase to 2.25MHz when VOUT rises above 0V.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant frequency architectures by preventing subharmonic oscillations at high duty cycles. It is accomplished internally by
adding a compensating ramp to the inductor current
signal at duty cycles in excess of 40%. Normally, this
results in a reduction of maximum inductor peak current
for duty cycles > 40%. However, the LTC3410-1.875 uses
a patented scheme that counteracts this compensating
ramp, which allows the maximum inductor peak current
to remain unaffected throughout all duty cycles.
Burst Mode Operation
The LTC3410-1.875 is capable of Burst Mode operation in
which the internal power MOSFETs operate intermittently
based on load demand.
When the converter is in Burst Mode operation, the peak
current of the inductor is set to approximately 70mA 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 off, reducing the quiescent current to
26µA. In this sleep state, the load current is being supplied
solely from the output capacitor. As the output voltage
droops, the EA amplifier’s output rises above the sleep
threshold signaling the BURST comparator to trip and turn
the top MOSFET on. This process repeats at a rate that is
dependent on the load demand.
34101875f
7
LTC3410-1.875
U
W
U U
APPLICATIO S I FOR ATIO
The basic LTC3410-1.875 application circuit is shown in
Figure 1. External component selection is driven by the
load requirement and begins with the selection of L followed by CIN and COUT.
4.7µH
VIN
2.7V
TO 5.5V
CIN
4.7µF
CER
VIN
VOUT
1.875V
SW
COUT
4.7µF
CER
LTC3410-1.875
RUN
VOUT
GND
34101875 F01
Figure 1. High Efficiency Step-Down Converter
MANUFACTURER PART NUMBER
Taiyo Yuden
For most applications, the value of the inductor will fall in
the range of 2.2µH 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 = 120mA (40% of 300mA).
⎛ V ⎞
1
VOUT ⎜ 1− OUT ⎟
( f)(L) ⎝ VIN ⎠
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 do not 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 LTC3410-1.875 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3410-1.875 applications.
Table 1. Representative Surface Mount Inductors
Inductor Selection
∆IL =
Inductor Core Selection
(1)
MAX DC
VALUE CURRENT DCR HEIGHT
CB2016T2R2M
CB2012T2R2M
LBC2016T3R3M
2.2µH
2.2µH
3.3µH
510mA
530mA
410mA
0.13Ω 1.6mm
0.33Ω 1.25mm
0.27Ω 1.6mm
Panasonic
ELT5KT4R7M
4.7µH
950mA
0.2Ω 1.2mm
Sumida
CDRH2D18/LD
4.7µH
630mA 0.086Ω 2mm
Murata
LQH32CN4R7M23 4.7µH
450mA
Taiyo Yuden
NR30102R2M
NR30104R7M
2.2µH
4.7µH
1100mA 0.1Ω 1mm
750mA 0.19Ω 1mm
FDK
FDKMIPF2520D
FDKMIPF2520D
FDKMIPF2520D
4.7µH
3.3µH
2.2µH
1100mA 0.11Ω 1mm
1200mA 0.1Ω 1mm
1300mA 0.08Ω 1mm
0.2Ω
2mm
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 360mA rated
inductor should be enough for most applications (300mA
+ 60mA). 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.
34101875f
8
LTC3410-1.875
U
W
U U
APPLICATIO S I FOR ATIO
CIN and COUT Selection
Using Ceramic Input and Output Capacitors
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:
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 LTC34101.875’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.
CIN required IRMS ≅ IOMAX
[VOUT (VIN − VOUT )]1/ 2
VIN
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.
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.
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:
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.
⎛
1 ⎞
∆VOUT ≅ ∆IL ⎜ ESR +
⎟
⎝
8fC OUT ⎠
The recommended capacitance value to use is 4.7µF for
both the input and output capacitors.
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.
If tantalum capacitors are used, 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.
34101875f
9
LTC3410-1.875
U
W
U U
APPLICATIO S I FOR ATIO
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.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of the
losses in LTC3410-1.875 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. 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.
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:
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.
Other losses including CIN and COUT ESR dissipative
losses and inductor core losses generally account for less
than 2% total additional loss.
1
VIN = 3.6V
POWER LOSS (W)
0.1
0.01
0.001
0.0001
0.00001
0.1
10
100
1
LOAD CURRENT (mA)
1000
34101875 F02
Figure 2. Power Loss vs Load Current
34101875f
10
LTC3410-1.875
U
W
U U
APPLICATIO S I FOR ATIO
Thermal Considerations
In most applications the LTC3410-1.875 does not dissipate much heat due to its high efficiency. But, in applications where the LTC3410-1.875 is running at high ambient
temperature with low supply voltage, 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 prevent the LTC3410-1.875 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
θJAis the thermal resistance from the junction of the die to
the ambient temperature.
The junction temperature, TJ, is given by:
TJ = TA + TR
where TA is the ambient temperature.
As an example, consider the LTC3410-1.875 with an input
voltage of 2.7V, a load current of 300mA and an ambient
temperature of 70°C. From the typical performance
graph of switch resistance, the R DS(ON) of the
P-channel switch at 70°C is approximately 1.05Ω and
the RDS(ON) of the N-channel synchronous switch is approximately 0.75Ω. The series resistance looking into the
SW pin is:
RSW = 1.05Ω (0.69) + 0.75Ω (0.31) = 0.96Ω
For the SC70 package, the θJA is 250°C/ W. Thus, the
junction temperature of the regulator is:
TJ = 70°C + (0.0864)(250) = 91.6°C
which is well 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.
Therefore, power dissipated by the part is:
PD = ILOAD2 • RDS(ON) = 86.4mW
34101875f
11
LTC3410-1.875
U
W
U U
APPLICATIO S I FOR ATIO
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3410-1.875. 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 and the VIN trace should be kept short, direct and
wide.
2. Does the (+) plate of CIN connect to VIN as closely as
possible? This capacitor provides the AC current to the
internal power MOSFETs.
3. Keep the (–) plates of CIN and COUT as close as possible.
Design Example
As a design example, assume the LTC3410-1.875 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 0.3A but most of the time it will be in
standby mode, requiring only 2mA. Efficiency at both low
1
and high load currents is important. With this information we can calculate L using Equation (1),
L=
⎛ V ⎞
1
VOUT ⎜ 1− OUT ⎟
( f)(∆IL ) ⎝ VIN ⎠
Substituting VOUT = 1.875V, VIN = 4.2V, ∆IL = 100mA
and f = 2.25MHz in Equation (3) gives:
L=
1 . 875V
⎛ 1 . 875V ⎞
1−
= 4 .66µH
2 . 25MHz(100mA) ⎜⎝
4 . 2V ⎟⎠
A 4.7µH inductor works well for this application. For best
efficiency choose a 360mA or greater inductor with less
than 0.3Ω series resistance.
CIN will require an RMS current rating of at least 0.125A ≅
ILOAD(MAX)/2 at temperature and COUT will require an ESR
of less than 0.5Ω. In most cases, a ceramic capacitor will
satisfy this requirement.
Figure 5 shows the complete circuit along with its
efficiency curve.
VOUT
RUN
(3)
VIN
VIA TO VIN
LTC3410-1.875
2
–
VOUT
GND
6
PIN 1
L1
COUT
VOUT
+
3
L1
SW
VIN
5
LTC34101.875
4
SW
CIN
VIN
COUT
CIN
34101875 F03
BOLD LINES INDICATE HIGH CURRENT PATHS
Figure 3. LTC3410-1.875 Layout Diagram
34101875 F04
Figure 4. LTC3410-1.875 Suggested Layout
34101875f
12
LTC3410-1.875
U
W
U U
APPLICATIO S I FOR ATIO
VIN
2.7V
TO 4.2V
4
†
CIN
4.7µF
CER
VIN
SW
3
4.7µH*
COUT†
4.7µF
CER
LTC3410-1.875
1
RUN
VOUT
6
VOUT
1.875V
GND
†
TAIYO YUDEN JMK212BJ475
*MURATA LQH32CN4R7M23
2, 5
34101875 F05a
Figure 5a
100
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
0.1
EFFICIENCY, VIN = 2.7V
EFFICIENCY, VIN = 3.6V
EFFICIENCY, VIN = 4.2V
1
10
LOAD (mA)
100
1000
34101875 F05b
Figure 5b
VOUT
100mV/DIV
AC COUPLED
IL
200mA/DIV
ILOAD
200mA/DIV
20µs/DIV
VIN = 3.6V
ILOAD = 100mA TO 300mA
34101875 F05C
Figure 5c
34101875f
13
LTC3410-1.875
U
TYPICAL APPLICATIO
Using Low Profile Components, <1mm Height
VIN
2.7V
TO 4.2V
4
CIN†
4.7µF
VIN
SW
3
4.7µH*
COUT†
4.7µF
CER
LTC3410-1.875
1
RUN
VOUT
6
VOUT
1.875V
GND
2, 5
†
TAIYO YUDEN JMK212BJ475
*FDK MIPF2520D
34101875 TA03
Low Profile Efficiency
100
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
EFFICIENCY (%)
90
80
70
60
50
0.1
1
10
LOAD (mA)
100
1000
34101875 TA04
Load Step
VOUT
100mV/DIV
AC COUPLED
IL
200mA/DIV
ILOAD
200mA/DIV
20µs/DIV
VIN = 3.6V
ILOAD = 100mA TO 300mA
34101875 TA05
34101875f
14
LTC3410-1.875
U
PACKAGE DESCRIPTIO
SC6 Package
6-Lead Plastic SC70
(Reference LTC DWG # 05-08-1638)
0.47
MAX
0.65
REF
1.80 – 2.20
(NOTE 4)
1.00 REF
INDEX AREA
(NOTE 6)
1.80 – 2.40 1.15 – 1.35
(NOTE 4)
2.8 BSC 1.8 REF
PIN 1
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
0.10 – 0.40
0.65 BSC
0.15 – 0.30
6 PLCS (NOTE 3)
0.80 – 1.00
0.00 – 0.10
REF
1.00 MAX
GAUGE PLANE
0.15 BSC
0.26 – 0.46
0.10 – 0.18
(NOTE 3)
SC6 SC70 1205 REV B
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. DETAILS OF THE PIN 1 INDENTIFIER ARE OPTIONAL,
BUT MUST BE LOCATED WITHIN THE INDEX AREA
7. EIAJ PACKAGE REFERENCE IS EIAJ SC-70
8. JEDEC PACKAGE REFERENCE IS MO-203 VARIATION AB
34101875f
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
LTC3410-1.875
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1616
500mA (IOUT), 1.4MHz, High Efficiency Step-Down
DC/DC Converter
90% Efficiency, VIN = 3.6V to 25V, VOUT = 1.25V, IQ = 1.9mA,
ISD = <1µA, ThinSOT Package
LT1676
450mA (IOUT), 100kHz, High Efficiency Step-Down
DC/DC Converter
90% Efficiency, VIN = 7.4V to 60V, VOUT = 1.24V, IQ = 3.2mA,
ISD = 2.5µA, S8 Package
LTC1701/LTC1701B
750mA (IOUT), 1MHz, High Efficiency Step-Down
DC/DC Converter
90% Efficiency, VIN = 2.5V to 5V, VOUT = 1.25V, IQ = 135µA,
ISD = 1µA, ThinSOT Package
LT1776
500mA (IOUT), 200kHz, High Efficiency Step-Down
DC/DC Converter
90% Efficiency, VIN = 7.4V to 40V, VOUT = 1.24V, IQ = 3.2mA,
ISD = 30µA, N8, S8 Packages
LTC1877
600mA (IOUT), 550kHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, VIN = 2.7V to 10V, VOUT = 0.8V, IQ = 10µA,
ISD = <1µA, MS8 Package
LTC1878
600mA (IOUT), 550kHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, VIN = 2.7V to 6V, VOUT = 0.8V, IQ = 10µA,
ISD = <1µA, MS8 Package
LTC1879
1.2A (IOUT), 550kHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, VIN = 2.7V to 10V, VOUT = 0.8V, IQ = 15µA,
ISD = <1µA, TSSOP-16 Package
LTC3403
600mA (IOUT), 1.5MHz, Synchronous Step-Down
DC/DC Converter with Bypass Transistor
96% Efficiency, VIN = 2.5V to 5.5V, VOUT = Dynamically Adjustable,
IQ = 20µA, ISD = <1µA, DFN Package
LTC3404
600mA (IOUT), 1.4MHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, VIN = 2.7V to 6V, VOUT = 0.8V, IQ = 10µA,
ISD = <1µA, MS8 Package
LTC3405/LTC3405A
300mA (IOUT), 1.5MHz, Synchronous Step-Down
DC/DC Converter
96% Efficiency, VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 20µA,
ISD = <1µA, ThinSOT Package
LTC3406/LTC3406B
600mA (IOUT), 1.5MHz, Synchronous Step-Down
DC/DC Converter
96% Efficiency, VIN = 2.5V to 5.5V, VOUT = 0.6V, IQ = 20µA,
ISD = <1µA, ThinSOT Package
LTC3409
600mA (IOUT), 1.5MHz/2.25MHz, Synchronous
Step-Down DC/DC Converter
95% Efficiency, VIN = 1.6V to 5.5V, VOUT = 0.613V, IQ = 65µA,
DD8 Package
LTC3410/LTC3410B
300mA (IOUT), 2.25MHz, Synchronous
Step-Down DC/DC Converter
96% Efficiency, VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 26µA,
SC70 Package
LTC3411
1.25A (IOUT), 4MHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60µA,
ISD = <1µA, MS Package
LTC3412
2.5A (IOUT), 4MHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60µA,
ISD = <1µA, TSSOP-16E Package
LTC3440
600mA (IOUT), 2MHz, Synchronous Buck-Boost
DC/DC Converter
95% Efficiency, VIN = 2.5V to 5.5V, VOUT = 2.5V, IQ = 25µA,
ISD = <1µA, MS Package
34101875f
16
Linear Technology Corporation
LT 0306 • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2005