LT3685EMSE [Linear]
36V, 2A, 2.4MHz Step-Down Switching Regulator;
| 型号： | LT3685EMSE |
| 厂家： | 
Linear |
| 描述： | 36V, 2A, 2.4MHz Step-Down Switching Regulator |
| 文件： | 总24页 (文件大小：281K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT3685
36V, 2A, 2.4MHz Step-Down
Switching Regulator
FEATURES
DESCRIPTION
TheLT®3685isanadjustablefrequency(200kHzto2.4MHz)
monolithic step-down switching regulator that accepts
input voltages up to 38V operating and 60V maximum. An
internal overvoltage protection circuit turns off the power
■
Wide Input Range:
Operation From 3.6V to 36V
Overvoltage Lockout Protects Circuit through
60V Transients
■
2A Maximum Output Current
switchwhenV isabove38Vtypical(36Vminimum)which
IN
■
Adjustable Switching Frequency: 200kHz to 2.4MHz
Low Shutdown Current: I < 1μA
Integrated Boost Diode
Synchronizable Between 250kHz to 2MHz
Power Good Flag
Saturating Switch Design: 0.25 On-Resistance
0.790V Feedback Reference Voltage
Output Voltage: 0.79V to 20V
then allows the part to safely withstand 60V transients. A
high efficiency 0.25 switch is included on the die along
with a boost Schottky diode and the necessary oscillator,
control, and logic circuitry. Current mode topology is
used for fast transient response and good loop stability.
The LT3685’s high operating frequency allows the use of
small, low cost inductors and ceramic capacitors result-
ing in low output ripple while keeping total solution size
to a minimum. The low current shutdown mode reduces
input supply current to less than 1μA while a resistor and
capacitor on the RUN/SS pin provide a controlled output
voltage ramp (soft-start). A power good flag signals when
■
Q
■
■
■
■
■
■
■
■
Soft-Start Capability
Small 10-Pin Thermally Enhanced MSOP and
(3mm × 3mm) DFN Packages
V
reaches89%oftheprogrammedoutputvoltage.The
OUT
APPLICATIONS
LT3685 is available in 10-Pin MSOP and 3mm × 3mm DFN
■
Automotive Battery Regulation
packages with exposed pads for low thermal resistance.
■
Set Top Box
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
■
Distributed Supply Regulation
■
Industrial Supplies
■
Wall Transformer Regulation
TYPICAL APPLICATION
3.3V Step-Down Converter
Efficiency
100
90
80
70
60
50
V
V
3.3V
2A
IN
OUT
4.5V TO 36V
TRANSIENT
TO 60V
V
= 5V
OUT
V
BD
IN
RUN/SS
BOOST
OFF ON
14k
V
= 3.3V
OUT
0.47μF
4.7μH
V
C
SW
LT3685
GND
4.7μF
R
T
470pF
PG
316k
V
= 12V
IN
40.2k
SYNC
FB
L = 5.6μH
F = 800 kHz
100k
22μF
0
0.5
1.0
LOAD CURRENT (A)
1.5
2
3685 TA01b
3685 TA01
3685fb
1
LT3685
(Note 1)
ABSOLUTE MAXIMUM RATINGS
Operating Junction Temperature Range (Note 2)
LT3685E............................................. –40°C to 125°C
LT3685I.............................................. –40°C to 125°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
V , RUN/SS Voltage (Note 5)...................................60V
IN
BOOST Pin Voltage ...................................................56V
BOOST Pin Above SW Pin.........................................30V
FB, RT, V Voltage .......................................................5V
C
PG, BD, SYNC Voltage ..............................................30V
(MSE Only) ....................................................... 300°C
PIN CONFIGURATION
TOP VIEW
TOP VIEW
BD
BOOST
SW
1
2
3
4
5
10
9
R
T
BD
BOOST
SW
1
2
3
4
5
10
9
R
T
V
C
V
C
11
8
FB
11
8
FB
V
7
6
PG
IN
V
IN
7
PG
RUN/SS
SYNC
RUN/SS
6
SYNC
MSE PACKAGE
10-LEAD PLASTIC MSOP
DD PACKAGE
= 45°C/W,
= 10°C/W
JC
JA
10-LEAD (3mm s 3mm) PLASTIC DFN
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
= 45°C/W,
= 10°C/W
JC
JA
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3685EDD#PBF
LT3685IDD#PBF
LT3685EMSE#PBF
LT3685IMSE#PBF
LT3685EDD#TRPBF
LT3685IDD#TRPBF
LT3685EMSE#TRPBF
LT3685IMSE#TRPBF
LCYG
LCYG
LTCYF
LTCYF
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
–40°C to 125°C
10-Lead (3mm × 3mm) Plastic DFN
10-Lead (3mm × 3mm) Plastic DFN
10-Lead Plastic MSOP
10-Lead Plastic MSOP
Consult LTC Marketing for parts specified with wider operating temperature ranges.
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 = 10V, VRUN/SS = 10V, VBOOST = 15V, VBD = 3.3V unless otherwise
noted. (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
3
MAX
3.6
UNITS
●
●
Minimum Input Voltage
V
V
V
IN
Overvoltage Lockout
36
38
40
Quiescent Current from V
V
V
V
= 0.2V
0.01
450
1.3
0.5
600
1.7
μA
μA
mA
IN
RUN/SS
= 3V, Not Switching
= 0, Not Switching
●
BD
BD
3685fb
2
LT3685
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VRUN/SS = 10V VBOOST = 15V, VBD = 3.3V unless otherwise
noted. (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Quiescent Current from BD
V
V
V
= 0.2V
0.01
0.9
1
0.5
1.3
5
μA
mA
μA
RUN/SS
= 3V, Not Switching
= 0, Not Switching
●
BD
BD
Minimum Bias Voltage (BD Pin)
Feedback Voltage
2.7
3
V
780
775
790
790
800
805
mV
mV
●
●
FB Pin Bias Current (Note 3)
FB Voltage Line Regulation
V = 1.2V
7
0.002
500
1000
45
30
nA
%/V
C
4V < V < 36V
0.01
IN
Error Amp g
μMho
m
Error Amp Gain
V Source Current
μA
μA
A/V
V
C
V Sink Current
C
45
V Pin to Switch Current Gain
C
3.5
2
V Clamp Voltage
C
Switching Frequency
R = 8.66k
2.1
0.9
160
2.4
1
200
2.7
1.15
240
MHz
MHz
kHz
T
R = 29.4k
T
R = 187k
T
●
●
Minimum Switch Off-Time
Switch Current Limit
60
3.7
500
0.02
1.5
22
150
4.2
nS
A
Duty Cycle = 5%
3.2
Switch V
I
= 2A
SW
mV
μA
V
CESAT
Boost Schottky Reverse Leakage
Minimum Boost Voltage (Note 4)
BOOST Pin Current
V
SW
= 10V, V = 0V
2
BD
2.1
35
10
2.5
I
= 1A
mA
μA
V
SW
RUN/SS Pin Current
V
= 2.5V
5
RUN/SS
RUN/SS Input Voltage High
RUN/SS Input Voltage Low
PG Threshold Offset from Feedback Voltage
PG Hysteresis
0.2
V
V
FB
Rising
90
12
mV
mV
μA
μA
V
PG Leakage
V
V
= 5V
0.1
600
1
PG
●
PG Sink Current
= 0.4V
100
0.5
PG
SYNC Low Threshold
SYNC High Threshold
SYNC Pin Bias Current
0.7
V
V
SYNC
= 0V
0.1
μA
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 3: Bias current flows out of the FB pin.
Note 4: This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the switch.
Note 5: Absolute Maximum Voltage at V and RUN/SS pins is 60V for
IN
Note 2: The LT3685E is guaranteed to meet performance specifications
from 0°C to 125°C. Specifications over the –40°C to 125°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LT3685I specifications are
guaranteed over the –40°C to 125°C temperature range.
nonrepetitive 1 second transients, and 40V for continuous operation.
3685fb
3
LT3685
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C unless otherwise noted.
Efficiency
Maximum Load Current
Efficiency
100
90
80
70
60
50
90
85
80
75
70
65
60
55
50
4.0
V
= 7V
IN
V
= 12V
V
= 12V
= 24V
IN
IN
3.5
TYPICAL
V
= 34V
IN
3.0
2.5
2.0
V
= 34V
IN
V
IN
V
IN
= 24V
MINIMUM
V
= 3.3V
1.5
1.0
OUT
L = 4.7μH
L: NEC PLC-0745-5R6
f: 800kHz
L: NEC PLC-0745-5R6
f: 800kHz
f = 800 kHz
V
= 5V
V
= 3.3V
OUT
OUT
5
10
15
20
25
30
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
LOAD CURRENT (A)
LOAD CURRENT (A)
INPUT VOLTAGE (V)
3685 G01
3685 G02
3685 G03
Switch Current Limit
Switch Current Limit
Maximum Load Current
4.0
3.5
3.0
3.5
3.0
2.5
2.0
1.5
1.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
TYPICAL
DUTY CYCLE = 10 %
DUTY CYCLE = 90 %
2.5
2.0
MINIMUM
V
= 5V
1.5
1.0
OUT
L = 4.7μH
f = 800kHz
20
60
80
100
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3685 G06
10
20
15
INPUT VOLTAGE (V)
25
30
0
40
5
DUTY CYCLE (%)
3685 G05
3685 G04
Boost Pin Current
Switch Voltage Drop
80
70
60
50
40
30
20
10
0
700
600
500
400
300
200
100
0
0
500
1000
1500
2000
2500
0
500
1000
1500
2000
2500
SWITCH CURRENT (mA)
SWITCH CURRENT (mA)
3685 G08
3685 G07
3685fb
4
LT3685
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C unless otherwise noted.
Switching Frequency
Frequency Foldback
Feedback Voltage
1200
840
820
800
780
760
1.20
1.15
1.10
1.05
1.00
0.95
0.90
0.85
0.80
R
= 29.4k
R = 29.4k
T
T
1000
800
600
400
200
0
700 800 900
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3685 G09
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3685 G10
0
100 200 300 400 500 600
FB PIN VOLTAGE (mV)
3685 G11
Soft-Start
RUN/SS Pin Current
Minimum Switch On-Time
4.0
12
10
8
140
120
3.5
3.0
100
2.5
2.0
1.5
1.0
0.5
80
60
40
20
6
4
2
0
0
0
0.5
1
2
2.5
3
3.5
20
30
35
0
1.5
0
15
25
5
10
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (˚C)
3685 G12
RUN/SS PIN VOLTAGE (V)
RUN/SS PIN VOLTAGE (V)
3685 G13
3685 G14
Error Amp Output Current
Boost Diode
1.4
50
40
1.2
1.0
0.8
0.6
0.4
0.2
0
30
20
10
0
–10
–20
–30
–40
–50
0
0.5
1.0
1.5
2.0
–200
–100
0
100
200
BOOST DIODE CURRENT (A)
FB PIN ERROR VOLTAGE (V)
3685 G15
3685 G16
3685fb
5
LT3685
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C unless otherwise noted.
VC Voltages
Minimum Input Voltage
Minimum Input Voltage
2.50
5.0
4.5
4.0
3.5
3.0
2.5
2.0
6.5
6.0
5.5
5.0
2.00
CURRENT LIMIT CLAMP
1.50
1.00
0.50
0
SWITCHING THRESHOLD
4.5
4.0
V
= 5V
V
= 3.3V
OUT
OUT
L = 4.7μH
f = 800kHz
L = 4.7μH
f = 800kHz
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1
10
100
1000
10000
1
10
100
1000
10000
LOAD CURRENT (A)
LOAD CURRENT (A)
3685 G19
3685 G17
3685 G18
Switching Waveforms;
Discontinuous Operation
Switching Waveforms;
Continuous Operation
Power Good Threshold
95
90
85
80
75
V
SW
5V/DIV
V
SW
5V/DIV
I
L
I
L
1A/DIV
0.5A/DIV
V
V
OUT
OUT
10mV/DIV
10mV/DIV
3685 G22
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3685 G20
3685 G21
1μs/DIV
1μs/DIV
V
I
= 12V; FRONT PAGE APPLICATION
= 1A
V
I
= 12V; FRONT PAGE APPLICATION
= 110mA
IN
LOAD
IN
LOAD
3685fb
6
LT3685
PIN FUNCTIONS
BD (Pin 1): This pin connects to the anode of the boost
Schottky diode. BD also supplies current to the internal
regulator.
SYNC (Pin 6): This is the external clock synchronization
input. GroundthispinwhenSYNCfunctionisnotused. Tie
to a clock source for synchronization. Clock edges should
have rise and fall times faster than 1μs. See synchronizing
section in Applications Information.
BOOST (Pin 2): This pin is used to provide a drive
voltage,higherthantheinputvoltage,totheinternalbipolar
NPN power switch.
PG (Pin 7): The PG pin is the open collector output of an
internal comparator. PG remains low until the FB pin is
within 11% of the final regulation voltage. PG output is
SW (Pin 3): The SW pin is the output of the internal power
switch. Connect this pin to the inductor, catch diode and
boost capacitor.
valid when V is above 3.6V and RUN/SS is high.
IN
FB (Pin 8): The LT3685 regulates the FB pin to 0.790V.
Connect the feedback resistor divider tap to this pin.
V (Pin 4): The V pin supplies current to the LT3685’s
IN
IN
internal regulator and to the internal power switch. This
pin must be locally bypassed.
V (Pin 9): The V pin is the output of the internal error
C
C
amplifier. The voltage on this pin controls the peak switch
current. Tie an RC network from this pin to ground to
compensate the control loop.
RUN/SS (Pin 5): The RUN/SS pin is used to put the
LT3685 in shutdown mode. Tie to ground to shut down
the LT3685. Tie to 2.5V or more for normal operation. If
the shutdown feature is not used, tie this pin to the V
R (Pin10):OscillatorResistorInput.Connectingaresistor
IN
T
pin. RUN/SS also provides a soft-start function; see the
Applications Information section.
to ground from this pin sets the switching frequency.
Exposed Pad (Pin 11): Ground. The Exposed Pad must
be soldered to PCB.
BLOCK DIAGRAM
V
IN
V
4
IN
C1
–
+
INTERNAL 0.79V REF
BD
1
RUN/SS
5
SLOPE COMP
S
SWITCH
BOOST
2
3
LATCH
C3
R
R
T
OSCILLATOR
200kHz–2.4MHz
Q
10
6
S
L1
SW
V
R
OUT
T
SYNC
PG
C2
D1
SOFT-START
7
V
CLAMP
ERROR AMP
C
+ 0.7V
–
+
–
V
C
9
C
C
C
F
R
C
GND
11
FB
8
R2
R1
3685 BD
3685fb
7
LT3685
OPERATION
The LT3685 is a constant frequency, current mode step-
down regulator. An oscillator, with frequency set by RT,
enables an RS flip-flop, turning on the internal power
switch. An amplifier and comparator monitor the current
The switch driver operates from either the input or from
the BOOST pin. An external capacitor and diode are used
to generate a voltage at the BOOST pin that is higher than
the input supply. This allows the driver to fully saturate the
internal bipolar NPN power switch for efficient operation.
flowing between the V and SW pins, turning the switch
IN
off when this current reaches a level determined by the
The oscillator reduces the LT3685’s operating frequency
when the voltage at the FB pin is low. This frequency
foldbackhelpstocontroltheoutputcurrentduringstartup
and overload.
voltage at V . An error amplifier measures the output
C
voltage through an external resistor divider tied to the FB
pin and servos the V pin. If the error amplifier’s output
C
increases, more current is delivered to the output; if it
TheLT3685containsapowergoodcomparatorwhichtrips
when the FB pin is at 89% of its regulated value. The PG
output is an open-collector transistor that is off when the
output is in regulation, allowing an external resistor to pull
the PG pin high. Power good is valid when the LT3685 is
decreases,lesscurrentisdelivered.Anactiveclamponthe
V pinprovidescurrentlimit. TheV pinisalsoclampedto
C
C
the voltage on the RUN/SS pin; soft-start is implemented
by generating a voltage ramp at the RUN/SS pin using an
external resistor and capacitor.
enabled and V is above 3.6V.
IN
Aninternalregulatorprovidespowertothecontrolcircuitry.
The bias regulator normally draws power from the V pin,
The LT3685 has an overvoltage protection feature which
IN
disables switching action when the V goes above 38V
but if the BD pin is connected to an external voltage higher
than 3V bias power will be drawn from the external source
(typically the regulated output voltage). This improves
efficiency. The RUN/SS pin is used to place the LT3685
in shutdown, disconnecting the output and reducing the
input current to less than 1μA.
IN
typical (36V minimum). When switching is disabled, the
LT3685 can safely sustain input voltages up to 60V.
3685fb
8
LT3685
APPLICATIONS INFORMATION
FB Resistor Network
where V is the typical input voltage, V
is the output
IN
OUT
voltage, V is the catch diode drop (~0.5V) and V is the
D
SW
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
internal switch drop (~0.5V at max load). This equation
shows that slower switching frequency is necessary to
safely accommodate high V /V
ratio. Also, as shown
IN OUT
VOUT
0.79V
⎛
⎞
⎠
inthenextsection,lowerfrequencyallowsalowerdropout
voltage. The reason input voltage range depends on the
switchingfrequencyisbecausetheLT3685switchhasfinite
minimum on and off times. The switch can turn on for a
minimumof~150nsandturnoffforaminimumof~150ns.
Typical minimum on time at 25°C is 80ns. This means that
the minimum and maximum duty cycles are:
R1=R2
–1
⎜
⎝
⎟
Reference designators refer to the Block Diagram.
Setting the Switching Frequency
The LT3685 uses a constant frequency PWM architecture
thatcanbeprogrammedtoswitchfrom200kHzto2.4MHz
DCMIN = fSW ON(MIN)
t
by using a resistor tied from the R pin to ground. A table
T
DCMAX =1– fSW OFF(MIN)
t
showing the necessary R value for a desired switching
T
frequency is in Figure 1.
where f is the switching frequency, the t
is the
ON(MIN)
SW
minimum switch on time (~150ns), and the t
is
OFF(MIN)
SWITCHING FREQUENCY (MHz)
R VALUE (kΩ)
T
the minimum switch off time (~150ns). These equations
show that duty cycle range increases when switching
frequency is decreased.
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
187
121
88.7
68.1
56.2
46.4
40.2
34
A good choice of switching frequency should allow ad-
equate input voltage range (see next section) and keep
the inductor and capacitor values small.
29.4
23.7
19.1
16.2
13.3
11.5
9.76
8.66
Input Voltage Range
The maximum input voltage for LT3685 applications
depends on switching frequency, the Absolute Maximum
Ratings of the V and BOOST pins, and the operating
IN
mode.
Figure 1. Switching Frequency vs. RT Value
The LT3685 can operate from input voltages up to 38V,
and safely withstand input voltages up 60V. Note that
Operating Frequency Tradeoffs
while V > 38V (typical), the LT3685 will stop switching,
IN
Selection of the operating frequency is a tradeoff between
efficiency,componentsize,minimumdropoutvoltage,and
maximum input voltage. The advantage of high frequency
operationisthatsmallerinductorandcapacitorvaluesmay
be used. The disadvantages are lower efficiency, lower
maximum input voltage, and higher dropout voltage. The
allowing the output to fall out of regulation.
While the output is in start-up, short-circuit, or other
overload conditions, the switching frequency should be
chosen according to the following discussion.
For safe operation at inputs up to 60V the switching fre-
highest acceptable switching frequency (f
) for a
quency must be set low enough to satisfy V
≥ 40V
IN(MAX)
SW(MAX)
IN(MAX)
given application can be calculated as follows:
according to the following equation. If lower V
is
desired, this equation can be used directly.
VD + VOUT
fSW(MAX)
=
tON(MIN) V + V – V
(
)
D
IN
SW
3685fb
9
LT3685
APPLICATIONS INFORMATION
frequency. A reasonable starting point for selecting the
ripple current is:
VOUT + VD
VIN(MAX)
=
– VD + VSW
fSW ON(MIN)
t
ΔI = 0.4(I
)
L
OUT(MAX)
where V
OUT
is the maximum operating input voltage,
IN(MAX)
where I
is the maximum output load current. To
OUT(MAX)
V
is the output voltage, V is the catch diode drop
D
guarantee sufficient output current, peak inductor current
(~0.5V), V is the internal switch drop (~0.5V at max
SW
mustbelowerthantheLT3685’sswitchcurrentlimit(I ).
The peak inductor current is:
LIM
load), f is the switching frequency (set by R ), and
SW
ON(MIN)
T
t
istheminimumswitchontime(~150ns).Notethat
I
= I + ΔI /2
OUT(MAX) L
a higher switching frequency will depress the maximum
operating input voltage. Conversely, a lower switching
frequency will be necessary to achieve safe operation at
high input voltages.
L(PEAK)
where I
is the peak inductor current, I
is
L(PEAK)
OUT(MAX)
the maximum output load current, and ΔI is the inductor
L
ripple current. The LT3685’s switch current limit (I ) is
LIM
at least 3.5A at low duty cycles and decreases linearly to
2.5A at DC = 0.8. The maximum output current is a func-
tion of the inductor ripple current:
If the output is in regulation and no short-circuit, start-
up, or overload events are expected, then input voltage
transients of up to 60V are acceptable regardless of the
switching frequency. In this mode, the LT3685 may enter
pulse skipping operation where some switching pulses
are skipped to maintain output regulation. In this mode
the output voltage ripple and inductor current ripple will
be higher than in normal operation. Above 38V switching
will stop.
I
= I – ΔI /2
LIM L
OUT(MAX)
Be sure to pick an inductor ripple current that provides
sufficient maximum output current (I ).
OUT(MAX)
The largest inductor ripple current occurs at the highest
V . To guarantee that the ripple current stays below the
IN
specified maximum, the inductor value should be chosen
The minimum input voltage is determined by either the
LT3685’s minimum operating voltage of ~3.6V or by its
maximum duty cycle (see equation in previous section).
The minimum input voltage due to duty cycle is:
according to the following equation:
⎛
⎞
⎛
⎞
VOUT + VD
fSWΔIL
VOUT + VD
VIN(MAX)
L =
1–
⎜
⎟
⎜
⎟
⎝
⎠
⎝
⎠
VOUT + VD
VIN(MIN)
=
– VD + VSW
1– fSW OFF(MIN)
t
where V is the voltage drop of the catch diode (~0.4V),
D
V
is the maximum input voltage, V
is the output
IN(MAX)
OUT
whereV
istheminimuminputvoltage,andt
voltage, f is the switching frequency (set by RT), and
IN(MIN)
OFF(MIN)
SW
is the minimum switch off time (150ns). Note that higher
switching frequency will increase the minimum input
voltage. If a lower dropout voltage is desired, a lower
switching frequency should be used.
L is in the inductor value.
Theinductor’sRMScurrentratingmustbegreaterthanthe
maximumloadcurrentanditssaturationcurrentshouldbe
about 30% higher. For robust operation in fault conditions
(start-up or short circuit) and high input voltage (>30V),
the saturation current should be above 3.5A. To keep the
efficiency high, the series resistance (DCR) should be less
than 0.1 , and the core material should be intended for
high frequency applications. Table 1 lists several vendors
and suitable types.
Inductor Selection
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current ΔI increases with higher V or V
L
IN
OUT
anddecreaseswithhigherinductanceandfasterswitching
3685fb
10
LT3685
APPLICATIONS INFORMATION
Table 1. Inductor Vendors
high impedance, or there is significant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a lower performance
electrolytic capacitor.
VENDOR
Murata
TDK
URL
PART SERIES
TYPE
www.murata.com
LQH55D
Open
www.componenttdk.com SLF7045
SLF10145
Shielded
Shielded
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT3685 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 4.7μF capacitor is capable of this task, but only if it is
placed close to the LT3685 and the catch diode (see the
PCB Layout section). A second precaution regarding the
ceramic input capacitor concerns the maximum input
voltage rating of the LT3685. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT3685 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3685’s
voltage rating. This situation is easily avoided (see the Hot
Plugging Safety section).
Toko
www.toko.com
D62CB
D63CB
D75C
Shielded
Shielded
Shielded
Open
D75F
Sumida
www.sumida.com
CR54
Open
CDRH74
CDRH6D38
CR75
Shielded
Shielded
Open
Of course, such a simple design guide will not always re-
sult in the optimum inductor for your application. A larger
value inductor provides a slightly higher maximum load
current and will reduce the output voltage ripple. If your
load is lower than 2A, then you can decrease the value of
the inductor and operate with higher ripple current. This
allows you to use a physically smaller inductor, or one
with a lower DCR resulting in higher efficiency. There are
several graphs in the Typical Performance Characteristics
section of this data sheet that show the maximum load
current as a function of input voltage and inductor value
for several popular output voltages. Low inductance may
result in discontinuous mode operation, which is okay
but further reduces maximum load current. For details of
maximum output current and discontinuous mode opera-
tion, see Linear Technology Application Note 44. Finally,
For space sensitive applications, a 2.2μF ceramic capaci-
tor can be used for local bypassing of the LT3685 input.
However, the lower input capacitance will result in in-
creased input current ripple and input voltage ripple, and
may couple noise into other circuitry. Also, the increased
voltage ripple will raise the minimum operating voltage
of the LT3685 to ~3.7V.
Output Capacitor and Output Ripple
for duty cycles greater than 50% (V /V > 0.5), there
The output capacitor has two essential functions. Along
withtheinductor,itfiltersthesquarewavegeneratedbythe
LT3685toproducetheDCoutput. Inthisroleitdetermines
the output ripple, and low impedance at the switching
frequency is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3685’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
OUT IN
is a minimum inductance required to avoid subharmonic
oscillations. See AN19.
Input Capacitor
BypasstheinputoftheLT3685circuitwithaceramiccapaci-
tor of X7R or X5R type. Y5V types have poor performance
over temperature and applied voltage, and should not be
used. A 4.7μF to 10μF ceramic capacitor is adequate to
bypasstheLT3685andwilleasilyhandletheripplecurrent.
Notethatlargerinputcapacitanceisrequiredwhenalower
switching frequency is used. If the input power source has
100
COUT
=
VOUT SW
f
3685fb
11
LT3685
APPLICATIONS INFORMATION
Table 2. Capacitor Vendors
VENDOR
PHONE
URL
PART SERIES
Ceramic,
Polymer,
Tantalum
Ceramic,
Tantalum
Ceramic,
Polymer,
Tantalum
Ceramic
COMMANDS
Panasonic
(714) 373-7366
www.panasonic.com
EEF Series
Kemet
Sanyo
(864) 963-6300
(408) 749-9714
www.kemet.com
T494, T495
POSCAP
www.sanyovideo.com
Murata
AVX
(408) 436-1300
(864) 963-6300
www.murata.com
www.avxcorp.com
Ceramic,
Tantalum
Ceramic
TPS Series
Taiyo Yuden
www.taiyo-yuden.com
where f is in MHz, and C
is the recommended output
where I
is the output load current. The only reason to
SW
OUT
OUT
capacitance in μF. Use X5R or X7R types. This choice will
provide low output ripple and good transient response.
Transient performance can be improved with a higher value
capacitor if the compensation network is also adjusted
to maintain the loop bandwidth. A lower value of output
capacitor can be used to save space and cost but transient
performance will suffer. See the Frequency Compensation
section to choose an appropriate compensation network.
consideradiodewithalargercurrentratingthannecessary
for nominal operation is for the worst-case condition of
shorted output. The diode current will then increase to the
typical peak switch current. Peak reverse voltage is equal
to the regulator input voltage. Use a Schottky diode with a
reverse voltage rating greater than the input voltage. The
overvoltage protection feature in the LT3685 will keep the
switch off when V > 38V which allows the use of a 40V
IN
rated Schottky even when V ranges up to 60V. Table 3
IN
When choosing a capacitor, look carefully through the
data sheet to find out what the actual capacitance is under
operating conditions (applied voltage and temperature).
A physically larger capacitor, or one with a higher voltage
rating, may be required. High performance tantalum or
electrolytic capacitors can be used for the output capacitor.
Low ESR is important, so choose one that is intended for
useinswitchingregulators. TheESRshouldbespecifiedby
the supplier, and should be 0.05 or less. Such a capacitor
will be larger than a ceramic capacitor and will have a larger
capacitance, becausethecapacitormustbelargetoachieve
low ESR. Table 2 lists several capacitor vendors.
lists several Schottky diodes and their manufacturers.
Table 3. Diode Vendors
V
I
V AT 1A
V AT 2A
R
AVE
F
F
PART NUMBER
(V)
(A)
(mV)
(mV)
On Semicnductor
MBRM120E
MBRM140
20
40
1
1
530
550
595
Diodes Inc.
B220
20
30
40
2
2
2
500
500
500
B230
DFLS240L
International Rectifier
10BQ030
20BQ030
30
30
1
2
420
470
470
Catch Diode
The catch diode conducts current only during switch off
time. Average forward current in normal operation can
be calculated from:
Ceramic Capacitors
A precaution regarding ceramic capacitors concerns the
maximum input voltage rating of the LT3685. A ceramic
input capacitor combined with trace or cable inductance
I
= I (V – V )/V
OUT IN OUT IN
D(AVG)
3685fb
12
LT3685
APPLICATIONS INFORMATION
forms a high quality (under damped) tank circuit. If the
LT3685 circuit is plugged into a live supply, the input volt-
agecanringtotwiceitsnominalvalue, possiblyexceeding
the LT3685’s rating. This situation is easily avoided (see
the Hot Plugging Safely section).
most cases a zero is required and comes from either the
output capacitor ESR or from a resistor R in series with
C
C . This simple model works well as long as the value
C
of the inductor is not too high and the loop crossover
frequency is much lower than the switching frequency.
A phase lead capacitor (C ) across the feedback divider
PL
Frequency Compensation
may improve the transient response. Figure 3 shows the
transient response when the load current is stepped from
500mA to 1500mA and back to 500mA.
The LT3685 uses current mode control to regulate the
output.Thissimplifiesloopcompensation.Inparticular,the
LT3685 does not require the ESR of the output capacitor
for stability, so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size. Frequency
compensation is provided by the components tied to the
V pin, as shown in Figure 2. Generally a capacitor (C )
LT3685
CURRENT MODE
POWER STAGE
m
SW
FB
OUTPUT
ERROR
g
= 3.5mho
AMPLIFIER
C
C
C
R1
PL
and a resistor (R ) in series to ground are used. In addi-
–
C
g
=
m
tion, there may be lower value capacitor in parallel. This
420μmho
ESR
+
0.8V
capacitor (C ) is not part of the loop compensation but
C1
F
+
3M
is used to filter noise at the switching frequency, and is
required only if a phase-lead capacitor is used or if the
output capacitor has high ESR.
C1
POLYMER
OR
TANTALUM
CERAMIC
V
C
GND
Loop compensation determines the stability and transient
performance. Designing the compensation network is a
bit complicated and the best values depend on the ap-
plication and in particular the type of output capacitor. A
practical approach is to start with one of the circuits in
this data sheet that is similar to your application and tune
the compensation network to optimize the performance.
Stability should then be checked across all operating
conditions, including load current, input voltage and
temperature. The LT1375 data sheet contains a more
thorough discussion of loop compensation and describes
how to test the stability using a transient load. Figure 2
shows an equivalent circuit for the LT3685 control loop.
The error amplifier is a transconductance amplifier with
finite output impedance. The power section, consisting of
the modulator, power switch and inductor, is modeled as
a transconductance amplifier generating an output cur-
R
C
R2
C
F
C
C
3685 F02
Figure 2. Model for Loop Response
V
OUT
100mV/DIV
I
L
0.5A/DIV
V
= 12V; FRONT PAGE APPLICATION
10μs/DIV
IN
3685 F03
rent proportional to the voltage at the V pin. Note that
C
the output capacitor integrates this current, and that the
Figure 3. Transient Load Response of the LT3685 Front Page
Application as the Load Current is Stepped from 500mA to
1500mA. VOUT = 3.3V
capacitor on the V pin (C ) integrates the error ampli-
C
C
fier output current, resulting in two poles in the loop. In
3685fb
13
LT3685
APPLICATIONS INFORMATION
BOOST and BIAS Pin Considerations
V
OUT
BD
Capacitor C3 and the internal boost Schottky diode (see
the Block Diagram) are used to generate a boost volt-
age that is higher than the input voltage. In most cases
a 0.22μF capacitor will work well. Figure 2 shows three
ways to arrange the boost circuit. The BOOST pin must be
more than 2.3V above the SW pin for best efficiency. For
outputs of 3V and above, the standard circuit (Figure 4a)
is best. For outputs between 2.8V and 3V, use a 1μF boost
capacitor. A 2.5V output presents a special case because it
is marginally adequate to support the boosted drive stage
whileusingtheinternalboostdiode.ForreliableBOOSTpin
operation with 2.5V outputs use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1μF boost
capacitor (see Figure 4b). For lower output voltages the
boost diode can be tied to the input (Figure 4c), or to
BOOST
V
V
IN
LT3685
GND
IN
C3
SW
4.7μF
(4a) For V
> 2.8V
OUT
V
OUT
D2
BD
BOOST
V
V
IN
LT3685
IN
C3
SW
GND
4.7μF
another supply greater than 2.8V. Tying BD to V reduces
IN
the maximum input voltage to 30V. The circuit in Figure 4a
is more efficient because the BOOST pin current and BD
pin quiescent current comes from a lower voltage source.
You must also be sure that the maximum voltage ratings
of the BOOST and BD pins are not exceeded.
(4b) For 2.5V < V
< 2.8V
OUT
V
OUT
BD
BOOST
V
V
IN
LT3685
IN
C3
The minimum operating voltage of an LT3685 application
is limited by the minimum input voltage (3.6V) and by the
maximum duty cycle as outlined in a previous section. For
proper startup, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3685 is turned on with its RUN/SS pin when the
output is already in regulation, then the boost capacitor
may not be fully charged. Because the boost capacitor is
charged with the energy stored in the inductor, the circuit
will rely on some minimum load current to get the boost
circuit running properly. This minimum load will depend
on input and output voltages, and on the arrangement of
the boost circuit. The minimum load generally goes to
zero once the circuit has started. Figure 5 shows a plot
of minimum load to start and to run as a function of input
SW
GND
4.7μF
3685 FO4
(4c) For V
< 2.5V; V
= 30V
IN(MAX)
OUT
Figure 4. Three Circuits For Generating The Boost Voltage
voltage. In many cases the discharged output capacitor
will present a load to the switcher, which will allow it to
start. The plots show the worst-case situation where V
IN
is ramping very slowly. For lower start-up voltage, the
boost diode can be tied to V ; however, this restricts the
IN
input range to one-half of the absolute maximum rating
of the BOOST pin.
3685fb
14
LT3685
APPLICATIONS INFORMATION
6.0
5.5
5.0
4.5
4.0
3.5
3.0
TO START
(WORST CASE)
I
L
RUN
15k
1A/DIV
RUN/SS
GND
V
RUN/SS
2V/DIV
TO RUN
0.22μF
V
OUT
2V/DIV
V
A
= 3.3V
OUT
T
= 25°C
2.5
2.0
L = 8.2μH
f = 700kHz
3685 F06
2ms/DIV
1
10
100
1000
10000
LOAD CURRENT (A)
Figure 6. To Soft-Start the LT3685, Add a Resisitor
and Capacitor to the RUN/SS Pin
8.0
7.0
6.0
5.0
4.0
3.0
2.0
TO START
(WORST CASE)
By choosing a large RC time constant, the peak start-up
current can be reduced to the current that is required to
regulate the output, with no overshoot. Choose the value
oftheresistorsothatitcansupply20μAwhentheRUN/SS
pin reaches 2.5V.
TO RUN
V
T
= 5V
OUT
A
Synchronization
= 25°C
L = 8.2μH
f = 700kHz
Synchronizing the LT3685 oscillator to an external fre-
quency can be done by connecting a square wave (with
20% to 80% duty cycle) to the SYNC pin. The square
wave amplitude should have valleys that are below 0.3V
and peaks that are above 0.8V (up to 6V).
1
10
100
1000
10000
LOAD CURRENT (A)
3685 F05
Figure 5. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
The LT3685 may be synchronized over a 250kHz to 2MHz
At light loads, the inductor current becomes discontinu-
ous and the effective duty cycle can be very high. This
reduces the minimum input voltage to approximately
range. The R resistor should be chosen to set the LT3685
T
switching frequency 20% below the lowest synchronization
input.Forexample,ifthesynchronizationsignalwillbe250kHz
300mV above V . At higher load currents, the inductor
OUT
and higher, the R should be chosen for 200kHz. To assure
T
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT3685, requiring a higher
input voltage to maintain regulation.
reliable and safe operation the LT3685 will only synchronize
whentheoutputvoltageisnearregulationasindicatedbythe
PG flag. It is therefore necessary to choose a large enough
inductor value to supply the required output current at the
Soft-Start
frequency set by the R resistor. See Inductor Selection sec-
T
The RUN/SS pin can be used to soft-start the LT3685,
reducing the maximum input current during start-up.
The RUN/SS pin is driven through an external RC filter to
create a voltage ramp at this pin. Figure 6 shows the start-
up and shut-down waveforms with the soft-start circuit.
tion. It is also important to note that slope compensation
is set by the R value: When the sync frequency is much
T
higher than the one set by R , the slope compensation will
T
be significantly reduced which may require a larger inductor
value to prevent subharmonic oscillation.
3685fb
15
LT3685
APPLICATIONS INFORMATION
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3685 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT3685 is absent. This may occur in battery charging ap-
plications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT3685’s
L1
C2
V
OUT
C
C
R
RT
output. If the V pin is allowed to float and the RUN/SS
R
C
IN
pin is held high (either by a logic signal or because it is
R2
tied to V ), then the LT3685’s internal circuitry will pull
IN
R1
its quiescent current through its SW pin. This is fine if
your system can tolerate a few mA in this state. If you
ground the RUN/SS pin, the SW pin current will drop to
C1
D1
R
PG
essentially zero. However, if the V pin is grounded while
GND
IN
the output is held high, then parasitic diodes inside the
3685 F08
LT3685 can pull large currents from the output through
VIAS TO V
IN
VIAS TO LOCAL GROUND PLANE
VIAS TO V
VIAS TO RUN/SS
VIAS TO PG
the SW pin and the V pin. Figure 7 shows a circuit that
IN
OUTLINE OF LOCAL
GROUND PLANE
VIAS TO SYNC
OUT
will run only when the input voltage is present and that
protects against a shorted or reversed input.
Figure 8. A Good PCB Layout Ensures Proper, Low EMI Operation
D4
MBRS140
bythesecomponentsshouldbeassmallaspossible.These
components,alongwiththeinductorandoutputcapacitor,
should be placed on the same side of the circuit board,
and their connections should be made on that layer. Place
a local, unbroken ground plane below these components.
The SW and BOOST nodes should be as small as possible.
V
IN
V
BOOST
SW
IN
LT3685
V
RUN/SS
OUT
V
C
GND FB
BACKUP
Finally, keep the FB and V nodes small so that the ground
C
traces will shield them from the SW and BOOST nodes.
The Exposed Pad on the bottom of the package must be
soldered to ground so that the pad acts as a heat sink. To
keep thermal resistance low, extend the ground plane as
much as possible, and add thermal vias under and near
the LT3685 to additional ground planes within the circuit
board and on the bottom side.
3685 F07
Figure 7. Diode D4 Prevents a Shorted Input from
Discharging a Backup Battery Tied to the Output. It Also
Protects the Circuit from a Reversed Input. The LT3685
Runs Only When the Input is Present
PCB Layout
Hot Plugging Safely
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 8 shows
the recommended component placement with trace,
ground plane and via locations. Note that large, switched
currents flow in the LT3685’s V and SW pins, the catch
diode (D1) and the input capacitor (C1). The loop formed
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypasscapacitorofLT3685circuits.However,thesecapaci-
tors can cause problems if the LT3685 is plugged into a
live supply (see Linear Technology Application Note 88 for
IN
3685fb
16
LT3685
APPLICATIONS INFORMATION
a complete discussion). The low loss ceramic capacitor,
combined with stray inductance in series with the power
source, forms an under damped tank circuit, and the
additional vias can reduce thermal resistance further. With
these steps, the thermal resistance from die (or junction)
to ambient can be reduced to
= 35°C/W or less. With
JA
voltage at the V pin of the LT3685 can ring to twice the
100 LFPM airflow, this resistance can fall by another 25%.
Further increases in airflow will lead to lower thermal re-
sistance. Because of the large output current capability of
the LT3685, it is possible to dissipate enough heat to raise
thejunctiontemperaturebeyondtheabsolutemaximumof
125°C. When operating at high ambient temperatures, the
maximum load current should be derated as the ambient
temperature approaches 125°C.
IN
nominal input voltage, possibly exceeding the LT3685’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LT3685 into an
energized supply, the input network should be designed
to prevent this overshoot. Figure 9 shows the waveforms
that result when an LT3685 circuit is connected to a 24V
supply through six feet of 24-gauge twisted pair. The
first plot is the response with a 4.7μF ceramic capacitor
at the input. The input voltage rings as high as 50V and
the input current peaks at 26A. A good solution is shown
in Figure 9b. A 0.7 resistor is added in series with the
input to eliminate the voltage overshoot (it also reduces
the peak input current). A 0.1μF capacitor improves high
frequency filtering. For high input voltages its impact on
efficiency is minor, reducing efficiency by 1.5 percent for
a 5V output at full load operating from 24V.
Power dissipation within the LT3685 can be estimated by
calculatingthetotalpowerlossfromanefficiencymeasure-
ment and subtracting the catch diode loss and inductor
loss. The die temperature is calculated by multiplying the
LT3685 power dissipation by the thermal resistance from
junction to ambient.
Other Linear Technology Publications
Application Notes 19, 35 and 44 contain more detailed de-
scriptions and design information for buck regulators and
otherswitchingregulators.TheLT1376datasheethasamore
extensivediscussionofoutputripple,loopcompensationand
stability testing. Design Note 100 shows how to generate a
bipolar output supply using a buck regulator.
High Temperature Considerations
The PCB must provide heat sinking to keep the LT3685
cool. The Exposed Pad on the bottom of the package must
be soldered to a ground plane. This ground should be tied
to large copper layers below with thermal vias; these lay-
ers will spread the heat dissipated by the LT3685. Place
3685fb
17
LT3685
APPLICATIONS INFORMATION
CLOSING SWITCH
DANGER
SIMULATES HOT PLUG
V
IN
I
IN
20V/DIV
V
IN
RINGING V MAY EXCEED
LT3685
4.7μF
IN
ABSOLUTE MAXIMUM RATING
+
I
IN
LOW
STRAY
10A/DIV
IMPEDANCE
ENERGIZED
24V SUPPLY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
20μs/DIV
(9a)
0.7Ω
V
IN
20V/DIV
LT3685
4.7μF
+
0.1μF
I
IN
10A/DIV
20μs/DIV
(9b)
V
IN
20V/DIV
LT3685
4.7μF
+
+
22μF
35V
AI.EI.
I
IN
10A/DIV
3685 F09
20μs/DIV
(9c)
Figure 9. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation when the LT3685 is Connected to a Live Supply
3685fb
18
LT3685
TYPICAL APPLICATIONS
5V Step-Down Converter
V
V
5V
2A
IN
OUT
6.8V TO 36V
TRANSIENT
TO 60V
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
6.8μH
V
SW
C
LT3685
GND
4.7μF
R
T
16.2k
PG
536k
SYNC
40.2k
FB
470pF
22μF
100k
f = 800kHz
3685 TA02
D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB6R8M
3.3V Step-Down Converter
V
V
3.3V
2A
IN
OUT
4.4V TO 36V
TRANSIENT
TO 60V
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
4.7μH
V
SW
C
LT3685
GND
4.7μF
R
T
14k
PG
316k
SYNC
40.2k
FB
470pF
22μF
100k
f = 800kHz
3685 TA03
D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB4R7M
3685fb
19
LT3685
TYPICAL APPLICATIONS
2.5V Step-Down Converter
V
V
2.5V
2A
IN
OUT
4V TO 36V
TRANSIENT
TO 60V
V
BD
D2
IN
RUN/SS
BOOST
ON OFF
L
1μF
D1
4.7μH
V
SW
C
4.7μF
LT3685
GND
R
T
20k
PG
215k
56.2k
SYNC
FB
330pF
47μF
100k
f = 600kHz
3685 TA04
D1: DIODES INC. DFLS240L
D2: MBR0540
L: TAIYO YUDEN NP06DZB4R7M
5V, 2MHz Step-Down Converter
V
V
5V
2A
IN
OUT
8.6V TO 22V
TRANSIENT TO 36V
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
2.2μH
V
SW
C
LT3685
GND
2.2μF
D
R
T
14k
PG
536k
SYNC
11.5k
FB
470pF
22μF
100k
f = 2MHz
3685 TA05
D: DIODES INC. DFLS240L
L: SUMIDA CDRH4D22/HP-2R2
3685fb
20
LT3685
TYPICAL APPLICATIONS
12V Step-Down Converter
V
V
12V
2A
IN
OUT
15V TO 36V
TRANSIENT
TO 60V*
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
10μH
V
SW
C
LT3685
GND
10μF
R
T
26.1k
PG
715k
SYNC
40.2k
FB
330pF
22μF
50k
f = 800kHz
3685 TA06
D: DIODES INC. DFLS240L
L: NEC/TOKIN PLC-0755-100
*USE SCHOTTKY DIODE RATED AT V >45V.
R
1.8V Step-Down Converter
V
V
1.8V
2A
IN
OUT
3.5V TO 27V
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
3.3μH
V
SW
C
LT3685
GND
4.7μF
R
T
18.2k
PG
127k
SYNC
68.1k
FB
330pF
47μF
100k
f = 500kHz
3685 TA08
D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB3R3M
3685fb
21
LT3685
PACKAGE DESCRIPTION
DD Package
10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1699)
R = 0.115
TYP
6
0.38 p 0.10
10
0.675 p 0.05
3.50 p 0.05
2.15 p 0.05 (2 SIDES)
1.65 p 0.05
3.00 p 0.10 1.65 p 0.10
(4 SIDES)
(2 SIDES)
PIN 1
PACKAGE
OUTLINE
TOP MARK
(SEE NOTE 6)
(DD) DFN 1103
5
1
0.25 p 0.05
0.50 BSC
0.75 p 0.05
0.200 REF
0.25 p 0.05
0.50
BSC
2.38 p 0.10
(2 SIDES)
2.38 p 0.05
(2 SIDES)
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2).
CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
3685fb
22
LT3685
PACKAGE DESCRIPTION
MSE Package
10-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1663)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.06 p 0.102
2.794 p 0.102
(.110 p .004)
0.889 p 0.127
(.035 p .005)
(.081 p .004)
1
1.83 p 0.102
(.072 p .004)
5.23
(.206)
MIN
2.083 p 0.102 3.20 – 3.45
(.082 p .004) (.126 – .136)
10
0.50
(.0197)
BSC
0.305 p 0.038
(.0120 p .0015)
TYP
3.00 p 0.102
(.118 p .004)
(NOTE 3)
0.497 p 0.076
(.0196 p .003)
REF
10 9
8
7 6
RECOMMENDED SOLDER PAD LAYOUT
3.00 p 0.102
(.118 p .004)
(NOTE 4)
4.90 p 0.152
(.193 p .006)
DETAIL “A”
0.254
(.010)
0o – 6o TYP
1
2
3
4 5
GAUGE PLANE
0.53 p 0.152
(.021 p .006)
0.86
(.034)
REF
1.10
(.043)
MAX
DETAIL “A”
0.18
(.007)
SEATING
PLANE
0.17 – 0.27
(.007 – .011)
TYP
0.1016 p 0.0508
(.004 p .002)
0.50
(.0197)
BSC
MSOP (MSE) 0307 REV B
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
3685fb
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 representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
23
LT3685
TYPICAL APPLICATION
1.2V Step-Down Converter
V
V
1.2V
2A
IN
OUT
3.6V TO 27V
V
BD
IN
RUN/SS
BOOST
ON OFF
L
0.47μF
D
3.3μH
V
SW
C
LT3685
GND
4.7μF
R
T
16.2k
PG
52.3k
SYNC
68.1k
FB
330pF
100k
47μF
f = 500kHz
3685 TA09
D: DIODES INC. DFLS240L
L: TAIYO YUDEN NP06DZB3R3M
RELATED PARTS
PART NUMBER DESCRIPTION
COMMENTS
LT1933
LT3437
LT1936
LT3493
500mA (I ), 500kHz Step-Down Switching Regulator in V : 3.6V to 36V, V
= 1.2V, I = 1.6mA, I <1μA, ThinSOT Package
Q SD
OUT
IN
OUT(MIN)
OUT(MIN)
SOT-23
60V, 400mA (I ), MicroPower Step-Down DC/DC
V : 3.3V to 80V, V
= 1.25V, I = 100μA, I <1μA, 10-Pin 3mm x
Q SD
OUT
IN
Converter with Burst Mode
3mm DFN and 16-Pin TSSOP Packages
36V, 1.4A (I ), 500kHz High Efficiency Step-Down
V : 3.6V to 36V, V
= 1.2V, I = 1.9mA, I <1μA, MS8E Package
Q SD
OUT
IN
OUT(MIN)
OUT(MIN)
OUT(MIN)
DC/DC Converter
36V, 1.2A (I ), 750kHz High Efficiency Step-Down
V : 3.6V to 40V, V
= 0.8V, I = 1.9mA, I <1μA, 6-Pin 2mm x 3mm
Q SD
OUT
IN
DC/DC Converter
DFN Package
LT1976/LT1977 60V, 1.2A (I ), 200kHz/500kHz, High Efficiency Step-
V : 3.3V to 60V, V
= 1.2V, I = 100μA, I <1μA, 16-Pin TSSOP
Q SD
OUT
IN
Down DC/DC Converter with Burst Mode
Package
LT1767
LT1940
LT1766
25V, 1.2A (I ), 1.1MHz, High Efficiency Step-Down
V : 3V to 25V, V
= 1.2V, I = 1mA, I <6μA, MS8E Package
OUT
IN
OUT(MIN) Q SD
DC/DC Converter
Dual 25V, 1.4A (I ), 1.1MHz, High Efficiency Step-Down V : 3.6V to 25V, V
= 1.2V, I = 3.8mA, I <30μA, 16-Pin TSSOP
Q SD
OUT
IN
OUT(MIN)
OUT(MIN)
OUT(MIN)
OUT(MIN)
DC/DC Converter
Package
60V, 1.2A (I ), 200kHz, High Efficiency Step-Down
V : 5.5V to 60V, V
= 1.2V, I = 2.5mA, I = 25μA, 16-Pin TSSOP
Q SD
OUT
IN
DC/DC Converter
Package
LT3434/LT3435 60V, 2.4A (I ), 200/500kHz, High Efficiency Step-Down V : 3.3V to 60V, V
= 1.2V, I = 100μA, I <1μA, 16-Pin TSSOP
Q SD
OUT
IN
DC/DC Converter with Burst Mode
Package
LT3480
LT3481
LT3684
38V, 2A (I ), 2.4MHz, High Efficiency Step-Down DC/DC V : 3.6V to 38V, V
= 0.79V, I = 70μA, I <1μA, 10-Pin 3mm x
Q SD
OUT
IN
Converter with Burst Mode
3mm DFN and 10-Pin MSOP Packages
36V, 2A (I ), 2.8MHz, High Efficiency Step-Down DC/DC V : 3.6V to 34V, V
= 1.26V, I = 50μA, I <1μA, 10-Pin 3mm x
OUT
IN
OUT(MIN)
Q
SD
Converter with Burst Mode
3mm DFN and 10-Pin MSOP Packages
36V, 2A (I ), 2.8MHz, High Efficiency Step-Down
V : 3.6V to 34V, V = 1.26V, I = 1.5mA, I <1μA, 10-Pin 3mm x
OUT
IN
OUT(MIN)
Q
SD
DC/DC Converter
3mm DFN and 10-Pin MSOP Packages
3685fb
LT 1208 REV B • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
24
●
●
© LINEAR TECHNOLOGY CORPORATION 2007
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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