Datasheet LT3685 Datasheet (LINEAR TECHNOLOGY)

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Page 1

36V, 2A, 2.4MHz Step-Down

Switching Regulator

FEATURES DESCRIPTION

LT3685

■

Wide Input Range:

Operation From 3.6V to 36V Overvoltage Lockout Protects Circuit through 60V Transients

■

2A Maximum Output Current

■

Adjustable Switching Frequency: 200kHz to 2.4MHz

■

Low Shutdown Current: IQ < 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

■

Soft-Start Capability

■

Small 10-Pin Thermally Enhanced MSOP and

(3mm × 3mm) DFN Packages

APPLICATIONS

■

Automotive Battery Regulation

■

Set Top Box

■

Distributed Supply Regulation

■

Industrial Supplies

■

Wall Transformer Regulation

The LT®3685 is an adjustable frequency (200kHz to 2.4MHz) 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 switch when V

is above 38V typical (36V minimum) which

then allows the part to safely withstand 60V transients. A high efﬁ ciency 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 resulting 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 ﬂ ag signals when

reaches 86% of the programmed output voltage. The

OUT

LT3685 is available in 10-Pin MSOP and 3mm × 3mm DFN packages with exposed pads for low thermal resistance.

, 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.

TYPICAL APPLICATION

3.3V Step-Down Converter

4.5V TO 36V TRANSIENT

TO 60V

OFF ON

14k

4.7μF

470pF

40.2k

RUN/SS BOOST

LT3685

SYNC

GND

Efﬁ ciency

OUT

3.3V 2A

0.47μF

4.7μH

100k

316k

22μF

3685 TA01

100

EFFICIENCY (%)

0.5 1.0 1.5 2 LOAD CURRENT (A)

= 5V

OUT

= 3.3V

OUT

VIN = 12V L = 5.6μH F = 800 kHz

3685 TA01b

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LT3685

ABSOLUTE MAXIMUM RATINGS

VIN, RUN/SS Voltage (Note 5) ...................................60V

BOOST Pin Voltage ...................................................56V

BOOST Pin Above SW Pin .........................................30V

FB, RT, V

Voltage .......................................................5V

PG, BD, SYNC Voltage ..............................................30V

PIN CONFIGURATION

TOP VIEW

BOOST

2 3

RUN/SS

10-LEAD (3mm s 3mm) PLASTIC DFN

EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB

DD PACKAGE

= 45°C/W, JC = 10°C/W

8 7

SYNC

(Note 1)

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)

(MSE Only) ....................................................... 300°C

TOP VIEW

BOOST

SW V

RUN/SS

10-LEAD PLASTIC MSOP

EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB

3 4 5

MSE PACKAGE

= 45°C/W, JC = 10°C/W

SYNC

ORDER INFORMATION

LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE

LT3685EDD#PBF LT3685IDD#PBF LT3685EMSE#PBF LT3685IMSE#PBF

Consult LTC Marketing for parts speciﬁ ed with wider operating temperature ranges. Consult LTC Marketing for information on non-standard lead based ﬁ nish parts.

*For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel speciﬁ cations, go to: http://www.linear.com/tapeandreel/

ELECTRICAL CHARACTERISTICS

The ● denotes the speciﬁ cations which apply over the full operating temperature range, otherwise speciﬁ cations are at TA = 25°C. VIN = 10V, V noted. (Note 2)

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum Input Voltage

Overvoltage Lockout

Quiescent Current from V

LT3685EDD#TRPBF LT3685IDD#TRPBF LT3685EMSE#TRPBF LT3685IMSE#TRPBF

LCYG LCYG LT C Y F LT C Y F

= 0.2V

RUN/SS

= 3V, Not Switching

= 0, Not Switching

10-Lead (3mm × 3mm) Plastic DFN 10-Lead (3mm × 3mm) Plastic DFN 10-Lead Plastic MSOP 10-Lead Plastic MSOP

RUN/SS

= 10V, V

= 15V, VBD = 3.3V unless otherwise

BOOST

●

–40°C to 125°C –40°C to 125°C –40°C to 125°C –40°C to 125°C

3 3.6 V

36 38 40 V

0.01 450

1.3

0.5

600

1.7

μA μA μA

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LT3685

ELECTRICAL CHARACTERISTICS

The ● denotes the speciﬁ cations which apply over the full operating temperature range, otherwise speciﬁ cations are at T noted. (Note 2)

PARAMETER CONDITIONS MIN TYP MAX UNITS

Quiescent Current from BD V

Minimum Bias Voltage (BD Pin) 2.7 3 V

Feedback Voltage

FB Pin Bias Current (Note 3) V

FB Voltage Line Regulation 4V < V

Error Amp g

Error Amp Gain 1000

Source Current 45 μA

Sink Current 45 μA

Pin to Switch Current Gain 3.5 A/V

Clamp Voltage 2V

Switching Frequency R

Minimum Switch Off-Time

Switch Current Limit Duty Cycle = 5% 3.2 3.7 4.2 A

Switch V

Boost Schottky Reverse Leakage V

Minimum Boost Voltage (Note 4)

BOOST Pin Current I

RUN/SS Pin Current V

RUN/SS Input Voltage High 2.5 V

RUN/SS Input Voltage Low 0.2 V

PG Threshold Offset from Feedback Voltage VFB Rising 90 mV

PG Hysteresis 12 mV

PG Leakage V

PG Sink Current V

SYNC Low Threshold 0.5 V

SYNC High Threshold 0.7 V

SYNC Pin Bias Current V

CESAT

= 25°C. VIN = 10V, V

= 0.2V

RUN/SS

= 3V, Not Switching

= 0, Not Switching

= 0.8V, VC = 0.4V

< 36V 0.002 0.01 %/V

RUN/SS

= 10V V

= 15V, VBD = 3.3V unless otherwise

BOOST

0.01

●

0.9

780 775

●

790 790

730 nA

0.5

1.3 5

800 805

μA

mV mV

500 μMho

= 8.66k

= 29.4k

= 187k

2.1

0.9

160

●

2.4 1

200

2.7

1.15 240

MHz MHz

kHz

60 150 nS

ISW = 2A 500 mV

= 10V, VBD = 0V 0.02 2 μA

●

= 1A 22 35 mA

= 2.5V 5 10 μA

RUN/SS

= 5V 0.1 1 μA

= 0.4V

= 0V 0.1 μA

SYNC

●

100 600 μA

1.5 2.1 V

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 LT3685E is guaranteed to meet performance speciﬁ cations from 0°C to 125°C. Speciﬁ cations over the –40°C to 125°C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LT3685I speciﬁ cations are guaranteed over the –40°C to 125°C temperature range.

Note 3: Bias current ﬂ ows 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

nonrepetitive 1 second transients, and 40V for continuous operation.

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LT3685

TYPICAL PERFORMANCE CHARACTERISTICS

Efﬁ ciency

100

EFFICIENCY (%)

VIN = 12V

VIN = 24V

= 5V

OUT

0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 2.01.0 LOAD CURRENT (A)

Maximum Load Current

3.5

TYPICAL

3.0

2.5

MINIMUM

2.0

LOAD CURRENT (A)

1.5

1.0 10 20

INPUT VOLTAGE (V)

VIN = 34V

L: NEC PLC-0745-5R6 f: 800kHz

3685 G01

= 5V

OUT

L = 4.7μH f = 800kHz

25 30

3685 G04

Efﬁ ciency

EFFICIENCY (%)

= 3.3V

OUT

0 0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 2.01.0

VIN = 12V

VIN = 24V

LOAD CURRENT (A)

VIN = 7V

VIN = 34V

L: NEC PLC-0745-5R6 f: 800kHz

Switch Current Limit Switch Current Limit

4.0

3.5

3.0

2.5

2.0

SWITCH CURRENT LIMIT(A)

1.5

1.0 20 60

DUTY CYCLE (%)

= 25°C unless otherwise noted.

Maximum Load Current

4.0

3.5

3.0

2.5

2.0

LOAD CURRENT (A)

1.5

1.0 5

3685 G02

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

SWITCH CURRENT LIMIT (A)

0.5

80 100

3685 G05

–50 25–25 0 50 75 100 150125

TYPICAL

MINIMUM

10 20

INPUT VOLTAGE (V)

DUTY CYCLE = 10 %

DUTY CYCLE = 90 %

TEMPERATURE (°C)

= 3.3V

OUT

L = 4.7μH f = 800 kHz

25 30

3685 G03

3685 G06

Switch Voltage Drop

700

600

500

400

300

VOLTAGE DROP (mV)

200

100

500 1000 2000 2500

SWITCH CURRENT (mA)

1500

3685 G07

Boost Pin Current

BOOST PIN CURRENT (mA)

0 1500500 1000 2000 2500

SWITCH CURRENT (mA)

3685 G08

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LT3685

TYPICAL PERFORMANCE CHARACTERISTICS

Feedback Voltage

840

820

800

FEEDBACK VOLTAGE (mV)

780

760

–50 25–25 0 50 75 100 150125

TEMPERATURE (°C)

Minimum Switch On-Time

140

120

100

MINIMUM SWITCH ON TIME (ns)

3685 G09

Switching Frequency Frequency Foldback

1.20 RT = 29.4k

1.15

1.10

1.05

1.00

0.95

FREQUENCY (MHz)

0.90

0.85

0.80

–50 25–25 0 50 75 100 150125

TEMPERATURE (°C)

Soft-Start

4.0

3.5

3.0

2.5

2.0

1.5

1.0

SWITCH CURRENT LIMIT (A)

0.5

= 25°C unless otherwise noted.

1200

RT = 29.4k

1000

800

600

400

SWITCHING FREQUENCY (kHz)

200

200 400

100 300

FB PIN VOLTAGE (mV)

3685 G10

RUN/SS Pin Current

RUN/SS PIN CURRENT (μA)

500

700 900

600

800

3685 G11

–50 25–25 0 50 75 100 15

TEMPERATURE (˚C)

125

3685 G12

Boost Diode

1.4

1.2

1.0

(V)

0.8

0.6

BOOST DIODE V

0.4

0.2

0.5 1.0 1.5

BOOST DIODE CURRENT (A)

0.5 1 2

2.0

3685 G15

1.5

RUN/SS PIN VOLTAGE (V)

2.5 3 3.5

Error Amp Output Current

–10

PIN CURRENT (μA)

–20

–30

–40

–50

–200

3685 G13

–100 100 FB PIN ERROR VOLTAGE (V)

020

510

15 25

RUN/SS PIN VOLTAGE (V)

3685 G16

20 30 35

3685 G14

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LT3685

TYPICAL PERFORMANCE CHARACTERISTICS

Minimum Input Voltage

5.0

4.5

4.0

3.5

3.0

INPUT VOLTAGE (V)

2.5

= 3.3V

OUT

L = 4.7μH f = 800kHz

2.0 10 100 1000

LOAD CURRENT (A)

Power Good Threshold

1000

3685 G17

Minimum Input Voltage

6.5

6.0

5.5

5.0

INPUT VOLTAGE (V)

4.5 V

= 5V

OUT

L = 4.7μH f = 800kHz

4.0

1 1000

10 100 1000

LOAD CURRENT (A)

Switching Waveforms; Discontinuous Operation

5V/DIV

= 25°C unless otherwise noted.

VC Voltages

2.50

3685 G18

2.00

1.50

VOLTAGE (V)

1.00

0.50

CURRENT LIMIT CLAMP

SWITCHING THRESHOLD

–50 25–25 0 50 75 100 150125

TEMPERATURE (°C)

Switching Waveforms; Continuous Operation

5V/DIV

3685 G19

THRESHOLD VOLTAGE (%)

–50 25–25 0 50 75 100 150125

TEMPERATURE (°C)

3685 G20

0.5A/DIV

OUT

10mV/DIV

1μs/DIV

VIN = 12V; FRONT PAGE APPLICATION I

= 110mA

LOAD

3685 G21

1A/DIV

OUT

10mV/DIV

1μs/DIV

VIN = 12V; FRONT PAGE APPLICATION I

= 1A

LOAD

3685 G22

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PIN FUNCTIONS

LT3685

BD (Pin 1): This pin connects to the anode of the boost Schottky diode. BD also supplies current to the internal regulator.

BOOST (Pin 2): This pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch.

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.

(Pin 4): The VIN pin supplies current to the LT3685’s

internal regulator and to the internal power switch. This pin must be locally bypassed.

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

pin. RUN/SS also provides a soft-start function; see the Applications Information section.

SYNC (Pin 6): This is the external clock synchronization input. Ground this pin when SYNC function is not used. Tie to a clock source for synchronization. Clock edges should have rise and fall times faster than 1μs. See synchronizing section in Applications Information.

PG (Pin 7): The PG pin is the open collector output of an internal comparator. PG remains low until the FB pin is within 14% of the ﬁ nal regulation voltage. PG output is valid when V

is above 3.6V and RUN/SS is high.

FB (Pin 8): The LT3685 regulates the FB pin to 0.790V. Connect the feedback resistor divider tap to this pin.

(Pin 9): The VC pin is the output of the internal error

ampliﬁ er. The voltage on this pin controls the peak switch current. Tie an RC network from this pin to ground to compensate the control loop.

(Pin 10): Oscillator Resistor Input. Connecting a resistor

to ground from this pin sets the switching frequency.

Exposed Pad (Pin 11): Ground. The Exposed Pad must be soldered to PCB.

BLOCK DIAGRAM

INTERNAL 0.79V REF

RUN/SS

SYNC

SOFT-START

GND

11 8

–

BOOST

OUT

3685 BD

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SLOPE COMP

OSCILLATOR

200kHz–2.4MHz

ERROR AMP

0.7V

–

+ –

SWITCH

LATCH

VC CLAMP

Page 8

LT3685

OPERATION

The LT3685 is a constant frequency, current mode stepdown regulator. An oscillator, with frequency set by RT, enables an RS ﬂ ip-ﬂ op, turning on the internal power switch. An ampliﬁ er and comparator monitor the current ﬂ owing between the V off when this current reaches a level determined by the voltage at V voltage through an external resistor divider tied to the FB pin and servos the V increases, more current is delivered to the output; if it decreases, less current is delivered. An active clamp on the

pin provides current limit. The VC pin is also clamped to

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.

An internal regulator provides power to the control circuitry. The bias regulator normally draws power from the V 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 efﬁ ciency. The RUN/SS pin is used to place the LT3685 in shutdown, disconnecting the output and reducing the input current to less than 1μA.

. An error ampliﬁ er measures the output

and SW pins, turning the switch

pin. If the error ampliﬁ er’s output

pin,

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 efﬁ cient operation.

The oscillator reduces the LT3685’s operating frequency when the voltage at the FB pin is low. This frequency foldback helps to control the output current during startup and overload.

The LT3685 contains a power good comparator which trips when the FB pin is at 86% 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 enabled and V

The LT3685 has an overvoltage protection feature which disables switching action when the V typical (36V minimum). When switching is disabled, the LT3685 can safely sustain input voltages up to 60V.

is above 3.6V.

goes above 38V

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APPLICATIONS INFORMATION

LT3685

FB Resistor Network

The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resistors according to:

⎛ ⎜

⎝

079

OUT

⎞

–

⎟

⎠

Reference designators refer to the Block Diagram.

Setting the Switching Frequency

The LT3685 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 2.4MHz by using a resistor tied from the R showing the necessary R

value for a desired switching

pin to ground. A table

frequency is in Figure 1.

SWITCHING FREQUENCY (MHz) RT VALUE (kΩ)

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

Figure 1. Switching Frequency vs. RT Value

187 121

88.7

68.1

56.2

46.4

40.2 34

29.4

23.7

19.1

16.2

13.3

11.5

9.76

8.66

Operating Frequency Tradeoffs

Selection of the operating frequency is a tradeoff between efﬁ ciency, component size, minimum dropout voltage, and maximum input voltage. The advantage of high frequency operation is that smaller inductor and capacitor values may be used. The disadvantages are lower efﬁ ciency, lower maximum input voltage, and higher dropout voltage. The highest acceptable switching frequency (f

SW(MAX)

) for a

given application can be calculated as follows:

D OUT

–

()

SW MAX

()

tVVV

ON MIN D IN SW

where VIN is the typical input voltage, V voltage, V

is the catch diode drop (~0.5V) and VSW is the

is the output

OUT

internal switch drop (~0.5V at max load). This equation shows that slower switching frequency is necessary to safely accommodate high V

IN/VOUT

ratio. Also, as shown in the next section, lower frequency allows a lower dropout voltage. The reason input voltage range depends on the switching frequency is because the LT3685 switch has ﬁ nite minimum on and off times. The switch can turn on for a minimum of ~150ns and turn off for a minimum of ~150ns. Typical minimum on time at 25°C is 80ns. This means that the minimum and maximum duty cycles are:

DC f t

where fSW is the switching frequency, the t minimum switch on time (~150ns), and the t

MIN SW ON MIN

MAX SW OFF MIN

–1

()

ON(MIN)

OFF(MIN)

is the

is the minimum switch off time (~150ns). These equations show that duty cycle range increases when switching frequency is decreased.

A good choice of switching frequency should allow adequate input voltage range (see next section) and keep the inductor and capacitor values small.

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

mode.

The LT3685 can operate from input voltages up to 38V, and safely withstand input voltages up 60V. Note that while V

> 38V (typical), the LT3685 will stop switching,

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 frequency must be set low enough to satisfy V

IN(MAX)

according to the following equation. If lower V

≥ 40V

IN(MAX)

desired, this equation can be used directly.

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LT3685

APPLICATIONS INFORMATION

IN MAX

where V V

OUT

IN(MAX)

is the output voltage, VD is the catch diode drop (~0.5V), V load), f t

ON(MIN)

is the minimum switch on time (~150ns). Note that

OUT D

SW ON MIN

()

–=

DSW()

is the maximum operating input voltage,

is the internal switch drop (~0.5V at max

is the switching frequency (set by RT), and

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.

If the output is in regulation and no short-circuit, startup, 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.

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:

IN MIN

where V

IN(MIN)

OUT D

–

SW OFF MIN

()

–=

DSW()

is the minimum input voltage, and t

OFF(MIN)

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.

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 VIN or V

OUT

and decreases with higher inductance and faster switching

frequency. A reasonable starting point for selecting the ripple current is:

ΔI

where I

= 0.4(I

OUT(MAX)

)

is the maximum output load current. To guarantee sufﬁ cient output current, peak inductor current must be lower than the LT3685’s switch current limit (I

LIM

The peak inductor current is:

L(PEAK)

= I

OUT(MAX)

is the peak inductor current, I

where I the maximum output load current, and ΔI ripple current. The LT3685’s switch current limit (I

+ ΔIL/2

LIM

) is

OUT(MAX)

is the inductor

at least 3.5A at low duty cycles and decreases linearly to

2.5A at DC = 0.8. The maximum output current is a function of the inductor ripple current:

= I

OUT(MAX)

LIM

– ΔIL/2

Be sure to pick an inductor ripple current that provides sufﬁ cient maximum output current (I

OUT(MAX)

The largest inductor ripple current occurs at the highest

. To guarantee that the ripple current stays below the

speciﬁ ed maximum, the inductor value should be chosen according to the following equation:

⎛

⎜ ⎝

OUT D

SW L

⎛

⎞

1–

⎜

⎟ ⎠

⎝

OUT D

IN MAX

⎞

⎟⎟ ⎠

()

where VD is the voltage drop of the catch diode (~0.4V), V

IN(MAX)

voltage, f

is the maximum input voltage, V

is the switching frequency (set by RT), and

is the output

OUT

L is in the inductor value.

The inductor’s RMS current rating must be greater than the maximum load current and its saturation current should be 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 efﬁ ciency 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.

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APPLICATIONS INFORMATION

LT3685

Table 1. Inductor Vendors

VENDOR URL PART SERIES TYPE

Murata www.murata.com LQH55D Open

TDK www.componenttdk.com SLF7045

SLF10145

Toko www.toko.com D62CB

D63CB D75C D75F

Sumida www.sumida.com CR54

CDRH74 CDRH6D38 CR75

Shielded Shielded

Shielded Shielded Shielded Open

Open Shielded Shielded Open

Of course, such a simple design guide will not always result 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 efﬁ ciency. 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 operation, see Linear Technology Application Note 44. Finally, for duty cycles greater than 50% (V

OUT/VIN

> 0.5), there is a minimum inductance required to avoid subharmonic oscillations. See AN19.

Input Capacitor

Bypass the input of the LT3685 circuit with a ceramic capacitor 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 bypass the LT3685 and will easily handle the ripple current. Note that larger input capacitance is required when a lower switching frequency is used. If the input power source has

high impedance, or there is signiﬁ cant inductance due to long wires or cables, additional bulk capacitance may be necessary. This can be provided with a lower performance electrolytic capacitor.

Step-down regulators draw current from the input supply 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).

For space sensitive applications, a 2.2μF ceramic capacitor can be used for local bypassing of the LT3685 input. However, the lower input capacitance will result in increased 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

The output capacitor has two essential functions. Along with the inductor, it ﬁ lters the square wave generated by the LT3685 to produce the DC output. In this role it determines 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

100

OUT SW

3685fa

Page 12

LT3685

APPLICATIONS INFORMATION

Table 2. Capacitor Vendors

VENDOR PHONE URL PART SERIES COMMANDS

Panasonic (714) 373-7366 www.panasonic.com Ceramic,

Polymer, Tantalum

Kemet (864) 963-6300 www.kemet.com Ceramic,

Tantalum T494, T495

Sanyo (408) 749-9714 www.sanyovideo.com Ceramic,

Polymer, Tantalum

Murata (408) 436-1300 www.murata.com Ceramic

AVX www.avxcorp.com Ceramic,

Tantalum TPS Series

Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com Ceramic

EEF Series

POSCAP

where fSW is in MHz, and C

is the recommended output

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.

When choosing a capacitor, look carefully through the data sheet to ﬁ nd 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 use in switching regulators. The ESR should be speciﬁ ed by 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, because the capacitor must be large to achieve low ESR. Table 2 lists several capacitor vendors.

Catch Diode

where I

is the output load current. The only reason to

OUT

consider a diode with a larger current rating than necessary 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 rated Schottky even when V

> 38V which allows the use of a 40V

ranges up to 60V. Table 3

lists several Schottky diodes and their manufacturers.

Table 3. Diode Vendors

PART NUMBER

On Semicnductor MBRM120E MBRM140

Diodes Inc. B220 B230 DFLS240L

International Rectiﬁ er 10BQ030 20BQ030

(V)

20 40

20 30 40

30 30

(A)

AVE

1 1

2 2 2

1 2

AT 1A

(mV)

530 550

420 470

VF AT 2A

(mV)

595

500 500 500

470

The catch diode conducts current only during switch off time. Average forward current in normal operation can be calculated from:

D(AVG)

= I

OUT

(VIN – V

OUT

)/V

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

3685fa

Page 13

APPLICATIONS INFORMATION

LT3685

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 rating. This situation is easily avoided (see the Hot Plugging Safely section).

Frequency Compensation

The LT3685 uses current mode control to regulate the output. This simpliﬁ es loop compensation. In particular, 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

pin, as shown in Figure 2. Generally a capacitor (CC)

and a resistor (R

) in series to ground are used. In addi-

tion, there may be lower value capacitor in parallel. This capacitor (C

) is not part of the loop compensation but

is used to ﬁ lter noise at the switching frequency, and is required only if a phase-lead capacitor is used or if the output capacitor has high ESR.

Loop compensation determines the stability and transient performance. Designing the compensation network is a bit complicated and the best values depend on the application 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 ampliﬁ er is a transconductance ampliﬁ er with ﬁ nite output impedance. The power section, consisting of the modulator, power switch and inductor, is modeled as a transconductance ampliﬁ er generating an output current proportional to the voltage at the V

pin. Note that

the output capacitor integrates this current, and that the capacitor on the V

pin (CC) integrates the error ampli-

ﬁ er output current, resulting in two poles in the loop. In

most cases a zero is required and comes from either the output capacitor ESR or from a resistor R

. This simple model works well as long as the value

in series with

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

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.

LT3685

CURRENT MODE

POWER STAGE

= 3.5mho

ERROR

AMPLIFIER

gm =

420μmho

Figure 2. Model for Loop Response

OUT

100mV/DIV

0.5A/DIV

VIN = 12V; FRONT PAGE APPLICATION

Figure 3. Transient Load Response of the LT3685 Front Page Application as the Load Current is Stepped from 500mA to 1500mA. V

OUT

= 3.3V

–

0.8V

POLYMER

GND

TANTALUM

10μs/DIV

ESR

OUTPUT

CERAMIC

3685 F02

3685 F03

3685fa

Page 14

LT3685

APPLICATIONS INFORMATION

BOOST and BIAS Pin Considerations

Capacitor C3 and the internal boost Schottky diode (see the Block Diagram) are used to generate a boost voltage 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 efﬁ ciency. 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 while using the internal boost diode. For reliable BOOST pin 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 another supply greater than 2.8V. Tying BD to V

reduces

the maximum input voltage to 30V. The circuit in Figure 4a is more efﬁ cient 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.

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

OUT

BOOST

4.7μF

LT3685

GND

(4a) For V

LT3685

GND

(4b) For 2.5V < V

LT3685

GND

(4c) For V

OUT

BOOST

< 2.5V; V

> 2.8V

< 2.8V

OUT

IN(MAX)

3685 FO4

= 30V

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

is ramping very slowly. For lower start-up voltage, the boost diode can be tied to V

; however, this restricts the

input range to one-half of the absolute maximum rating of the BOOST pin.

3685fa

Page 15

APPLICATIONS INFORMATION

6.0

5.5

TO START (WORST CASE)

5.0

4.5

4.0 TO RUN

3.5

INPUT VOLTAGE (V)

3.0

= 3.3V

OUT

= 25°C

2.5

L = 8.2μH f = 700kHz

2.0

8.0

7.0

6.0

5.0

4.0

INPUT VOLTAGE (V)

3.0

2.0

1 1000010 100 1000

10 100 1000

LOAD CURRENT (A)

TO START (WORST CASE)

TO RUN

= 5V

OUT

= 25°C

L = 8.2μH f = 700kHz

LOAD CURRENT (A)

Figure 5. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit

At light loads, the inductor current becomes discontinuous and the effective duty cycle can be very high. This reduces the minimum input voltage to approximately 300mV above V

. At higher load currents, the inductor

OUT

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.

Soft-Start

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 ﬁ lter to create a voltage ramp at this pin. Figure 6 shows the startup and shut-down waveforms with the soft-start circuit.

10000

3685 F05

LT3685

3685 F06

1A/DIV

RUN/SS

2V/DIV

OUT

2V/DIV

RUN

15k

RUN/SS

0.22μF

GND

2ms/DIV

Figure 6. To Soft-Start the LT3685, Add a Resisitor and Capacitor to the RUN/SS Pin

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 of the resistor so that it can supply 20μA when the RUN/SS pin reaches 2.5V.

Synchronization

Synchronizing the LT3685 oscillator to an external frequency 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).

The LT3685 may be synchronized over a 250kHz to 2MHz range. The R

resistor should be chosen to set the LT3685

switching frequency 20% below the lowest synchronization input. For example, if the synchronization signal will be 250kHz and higher, the R

should be chosen for 200kHz. To assure

reliable and safe operation the LT3685 will only synchronize when the output voltage is near regulation as indicated by the PG ﬂ ag. It is therefore necessary to choose a large enough inductor value to supply the required output current at the frequency set by the R

resistor. See Inductor Selection sec-

tion. It is also important to note that slope compensation is set by the R higher than the one set by R

value: When the sync frequency is much

, the slope compensation will

be signiﬁ cantly reduced which may require a larger inductor value to prevent subharmonic oscillation.

3685fa

Page 16

LT3685

APPLICATIONS INFORMATION

Shorted and Reversed Input Protection

If the inductor is chosen so that it won’t saturate excessively, 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 applications or in battery backup systems where a battery or some other supply is diode OR-ed with the LT3685’s output. If the V

pin is allowed to ﬂ oat and the RUN/SS

pin is held high (either by a logic signal or because it is tied to V

), then the LT3685’s internal circuitry will pull

its quiescent current through its SW pin. This is ﬁ ne 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 essentially zero. However, if the V

pin is grounded while

the output is held high, then parasitic diodes inside the LT3685 can pull large currents from the output through the SW pin and the V

pin. Figure 7 shows a circuit that

will run only when the input voltage is present and that protects against a shorted or reversed input.

MBRS140

RUN/SS

LT3685

GND FB

BOOST

3685 F07

OUT

BACKUP

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

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 ﬂ ow in the LT3685’s V

and SW pins, the catch

diode (D1) and the input capacitor (C1). The loop formed

VIAS TO LOCAL GROUND PLANE

VIAS TO V

OUT

VIAS TO SYNC

OUT

GND

VIAS TO RUN/SS

VIAS TO PG

VIAS TO V

OUTLINE OF LOCA

3685 F08

Figure 8. A Good PCB Layout Ensures Proper, Low EMI Operation

by these components should be as small as possible. These components, along with the inductor and output capacitor, 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. Finally, keep the FB and V

nodes small so that the ground

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.

Hot Plugging Safely

The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT3685 circuits. However, these capacitors can cause problems if the LT3685 is plugged into a live supply (see Linear Technology Application Note 88 for

3685fa

Page 17

APPLICATIONS INFORMATION

LT3685

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 voltage at the V 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 ﬁ rst 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 input to eliminate the voltage overshoot (it also reduces the peak input current). A 0.1μF capacitor improves high frequency ﬁ ltering. For high input voltages its impact on efﬁ ciency is minor, reducing efﬁ ciency by 1.5 percent for a 5V output at full load operating from 24V.

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 layers will spread the heat dissipated by the LT3685. Place

pin of the LT3685 can ring to twice the

resistor is added in series with the

additional vias can reduce thermal resistance further. With these steps, the thermal resistance from die (or junction) to ambient can be reduced to 100 LFPM airﬂ ow, this resistance can fall by another 25%. Further increases in airﬂ ow will lead to lower thermal resistance. Because of the large output current capability of the LT3685, it is possible to dissipate enough heat to raise the junction temperature beyond the absolute maximum of 125°C. When operating at high ambient temperatures, the maximum load current should be derated as the ambient temperature approaches 125°C.

Power dissipation within the LT3685 can be estimated by calculating the total power loss from an efﬁ ciency measurement 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 descriptions and design information for buck regulators and other switching regulators. The LT1376 data sheet has a more extensive discussion of output ripple, loop compensation and stability testing. Design Note 100 shows how to generate a bipolar output supply using a buck regulator.

= 35°C/W or less. With

3685fa

Page 18

LT3685

APPLICATIONS INFORMATION

LOW

IMPEDANCE

ENERGIZED

24V SUPPLY

CLOSING SWITCH

SIMULATES HOT PLUG

STRAY INDUCTANCE DUE TO 6 FEET (2 METERS) OF TWISTED PAIR

0.7Ω

LT3685

4.7μF

20V/DIV

10A/DIV

DANGER

20μs/DIV

MAY EXCEED

RINGING V ABSOLUTE MAXIMUM RATING

(9a)

(9b)

20V/DIV

10A/DIV

20μs/DIV

LT3685

4.7μF0.1μF

(9c)

20V/DIV

10A/DIV

20μs/DIV

LT3685

22μF

35V

AI.EI.

4.7μF

Figure 9. A Well Chosen Input Network Prevents Input Voltage Overshoot and Ensures Reliable Operation when the LT3685 is Connected to a Live Supply

3685 F09

3685fa

Page 19

TYPICAL APPLICATIONS

LT3685

5V Step-Down Converter

6.8V TO 36V TRANSIENT

TO 60V

4.7μF

4.4V TO 36V TRANSIENT

TO 60V

4.7μF

ON OFF

16.2k

40.2k

470pF

D: DIODES INC. DFLS240L L: TAIYO YUDEN NP06DZB6R8M

ON OFF

14k

40.2k

470pF

D: DIODES INC. DFLS240L L: TAIYO YUDEN NP06DZB4R7M

RUN/SS BOOST

SYNC

f = 800kHz

LT3685

GND

3.3V Step-Down Converter

RUN/SS BOOST

SYNC

f = 800kHz

LT3685

GND

0.47μF

100k

0.47μF

100k

536k

316k

6.8μH

4.7μH

3685 TA02

3685 TA03

V 5V 2A

22μF

OUT

3.3V 2A

22μF

OUT

3685fa

Page 20

LT3685

TYPICAL APPLICATIONS

2.5V Step-Down Converter

4V TO 36V

TRANSIENT

TO 60V

4.7μF

8.6V TO 22V

TRANSIENT TO 36V

2.2μF

ON OFF

20k

56.2k

330pF

D1: DIODES INC. DFLS240L D2: MBR0540 L: TAIYO YUDEN NP06DZB4R7M

f = 600kHz

5V, 2MHz Step-Down Converter

ON OFF

14k

11.5k

470pF

D: DIODES INC. DFLS240L L: SUMIDA CDRH4D22/HP-2R2

RUN/SS BOOST

SYNC

RUN/SS BOOST

SYNC

f = 2MHz

LT3685

GND

LT3685

GND

OUT

2.5V

3685 TA04

3685 TA05

47μF

OUT

5V 2A

22μF

1μF

4.7μH

100k

0.47μF

215k

2.2μH

536k

3685fa

Page 21

TYPICAL APPLICATIONS

LT3685

12V Step-Down Converter

V 15V TO 36V TRANSIENT

TO 60V*

10μF

3.5V TO 27V

4.7μF

ON OFF

26.1k

40.2k

330pF

D: DIODES INC. DFLS240L L: NEC/TOKIN PLC-0755-100 *USE SCHOTTKY DIODE RATED AT V

ON OFF

18.2k

68.1k

330pF

D: DIODES INC. DFLS240L L: TAIYO YUDEN NP06DZB3R3M

RUN/SS BOOST

SYNC

f = 800kHz

LT3685

GND

>45V.

1.8V Step-Down Converter

RUN/SS BOOST

SYNC

f = 500kHz

LT3685

GND

0.47μF

50k

0.47μF

100k

715k

127k

10μH

3.3μH

3685 TA06

3685 TA08

OUT

12V 2A

22μF

OUT

1.8V 2A

47μF

3685fa

Page 22

LT3685

PACKAGE DESCRIPTION

DD Package

10-Lead Plastic DFN (3mm × 3mm)

(Reference LTC DWG # 05-08-1699)

3.50 p0.05

0.675 p0.05

1.65 p0.05 (2 SIDES)2.15 p0.05

PACKAGE OUTLINE

0.25 p0.05

2.38 p0.05

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

(2 SIDES)

0.50 BSC

PIN 1

TOP MARK

(SEE NOTE 6)

0.200 REF

3.00 p0.10 (4 SIDES)

0.75 p0.05

1.65 p0.10 (2 SIDES)

0.00 – 0.05

R = 0.115

TYP

2.38 p0.10 (2 SIDES)

BOTTOM VIEW—EXPOSED PAD

106

0.25 p0.05

0.50 BSC

0.38 p0.10

(DD) DFN 1103

3685fa

Page 23

PACKAGE DESCRIPTION

2.794 p 0.102 (.110 p .004)

MSE Package

10-Lead Plastic MSOP

(Reference LTC DWG # 05-08-1663)

0.889 p 0.127

(.035 p .005)

BOTTOM VIEW OF

EXPOSED PAD OPTION

LT3685

2.06 p 0.102 (.081 p .004)

1.83 p 0.102 (.072 p .004)

5.23

(.206)

MIN

0.305 p 0.038

(.0120 p .0015)

TYP

RECOMMENDED SOLDER PAD LAYOUT

0.254

(.010)

GAUGE PLANE

0.18

(.007)

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

DETAIL “A”

2.083 p 0.102 (.082 p .004)

0.50

(.0197)

BSC

0o – 6o TYP

0.53 p 0.152

(.021 p .006)

3.20 – 3.45

(.126 – .136)

SEATING

PLANE

3.00 p 0.102

(.118 p .004)

(NOTE 3)

4.90 p 0.152 (.193 p .006)

1.10

(.043)

MAX

0.17 –0.27

(.007 – .011)

TYP

0.50

(.0197)

BSC

8910

0.497 p 0.076 (.0196 p .003)

REF

3.00 p 0.102

(.118 p .004)

(NOTE 4)

0.86

(.034)

REF

0.1016 p 0.0508 (.004 p .002)

MSOP (MSE) 0307 REV B

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.

3685fa

Page 24

LT3685

TYPICAL APPLICATION

1.2V Step-Down Converter

3.6V TO 27V

ON OFF

4.7μF

16.2k

68.1k

330pF

D: DIODES INC. DFLS240L L: TAIYO YUDEN NP06DZB3R3M

RUN/SS BOOST

LT3685

SYNC

GND

f = 500kHz

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

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), 500kHz Step-Down Switching Regulator in

OUT

SOT-23

LT3437 60V, 400mA (I

OUT

Converter with Burst Mode

LT1936 36V, 1.4A (I

OUT

DC/DC Converter

LT3493 36V, 1.2A (I

OUT

DC/DC Converter

LT1976/LT1977 60V, 1.2A (I

OUT

Down DC/DC Converter with Burst Mode

LT1767 25V, 1.2A (I

OUT

DC/DC Converter

LT1940 Dual 25V, 1.4A (I

DC/DC Converter

LT1766 60V, 1.2A (I

OUT

DC/DC Converter

LT3434/LT3435 60V, 2.4A (I

OUT

DC/DC Converter with Burst Mode

LT3480 38V, 2A (I

), 2.4MHz, High Efﬁ ciency Step-Down DC/DC

OUT

Converter with Burst Mode

LT3481 36V, 2A (I

), 2.8MHz, High Efﬁ ciency Step-Down DC/DC

OUT

Converter with Burst Mode

LT3684 36V, 2A (I

), 2.8MHz, High Efﬁ ciency Step-Down

OUT

DC/DC Converter

), MicroPower Step-Down DC/DC

), 500kHz High Efﬁ ciency Step-Down

), 750kHz High Efﬁ ciency Step-Down

), 200kHz/500kHz, High Efﬁ ciency Step-

), 1.1MHz, High Efﬁ ciency Step-Down

OUT

), 200kHz, High Efﬁ ciency Step-Down

), 200/500kHz, High Efﬁ ciency Step-Down

VIN: 3.6V to 36V, V

VIN: 3.3V to 80V, V 3mm DFN and 16-Pin TSSOP Packages

VIN: 3.6V to 36V, V

VIN: 3.6V to 40V, V DFN Package

VIN: 3.3V to 60V, V Package

VIN: 3V to 25V, V

VIN: 3.6V to 25V, V Package

VIN: 5.5V to 60V, V Package

VIN: 3.3V to 60V, V Package

VIN: 3.6V to 38V, V 3mm DFN and 10-Pin MSOP Packages

VIN: 3.6V to 34V, V 3mm DFN and 10-Pin MSOP Packages

OUT

1.2V 2A

0.47μF

100k

OUT(MIN)

3.3μH

52.3k

47μF

3685 TA09

= 1.2V, IQ = 1.6mA, ISD <1μA, ThinSOT Package

= 1.25V, IQ = 100μA, ISD <1μA, 10-Pin 3mm x

= 1.2V, IQ = 1.9mA, ISD <1μA, MS8E Package

= 0.8V, IQ = 1.9mA, ISD <1μA, 6-Pin 2mm x 3mm

= 1.2V, IQ = 100μA, ISD <1μA, 16-Pin TSSOP

= 1.2V, IQ = 1mA, ISD <6μA, MS8E Package

= 1.2V, IQ = 3.8mA, ISD <30μA, 16-Pin TSSOP

= 1.2V, IQ = 2.5mA, ISD = 25μA, 16-Pin TSSOP

= 1.2V, IQ = 100μA, ISD <1μA, 16-Pin TSSOP

= 0.79V, IQ = 70μA, ISD <1μA, 10-Pin 3mm x

= 1.26V, IQ = 50μA, ISD <1μA, 10-Pin 3mm x

= 1.26V, IQ = 1.5mA, ISD <1μA, 10-Pin 3mm x

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507

●

www.linear.com

3685fa

LT 0308 REV A • PRINTED IN USA