LTC3406B-2ES5 [Linear]
2.25MHz, 600mA Synchronous Step-Down Regulator in ThinSOT; 2.25MHz的， 600mA同步降压型稳压器采用ThinSOT
| 型号： | LTC3406B-2ES5 |
| 厂家： | 
Linear |
| 描述： | 2.25MHz, 600mA Synchronous Step-Down Regulator in ThinSOT |
| 文件： | 总16页 (文件大小：213K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC3406B-2
2.25MHz, 600mA
Synchronous Step-Down
Regulator in ThinSOTTM
U
FEATURES
DESCRIPTIO
TheLTC®3406B-2isahighefficiencymonolithicsynchro-
nous buck regulator using a constant frequency, current
mode architecture. Supply current with no load is 350µA,
dropping to <1µA in shutdown. The 2.5V to 5.5V input
voltage range makes the LTC3406B-2 ideally suited for
single Li-Ion battery-powered applications. 100% duty
cycle capability provides low dropout operation, extend-
ing battery life in portable systems. PWM pulse skipping
modeoperationprovidesverylowoutputripplevoltagefor
noise sensitive applications.
■
High Efficiency: Up to 96%
■
600mA Output Current at VIN = 3V
■
2.5V to 5.5V Input Voltage Range
■
2.25MHz Constant Frequency Operation
■
No Schottky Diode Required
■
Low Dropout Operation: 100% Duty Cycle
■
Low Quiescent Current: 350µA
■
0.6V Reference Allows Low Output Voltages
■
Shutdown Mode Draws <1µA Supply Current
■
Current Mode Operation for Excellent Line and
Load Transient Response
Theswitchingfrequencyisinternallysetat2.25MHz,allow-
ingtheuseoftinysurfacemountinductorsandcapacitors.
The internal synchronous switch increases efficiency and
eliminates the need for an external Schottky diode. Low
outputvoltagesareeasilysupportedwiththe0.6Vfeedback
referencevoltage.TheLTC3406B-2isavailableinalowpro-
file (1mm) SOT-23 package. Refer to LTC3406 for appli-
cations that require Burst Mode® operation.
■
Overtemperature Protected
■
Low Profile (1mm) SOT-23 Package
U
APPLICATIO S
■
Cellular Telephones
■
Personal Information Appliances
■
Wireless and DSL Modems
■
Digital Still Cameras
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
■
MP3 Players
■
Portable Instruments
Protected by U.S. Patents, including 6580258, 5481178.
U
TYPICAL APPLICATIO
High Efficiency Step-Down Converter
Efficiency vs Load Current
100
V
T
= 1.8V
OUT
A
2.2µH*
V
V
OUT
IN
= 25°C
90
80
70
60
50
40
30
20
10
1.8V
2.7V
V
SW
LTC3406B-2
RUN
IN
600mA
TO 5.5V
10µF
CER
4.7µF
CER
22pF
1M
V
= 3.6V
IN
V
FB
V
= 2.7V
IN
GND
499k
3406B TA01a
V
= 4.2V
1
IN
0.1
1000
10
100
OUTPUT CURRENT (mA)
3406B TA01b
sn3406b2 3406b2fs
1
LTC3406B-2
W W U W
U W
U
ABSOLUTE AXI U RATI GS
(Note 1)
PACKAGE/ORDER I FOR ATIO
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) ............. 800mA
N-Channel Switch Sink Current (DC) ................. 800mA
Peak SW Sink and Source Current ........................ 1.3A
Operating Temperature Range (Note 2) .. –40°C to 85°C
Maximum Junction Temperature (Notes 3, 6) ..... 125°C
Storage Temperature Range ................ –65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
ORDER PART
TOP VIEW
NUMBER
RUN 1
GND 2
SW 3
5 V
4 V
FB
LTC3406B-2ES5
IN
S5 PART MARKING
LTAGH
S5 PACKAGE
5-LEAD PLASTIC TSOT-23
TJMAX = 125°C, θJA = 250°C/ W, θJC = 90°C/ W
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
MIN
TYP
MAX
UNITS
I
Feedback Current
±30
nA
VFB
V
Regulated Feedback Voltage
(Note 4) T = 25°C
0.5880
0.5865
0.5850
0.6
0.6
0.6
0.6120
0.6135
0.6150
V
V
V
FB
A
(Note 4) 0°C ≤ T ≤ 85°C
A
(Note 4) –40°C ≤ T ≤ 85°C
●
●
●
A
∆V
∆V
Reference Voltage Line Regulation
∆Output Overvoltage Lockout
Peak Inductor Current
V
= 2.5V to 5.5V (Note 4)
IN
0.04
50
1
0.4
80
%/V
mV
A
FB
∆V
= V
– V , LTC3406B
20
OVL
OVL
OVL
FB
I
V
= 3V, V = 0.5V or V = 90%,
OUT
0.75
1.25
PK
IN
FB
Duty Cycle < 35%
V
V
Output Voltage Load Regulation
Input Voltage Range
0.5
%
V
LOADREG
IN
●
●
2.5
1.8
5.5
I
Input DC Bias Current
(Note 5)
S
V
V
= 0.5V or V
= 90%
OUT
350
0.1
500
1
µA
µA
FB
Shutdown
= 0V, V = 4.2V
RUN
IN
f
Oscillator Frequency
V
V
= 0.6V or V = 100%
OUT
= 0V or V
2.25
310
2.7
MHz
kHz
OSC
FB
FB
= 0V
OUT
R
R
R
R
of P-Channel FET
of N-Channel FET
I
I
= 100mA
0.4
0.35
±0.01
1
0.5
0.45
±1
Ω
Ω
PFET
NFET
LSW
DS(ON)
SW
SW
= –100mA
= 0V, V = 0V or 5V, V = 5V
DS(ON)
I
SW Leakage
V
µA
V
RUN
SW
IN
V
RUN Threshold
RUN Leakage Current
●
●
0.3
1.5
±1
RUN
I
±0.01
µA
RUN
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 4: The LTC3406B-2ES5 is tested in a proprietary test mode that
connects V to the output of the error amplifier.
FB
Note 2: The LTC3406B-2ES5 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 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 3: T is calculated from the ambient temperature T and power
J
A
dissipation P according to the following formula:
D
LTC3406B-2ES5: T = T + (P )(250°C/W)
J
A
D
sn3406b2 3406b2fs
2
LTC3406B-2
U W
TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure 1a Except for the Resistive Divider Resistor Values)
Efficiency vs Input Voltage
Efficiency vs Output Current
Efficiency vs Output Current
100
95
90
85
80
75
70
65
100
90
80
70
60
50
40
30
20
10
100
90
80
70
60
50
40
30
20
10
V
A
= 1.5V
T
= 25°C
OUT
A
V
T
= 1.2V
OUT
= 25°C
T
= 25°C
A
I
= 100mA
OUT
V
= 3.6V
IN
V
IN
= 3.6V
V
= 2.7V
IN
I
= 600mA
OUT
V
IN
= 2.7V
V
= 4.2V
V
IN
= 4.2V
IN
I
= 10mA
OUT
2
3
4
INPUT VOLTAGE (V)
5
6
0.1
1000
0.1
1000
1
10
100
1
10
100
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
3406B G01
3406B G03
3406B G02
Reference Voltage vs
Temperature
Oscillator Frequency vs
Temperature
Efficiency vs Output Current
0.614
0.609
0.604
0.599
0.594
0.589
0.584
2.55
2.40
2.25
2.10
1.95
100
90
80
70
60
50
40
30
20
10
V
A
= 2.5V
OUT
V
= 3.6V
V
= 3.6V
IN
IN
T
= 25°C
V
= 2.7V
V
= 4.2V
IN
IN
V
= 3.6V
IN
50
TEMPERATURE (°C)
100 125
–50 –25
0
25
75
50
TEMPERATURE (°C)
100 125
–50 –25
0
25
75
0.1
1000
1
10
100
OUTPUT CURRENT (mA)
3406B G05
3406B G06
3406B G04
Oscillator Frequency vs
Supply Voltage
RDS(ON) vs Input Voltage
Output Voltage vs Load Current
2.70
2.55
2.40
2.25
2.10
1.95
1.80
1.844
1.834
1.824
1.814
1.804
1.794
1.784
1.774
0.7
0.6
T
A
= 25°C
V
A
= 3.6V
T
= 25°C
A
IN
= 25°C
T
0.5
0.4
0.3
0.2
0.1
MAIN
SWITCH
SYNCHRONOUS
SWITCH
0
2
3
4
5
6
5
7
0
1
2
3
4
6
0
700
900
800
100 200 300 400 500 600
LOAD CURRENT (mA)
SUPPLY VOLTAGE (V)
INPUT VOLTAGE (V)
3406B G07
3406B G08
3406B G09
sn3406b2 3406b2fs
3
LTC3406B-2
U W
TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure 1a Except for the Resistive Divider Resistor Values)
Dynamic Supply Current vs
Supply Voltage
Dynamic Supply Current vs
Temperature
RDS(ON) vs Temperature
390
370
350
330
310
290
270
250
0.7
0.6
450
430
410
390
370
350
330
310
290
270
250
V
V
I
= 3.6V
V
I
A
= 1.8V
= 0A
IN
OUT
OUT
LOAD
= 25°C
V
= 2.7V
IN
= 1.8V
= 0A
T
LOAD
V
= 3.6V
IN
V
IN
= 4.2V
0.5
0.4
0.3
0.2
0.1
MAIN SWITCH
SYNCHRONOUS SWITCH
0
50
0
TEMPERATURE (°C)
100 125
50
TEMPERATURE (°C)
100 125
–50
–50 –25
0
25
75
2
2.5
3
3.5
4
4.5
5
5.5
6
SUPPLY VOLTAGE (V)
3406B G12
3406B G10
3406B G11
Switch Leakage vs Temperature
Switch Leakage vs Input Voltage
Discontinuous Operation
300
250
200
150
120
100
80
60
40
20
0
RUN = 0V
V
= 5.5V
IN
RUN = 0V
T
= 25°C
A
SW
2V/DIV
SYNCHRONOUS
SWITCH
V
OUT
10mV/DIV
AC COUPLED
MAIN
SWITCH
I
L
100mA/DIV
100
50
0
MAIN SWITCH
SYNCHRONOUS SWITCH
3406B G15
V
V
= 3.6V
1µs/DIV
IN
= 1.8V
OUT
I
= 50mA
LOAD
50
TEMPERATURE (°C)
100 125
0
2
3
4
5
6
–50 –25
0
25
75
1
INPUT VOLTAGE (V)
3406B G13
3406B G14
Start-Up from Shutdown
Load Step
Load Step
RUN
5V/DIV
V
V
OUT
OUT
100mV/DIV
100mV/DIV
AC COUPLED
AC COUPLED
V
OUT
1V/DIV
I
I
AC COUPLED
L
L
500mA/DIV
500mA/DIV
I
L
I
I
LOAD
LOAD
500mA/DIV
500mA/DIV
500mA/DIV
3406B G17
3406B G16
3406B G18
V
V
I
= 3.6V
V
V
I
= 3.6V
V
V
I
= 3.6V
20µs/DIV
40µs/DIV
20µs/DIV
IN
OUT
IN
OUT
IN
OUT
= 1.8V
= 1.8V
= 1.8V
= 0mA TO 600mA
= 600mA (LOAD: 3Ω RESISTOR)
= 50mA TO 600mA
LOAD
LOAD
LOAD
sn3406b2 3406b2fs
4
LTC3406B-2
U W
TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure 1a Except for the Resistive Divider Resistor Values)
Load Step
Load Step
V
V
OUT
OUT
100mV/DIV
100mV/DIV
AC COUPLED
AC COUPLED
I
I
L
L
500mA/DIV
500mA/DIV
I
I
LOAD
LOAD
500mA/DIV
500mA/DIV
3406B G19
3406B G20
V
V
I
= 3.6V
V
V
I
= 3.6V
20µs/DIV
20µs/DIV
IN
OUT
IN
OUT
= 1.8V
= 1.8V
= 100mA TO 600mA
= 200mA TO 600mA
LOAD
LOAD
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.
VIN (Pin 4): Main Supply Pin. Must be closely decoupled
to GND, Pin 2, with a 2.2µF or greater ceramic capacitor.
VFB (Pin 5): Feedback Pin. Receives the feedback voltage
from an external resistive divider across the output.
GND (Pin 2): Ground Pin.
SW (Pin 3): Switch Node Connection to Inductor. This pin
connects to the drains of the internal main and synchro-
nous power MOSFET switches.
sn3406b2 3406b2fs
5
LTC3406B-2
U
U
W
FU CTIO AL DIAGRA
SLOPE
COMP
OSC
V
IN
4
OSC
FREQ
5Ω
+
–
–
+
SHIFT
I
COMP
Q
Q
S
R
+
0.6V
V
FB
EA
–
5
SWITCHING
LOGIC
RS LATCH
ANTI-
SHOOT-
THRU
AND
V
IN
BLANKING
CIRCUIT
–
SW
3
OV
OVDET
+
RUN
1
0.65V
0.6V REF
+
–
SHUTDOWN
I
RCMP
2
GND
3406B BD
U
OPERATIO
(Refer to Functional Diagram)
Main Control Loop
The comparator OVDET guards against transient over-
shoots>6.25%byturningthemainswitchoffandkeeping
it off until the fault is removed.
TheLTC3406B-2usesaconstantfrequency,currentmode
step-down architecture. Both the main (P-channel
MOSFET)andsynchronous(N-channelMOSFET)switches
areinternal.Duringnormaloperation,theinternaltoppower
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
whichICOMP resetstheRSlatch,iscontrolledbytheoutput
of error amplifier EA. When the load current increases, it
causes a slight decrease in the feedback voltage, FB, rela-
tive to the 0.6V 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
eithertheinductorcurrentstartstoreverse,asindicatedby
the current reversal comparator IRCMP, or the beginning of
the next clock cycle.
Pulse Skipping Mode Operation
At light loads, the inductor current may reach zero or re-
verse on each pulse. The bottom MOSFET is turned off by
the current reversal comparator, IRCMP, and the switch
voltage will ring. This is discontinuous mode operation,
and is normal behavior for the switching regulator. At very
light loads, the LTC3406B-2 will automatically skip pulses
inpulseskippingmodeoperationtomaintainoutputregu-
lation. Refer to LTC3406 data sheet if Burst Mode opera-
tion is preferred.
Short-Circuit Protection
Whentheoutputisshortedtoground, thefrequencyofthe
oscillator is reduced to about 310kHz, 1/7 the nominal
sn3406b2 3406b2fs
6
LTC3406B-2
U
OPERATIO
(Refer to Functional Diagram)
frequency. This frequency foldback ensures that the in-
ductorcurrenthasmoretimetodecay, therebypreventing
runaway. The oscillator’s frequency will progressively
increase to 2.25MHz when VFB rises above 0V.
Animportantdetailtorememberisthatatlowinputsupply
voltages, the RDS(ON) of the P-channel switch increases
(see Typical Performance Characteristics). Therefore, the
user should calculate the power dissipation when the
LTC3406B-2 is used at 100% duty cycle with low input
voltage (See Thermal Considerations in the Applications
Information section).
Dropout Operation
Astheinputsupplyvoltagedecreasestoavalueapproach-
ing the output voltage, the duty cycle increases toward the
maximumon-time.Furtherreductionofthesupplyvoltage
forcesthemainswitchtoremainonformorethanonecycle
untilitreaches100%dutycycle.Theoutputvoltagewillthen
be determined by the input voltage minus the voltage drop
across the P-channel MOSFET and the inductor.
Low Supply Operation
TheLTC3406B-2willoperatewithinputsupplyvoltagesas
low as 2.5V, but the maximum allowable output current is
reduced at this low voltage. Figure 2 shows the reduction
in the maximum output current as a function of input
voltage for various output voltages.
2.2µH*
V
V
OUT
IN
Slope Compensation and Inductor Peak Current
1.8V
2.7V
V
SW
LTC3406B-2
RUN
IN
†
C
600mA
C
**
TO 5.5V
OUT
IN
22pF
10µF
4.7µF
Slope compensation provides stability in constant fre-
quency architectures by preventing subharmonic oscilla-
tions 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 LTC3406B-2 uses a
patent-pending scheme that counteracts this compensat-
ing ramp, which allows the maximum inductor peak
current to remain unaffected throughout all duty cycles.
CER
CER
1M
V
FB
GND
499k
3406B F01a
*MURATA LQH32CN2R2M33
**TAIYO YUDEN JMK212BJ475MG
†TAIYO YUDEN JMK316BJ106ML
Figure 1a. High Efficiency Step-Down Converter
1200
100
V
T
= 1.8V
OUT
A
= 25°C
90
80
70
60
50
40
30
20
10
1000
V
V
= 1.8V
= 1.5V
OUT
OUT
800
600
400
200
0
V
OUT
= 2.5V
V
= 3.6V
IN
V
IN
= 2.7V
V
IN
= 4.2V
1
2.5
3.5
4.0
4.5
5.0
5.5
3.0
0.1
1000
10
100
SUPPLY VOLTAGE (V)
OUTPUT CURRENT (mA)
3406B F02
3406B F01b
Figure 2. Maximum Output Current vs Input Voltage
Figure 1b. Efficiency vs Load Current
sn3406b2 3406b2fs
7
LTC3406B-2
W U U
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APPLICATIO S I FOR ATIO
The basic LTC3406B-2 application circuit is shown in
Figure 1. External component selection is driven by the
load requirement and begins with the selection of L fol-
Table 1. Representative Surface Mount Inductors
Part
Number
Value
(µH)
DCR
(ΩMAX)
MAX DC
Current (A)
Size
WxLxH (mm3)
lowed by CIN and COUT
.
Sumida
CDRH2D11
1.5
2.2
3.3
0.068
0.098
0.123
0.90
0.78
0.60
3.2 x 3.2 x 1.2
3.2 x 3.2 x 2.0
3.5 x 4.1 x 0.8
2.5 x 3.2 x 2.0
1.8 x 2.5 x 1.8
4.6 x 4.6 x 1.2
Inductor Selection
Sumida
CDRH2D18/LD
2.2
3.3
4.7
0.041
0.054
0.078
0.85
0.75
0.63
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
currentasshowninequation1. Areasonablestartingpoint
for setting ripple current is ∆IL = 240mA (40% of 600mA).
Sumida
CMD4D06
2.2
3.3
4.7
0.116
0.174
0.216
0.95
0.77
0.75
Murata
LQH32C
1.0
2.2
4.7
0.060
0.097
0.150
1.00
0.79
0.65
⎛
⎞
VOUT
V
IN
Taiyo Yuden
LQLBC2518
1.0
1.5
2.2
0.080
0.110
0.130
0.78
0.66
0.60
1
∆IL =
VOUT 1−
⎜
⎟
(1)
⎝
⎠
f L
( )( )
Toko
D412F
2.2
3.3
4.7
0.14
0.20
0.22
1.14
0.90
0.80
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
inductorshouldbeenoughformostapplications(600mA
+ 120mA). For better efficiency, choose a low DC-resis-
tance inductor.
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is com-
monlyusedfordesignbecauseevensignificantdeviations
do not offer much relief. Note that the capacitor
manufacturer’s ripple current ratings are often based on
2000hoursoflife.Thismakesitadvisabletofurtherderate
the capacitor, or choose a capacitor rated at a higher
temperature than required. Always consult the manufac-
turer if there is any question.
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 mate-
rials are small and don’t radiate much energy, but gener-
ally cost more than powdered iron core inductors with
similarelectricalcharacteristics. Thechoiceofwhichstyle
inductor to use often depends more on the price vs size
requirements and any radiated field/EMI requirements
than on what the LTC3406B-2 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3406B-2 applications.
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.Theoutputripple∆VOUT isdeter-
mined by:
CIN and COUT Selection
⎛
1
⎞
Incontinuousmode,thesourcecurrentofthetopMOSFET
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:
∆VOUT ≅ ∆I ESR +
⎜
⎟
L
⎝
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.
1/2
]
VOUT V − VOUT
(
IN
)
[
CIN required IRMS ≅ IOMAX
V
IN
sn3406b2 3406b2fs
8
LTC3406B-2
W U U
APPLICATIO S I FOR ATIO
U
0.6V ≤ V
≤ 5.5V
OUT
Aluminum electrolytic and dry tantalum capacitors are
bothavailableinsurfacemountconfigurations.Inthecase
oftantalum,itiscriticalthatthecapacitorsaresurgetested
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.
R2
V
FB
LTC3406B-2
GND
R1
3406B F03
Figure 3. Setting the LTC3406B-2 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
oftenusefultoanalyzeindividuallossestodeterminewhat
is limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as:
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
LTC3406B-2’scontrolloopdoesnotdependontheoutput
capacitor’s ESR for stable operation, ceramic capacitors
can be used freely to achieve very low output ripple and
small circuit size.
Efficiency = 100% – (L1 + L2 + L3 + ...)
whereL1, L2, etc. aretheindividuallossesasapercentage
of input power.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of the
losses in LTC3406B-2 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 4.
However, care must be taken when ceramic capacitors are
usedattheinputandtheoutput.Whenaceramiccapacitor
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.
1
When choosing the input and output ceramic capacitors,
choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage charac-
teristics of all the ceramics for a given value and size.
V
= 3.6V
IN
0.1
0.01
V
= 2.5V
= 1.8V
OUT
Output Voltage Programming
V
OUT
V
= 1.2V
100
OUT
The output voltage is set by a resistive divider according
to the following formula:
0.001
0.0001
V
= 1.5V
OUT
R2
R1
⎛
⎝
⎞
⎟
⎠
VOUT = 0.6 1+
⎜
(2)
0.1
1
10
1000
LOAD CURRENT (mA)
3406B F04
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 3.
Figure 4. Power Lost vs Load Current
sn3406b2 3406b2fs
9
LTC3406B-2
W U U
U
APPLICATIO S I FOR ATIO
To avoid the LTC3406B-2 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 tempera-
ture rise is given by:
1. The VIN quiescent current is due to two components:
the DC bias current as given in the electrical character-
istics 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/dtisthecurrentoutofVINthatistypicallylargerthan
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.
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.
The junction temperature, TJ, is given by:
TJ = TA + TR
where TA is the ambient temperature.
As an example, consider the LTC3406B-2 in dropout at an
input voltage of 2.7V, a load current of 600mA and an
ambient temperature of 70°C. From the typical perfor-
mance graph of switch resistance, the RDS(ON) of the
P-channel switch at 70°C is approximately 0.52Ω. There-
fore, power dissipated by the part is:
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:
PD = ILOAD2 • RDS(ON) = 187.2mW
For the SOT-23 package, the θJA is 250°C/W. Thus, the
junction temperature of the regulator is:
RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC)
TJ = 70°C + (0.1872)(250) = 116.8°C
The RDS(ON) for both the top and bottom MOSFETs can
beobtainedfromtheTypicalPerformanceCharateristics
curves. Thus, to obtain I2R losses, simply add RSW to
RL and multiply the result by the square of the average
output current.
which is below the maximum junction temperature of
125°C.
Note that at higher supply voltages, the junction tempera-
ture is lower due to reduced switch resistance (RDS(ON)).
Other losses including CIN and COUT ESR dissipative
losses and inductor core losses generally account for less
than 2% total additional loss.
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 moni-
toredforovershootorringingthatwouldindicateastability
problem. For a detailed explanation of switching control
loop theory, see Application Note 76.
Thermal Considerations
In most applications the LTC3406B-2 does not dissipate
much heat due to its high efficiency. But, in applications
where the LTC3406B-2 is running at high ambient tem-
perature with low supply voltage and high duty cycles,
such as in dropout, the heat dissipated may exceed the
maximumjunctiontemperatureofthepart.Ifthejunction
temperature reaches approximately 150°C, both power
switches will be turned off and the SW node will become
high impedance.
sn3406b2 3406b2fs
10
LTC3406B-2
W U U
APPLICATIO S I FOR ATIO
U
LTC3406B-2. These items are also illustrated graphically
in Figures 5 and 6. 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 VFB pin connect directly to the feedback
resistors? The resistive divider R1/R2 must be con-
nected between the (+) plate of COUT and ground.
3. Does the (+) plate of CIN connect to VIN as closely as
possible? This capacitor provides the AC current to the
internal power MOSFETs.
A second, more severe transient is caused by switching in
loads with large (>1µF) supply bypass capacitors. The
dischargedbypasscapacitorsareeffectivelyputinparallel
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.
4. Keep the switching node, SW, away from the sensitive
VFB node.
5. Keepthe(–)platesofCIN andCOUT ascloseaspossible.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
1
5
RUN
V
FB
LTC3406B-2
GND
R2
R1
2
3
–
+
C
V
OUT
L1
OUT
C
FWD
4
SW
V
IN
C
IN
+
V
IN
–
3406B F05
BOLD LINES INDICATE HIGH CURRENT PATHS
Figure 5. LTC3406B-2 Layout Diagram
VIA TO GND
R1
R2
V
IN
VIA TO V
IN
VIA TO V
OUT
PIN 1
C
FWD
LTC3406B-2
V
OUT
SW
L1
C
OUT
C
IN
GND
3406B F06
Figure 6. LTC3406B-2 Suggested Layout
sn3406b2 3406b2fs
11
LTC3406B-2
W U U
U
APPLICATIO S I FOR ATIO
Design Example
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.
As a design example, assume the LTC3406B-2 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
standbymode, requiringonly2mA. Efficiencyatbothlow
and high load currents is important. Output voltage is
2.5V. With this information we can calculate L using
equation (1),
For the feedback resistors, choose R1 = 316k. R2 can
then be calculated from equation (2) to be:
⎛
⎞
VOUT
0.6
⎛
⎞
1
VOUT
V
IN
R2 =
− 1 R1= 1000k
⎜
⎟
L =
VOUT 1−
⎝
⎠
⎜
⎟
(3)
⎝
⎠
f ∆IL
( )(
)
Figure 7 shows the complete circuit along with its effi-
ciency curve.
Substituting VOUT = 2.5V, VIN = 4.2V, ∆IL = 240mA and
f = 2.25MHz in equation (3) gives:
⎛
⎞
2.5V
1.5MHz(240mA)
2.5V
4.2V
L =
1−
= 1.87µH
⎜
⎟
⎝
⎠
2.2µH*
V
IN
4
1
3
5
V
OUT
2.7V
V
SW
LTC3406B-2
RUN
IN
2.5V
†
22pF
C
TO 4.2V
IN
C
**
4.7µF
CER
OUT
10µF
CER
V
FB
1M
GND
2
316k
3406B F07a
*MURATA LQH32CN2R2M33
**TAIYO YUDEN JHK316BJ106ML
† TAIYO YUDEN JMK212BJ475MG
Figure 7a
100
V
= 2.5V
OUT
= 25°C
A
T
90
80
70
60
50
40
30
20
10
V
IN
= 2.7V
V
IN
= 4.2V
V
IN
= 3.6V
0.1
1000
1
10
100
OUTPUT CURRENT (mA)
3406B G04
Figure 7b
sn3406b2 3406b2fs
12
LTC3406B-2
U
TYPICAL APPLICATIO S
Single Li-Ion 1.2V/600mA Regulator for
High Efficiency and Small Footprint
Load Step
2.2µH*
V
IN
4
1
3
5
V
OUT
2.7V
V
SW
LTC3406B-2
RUN
IN
1.2V
†
22pF
301k
V
C
TO 4.2V
OUT
IN
100mV/DIV
C
**
4.7µF
CER
OUT
AC COUPLED
10µF
CER
I
V
L
FB
500mA/DIV
GND
2
301k
I
LOAD
3406B TA09
500mA/DIV
*MURATA LQH32CN2R2M33
**TAIYO YUDEN JHK316BJ106ML
† TAIYO YUDEN JMK212BJ475MG
3406B TA11
V
V
LOAD
= 3.6V
20µs/DIV
IN
= 1.2V
OUT
I
= 0mA TO 600mA
Efficiency vs Output Current
Load Step
100
V
T
= 1.2V
OUT
A
= 25°C
90
80
70
60
50
40
30
20
10
V
OUT
100mV/DIV
AC COUPLED
I
L
V
= 3.6V
IN
500mA/DIV
V
= 2.7V
IN
I
LOAD
V
= 4.2V
IN
500mA/DIV
3406B TA12
V
V
LOAD
= 3.6V
20µs/DIV
IN
= 1.2V
OUT
I
= 100mA TO 600mA
0.1
1000
1
10
100
OUTPUT CURRENT (mA)
3406B G02
sn3406b2 3406b2fs
13
LTC3406B-2
U
TYPICAL APPLICATIO S
5V Input to 3.3V/0.6A Regulator
2.2µH*
Load Step
4
3
V
IN
5V
V
OUT
3.3V
V
SW
LTC3406B-2
RUN
IN
†
V
22pF
1M
OUT
C
IN
100mV/DIV
C
**
4.7µF
CER
OUT
AC COUPLED
10µF
CER
1
5
V
I
FB
L
500mA/DIV
GND
2
221k
I
LOAD
3406B TA13
500mA/DIV
*MURATA LQH32CN2R2M33
**TAIYO YUDEN JHK316BJ106ML
† TAIYO YUDEN JMK212BJ475MG
3406B TA15
V
V
LOAD
= 3.6V
20µs/DIV
IN
= 3.3V
OUT
I
= 0mA TO 600mA
Efficiency vs Output Current
Load Step
100
90
80
70
60
50
40
30
20
10
V
V
= 3.3V
OUT
IN
= 5V
V
OUT
100mV/DIV
AC COUPLED
I
L
500mA/DIV
I
LOAD
500mA/DIV
3406B TA16
V
V
LOAD
= 3.6V
20µs/DIV
IN
= 3.3V
OUT
I
= 100mA TO 600mA
0.1
1000
1
10
100
OUTPUT CURRENT (mA)
3406B TA14
sn3406b2 3406b2fs
14
LTC3406B-2
U
PACKAGE DESCRIPTIO
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
1.50 – 1.75
(NOTE 4)
2.80 BSC
1.4 MIN
3.85 MAX 2.62 REF
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
DATUM ‘A’
0.01 – 0.10
1.00 MAX
0.30 – 0.50 REF
1.90 BSC
0.09 – 0.20
(NOTE 3)
NOTE:
S5 TSOT-23 0302
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
sn3406b2 3406b2fs
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
15
LTC3406B-2
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
90% Efficiency, V : 3.6V to 25V, V
LT1616
500mA (I ), 1.4MHz, High Efficiency Step-Down
= 1.25V, I = 1.9mA,
Q
OUT
IN
OUT(MIN)
OUT(MIN)
OUT(MIN)
DC/DC Converter
I
< 1µA, ThinSOT Package
SD
LT1676
450mA (I ), 100kHz, High Efficiency Step-Down
90% Efficiency, V : 7.4V to 60V, V
I
= 1.24V, I = 3.2mA,
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IN
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= 2.5µA, S8 Package
SD
LTC1877
600mA (I ), 550kHz, Synchronous Step-Down
95% Efficiency, V : 2.7V to 10V, V
= 0.8V, I = 10µA,
Q
OUT
IN
DC/DC Converter
I
< 1µA, MS8 Package
SD
LTC1878
600mA (I ), 550kHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, V : 2.7V to 6V, V
= 0.8V, I = 10µA,
OUT(MIN) Q
OUT
IN
I
< 1µA, MS8 Package
SD
LTC1879
1.2A (I ), 550kHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, V : 2.7V to 10V, V
= 0.8V, I = 15µA,
OUT
IN
OUT(MIN)
Q
I
< 1µA, TSSOP-16 Package
SD
LTC3403
600mA (I ), 1.5MHz, Synchronous Step-Down
DC/DC Converter with Bypass Transistor
96% Efficiency, V : 2.5V to 5.5V, V = Dynamically
OUT(MIN)
OUT
IN
Adjustable, I = 20µA, I < 1µA, DFN Package
Q SD
LTC3404
600mA (I ), 1.4MHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, V : 2.7V to 6V, V
= 0.8V, I = 10µA,
OUT(MIN) Q
OUT
IN
I
< 1µA, MS8 Package
SD
LTC3405/LTC3405A
LTC3406/LTC3406B
LTC3407
300mA (I ), 1.5MHz, Synchronous Step-Down
DC/DC Converter
96% Efficiency, V : 2.5V to 5.5V, V
= 0.8V, I = 20µA,
Q
OUT
IN
OUT(MIN)
OUT(MIN)
OUT(MIN)
I
< 1µA, ThinSOT Package
SD
600mA (I ), 1.5MHz, Synchronous Step-Down
96% Efficiency, V : 2.5V to 5.5V, V
= 0.6V, I = 20µA,
Q
OUT
IN
DC/DC Converter
I
< 1µA, ThinSOT Package
SD
Dual Output (600mA × 2) 1.5MHz Synchronous
Step-Down DC/DC Converter
95% Efficiency V : 2.5V to 5.5V, V
= 0.6V, I = 40µA,
IN
Q
MS10E Package
LTC3408
600mA (I ), 1.5MHz Synchronous Step-Down
DC/DC Converter with 0.08Ω Bypass Transistor
96% Efficiency, V : 2.5V to 5.5V, V = Dynamically
OUT(MIN)
OUT
IN
Adjustable, I = 1.5mA, I < 1µA, DFN Package
Q SD
LTC3411
1.25A (I ), 4MHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, V : 2.5V to 5.5V, V
= 0.8V, I = 60µA,
Q
OUT
IN
OUT(MIN)
OUT(MIN)
I
< 1µA, MS Package
SD
LTC3412
2.5A (I ), 4MHz, Synchronous Step-Down
95% Efficiency, V : 2.5V to 5.5V, V
= 0.8V, I = 60µA,
Q
OUT
IN
DC/DC Converter
I
< 1µA, TSSOP-16E Package
SD
LTC3414
4A (I ), 4MHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, V : 2.25V to 5.5V, V
TSSOP-20E Package
= 0.8V, I = 64µA,
OUT(MIN) Q
OUT
IN
LTC3440
600mA (I ), 2MHz, Synchronous Buck-Boost
DC/DC Converter
95% Efficiency, V : 2.5V to 5.5V, V : 2.5V to 5.5V, I = 25µA,
OUT
IN
OUT
Q
I
< 1µA, MS Package
SD
LTC3441
1A (I ), 1MHz, Synchronous Buck-Boost
DC/DC Converter
95% Efficiency, V : 2.5V to 5.5V, V : 2.4V to 5.25V, I = 25µA,
DFN Package
OUT
IN
OUT
Q
sn3406b2 3406b2fs
LT/TP 0204 1K • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
16
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©LINEAR TECHNOLOGY CORPORATION 2003
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