LT3493IDCB#PBF - LINEAR TECHNOLOGY

LT3493

3493fb

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

1.2A, 750kHz Step-Down

Switching Regulator in

2mm × 3mm DFN

n Automotive Battery Regulation

n Industrial Control Supplies

n Wall Transformer Regulation

n Distributed Supply Regulation

n Battery-Powered Equipment

L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

3.3V Step-Down Converter

Efﬁ ciency

n Wide Input Range: 3.6V to 36V Operating,

40V Maximum

n 1.2A Output Current

n Fixed Frequency Operation: 750kHz

n Output Adjustable Down to 780mV

n Short-Circuit Robust

n Uses Tiny Capacitors and Inductors

n Soft-Start

n Internally Compensated

n Low Shutdown Current: <2μA

n Low VCESAT Switch: 330mV at 1A

n Thermally Enhanced, Low Proﬁ le DFN Package

The LT

3493 is a current mode PWM step-down DC/DC

converter with an internal 1.75A power switch. The wide

operating input range of 3.6V to 36V (40V maximum)

makes the LT3493 ideal for regulating power from a wide

variety of sources, including unregulated wall transform-

ers, 24V industrial supplies and automotive batteries.

Its high operating frequency allows the use of tiny, low

cost inductors and ceramic capacitors, resulting in low,

predictable output ripple.

Cycle-by-cycle current limit provides protection against

shorted outputs and soft-start eliminates input current

surge during start-up. The low current (<2μA) shutdown

mode provides output disconnect, enabling easy power

management in battery-powered systems.

VIN

4.2V TO 36V

ON OFF

0.1μF 10μH

32.4k

10μF

3493 TA01a

22pF

1μF 10k

VOUT

3.3V

1.2A, VIN > 12V

0.95A, VIN > 5V

VIN BOOST

GND FB

SHDN SW

LT3493

LOAD CURRENT (A)

EFFICIENCY (%)

3493 TA01b

50 0.4 0.8 1.2

0.20 0.6 1.0

VIN = 12V

VOUT = 3.3V

L = 10μH

LT3493

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PIN CONFIGURATION ABSOLUTE MAXIMUM RATINGS

(Note 1)

TOP VIEW

SHDN

VIN

GND

BOOST

DCB PACKAGE

6-LEAD (2mm s 3mm) PLASTIC DFN

TJMAX = 125°C, θJA = 64°C/W

EXPOSED PAD (PIN 7) IS GND, MUST BE SOLDERED TO PCB

ELECTRICAL CHARACTERISTICS

The l denotes the speciﬁ cations which apply over the full operating

temperature range, otherwise speciﬁ cations are at TA = 25°C. VIN = 12V, VBOOST = 17V, unless otherwise noted. (Note 2)

ORDER INFORMATION

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

LT3493EDCB#PBF LT3493EDCB#TRPBF LCGG 6-Lead (2mm × 3mm) Plastic DFN –40°C to 85°C

LT3493IDCB#PBF LT3493IDCB#TRPBF LCGH 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C

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

LT3493EDCB LT3493EDCB#TR LCGG 6-Lead (2mm × 3mm) Plastic DFN –40°C to 85°C

LT3493IDCB LT3493IDCB#TR LCGH 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C

Consult LTC Marketing for parts speciﬁ ed with wider operating temperature ranges.

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/

Input Voltage (VIN) ....................................................40V

BOOST Pin Voltage ..................................................50V

BOOST Pin Above SW Pin .........................................25V

SHDN Pin ..................................................................40V

FB Voltage ...................................................................6V

Operating Temperature Range (Note 2)

LT3493E .............................................. –40°C to 85°C

LT3493I ............................................. –40°C to 125°C

Maximum Junction Temperature .......................... 125°C

Storage Temperature Range ................... –65°C to 150°C

PARAMETER CONDITIONS MIN TYP MAX UNITS

VIN Operating Range 3.6 36 V

Undervoltage Lockout 3.1 3.4 3.6 V

Feedback Voltage l765 780 795 mV

FB Pin Bias Current VFB = Measured VREF + 10mV (Note 4) l50 150 nA

Quiescent Current Not Switching 1.9 2.5 mA

Quiescent Current in Shutdown VSHDN = 0V 0.01 2 μA

Reference Line Regulation VIN = 5V to 36V 0.007 %/V

Switching Frequency VFB = 0.7V

VFB = 0V

685 750

815 kHz

kHz

Maximum Duty Cycle

TA = 25°C

l88

LT3493

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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 LT3493E is guaranteed to meet performance speciﬁ cations

from 0°C to 85°C. Speciﬁ cations over the –40°C to 85°C operating

temperature range are assured by design, characterization and correlation

with statistical process controls. The LT3493I speciﬁ cations are

guaranteed over the –40°C to 125°C temperature range.

PARAMETER CONDITIONS MIN TYP MAX UNITS

Switch Current Limit (Note 3) 1.4 1.75 2.2 A

Switch VCESAT ISW = 1A 330 mV

Switch Leakage Current 2μA

Minimum Boost Voltage Above Switch ISW = 1A 1.85 2.2 V

BOOST Pin Current ISW = 1A 30 50 mA

SHDN Input Voltage High 2.3 V

SHDN Input Voltage Low 0.3 V

SHDN Bias Current VSHDN = 2.3V (Note 5)

VSHDN = 0V

0.01

0.1

μA

ELECTRICAL CHARACTERISTICS

The l denotes the speciﬁ cations which apply over the full operating

temperature range, otherwise speciﬁ cations are at TA = 25°C. VIN = 12V, VBOOST = 17V, unless otherwise noted. (Note 2)

Note 3: Current limit guaranteed by design and/or correlation to static test.

Slope compensation reduces current limit at higher duty cycle.

Note 4: Current ﬂ ows out of pin.

Note 5: Current ﬂ ows into pin.

TYPICAL PERFORMANCE CHARACTERISTICS

TA = 25°C unless otherwise noted.

Efﬁ ciency (VOUT = 5V, L = 10μH)

LOAD CURRENT (A)

EFFICIENCY (%)

0.8 1.0

3493 G01

0.2 0.4 0.6 1.2

VIN = 8V

VIN = 12V

VIN = 24V

LOAD CURRENT (A)

EFFICIENCY (%)

0.8 1.0

3493 G02

0.2 0.4 0.6 1.2

VIN = 8V

VIN = 12V

VIN = 24V

LOAD CURRENT (A)

EFFICIENCY (%)

0.8 1.0

3493 G03

0.2 0.4 0.6 1.2

VIN = 5V

VIN = 12V

Efﬁ ciency (VOUT = 3.3V, L = 10μH) Efﬁ ciency (VOUT = 1.8V, L = 4.7μH)

LT3493

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Maximum Load Current,

VOUT = 5V, L = 8.2μH

Maximum Load Current,

VOUT = 5V, L = 33μH

Maximum Load Current,

VOUT = 3.3V, L = 4.7μH

Maximum Load Current,

VOUT = 3.3V, L = 10μH Switch Voltage Drop Undervoltage Lockout

Switching Frequency Frequency Foldback Soft-Start

TYPICAL PERFORMANCE CHARACTERISTICS

TA = 25°C unless otherwise noted.

VIN (V)

1.60

1.50

1.40

1.30

1.20

1.10

1.00

0.90 20 28

3493 G04

12 16 24

OUTPUT CURRENT (A)

TYPICAL

MINIMUM

VIN (V)

1.60

1.50

1.40

1.30

1.20

1.10

1.00

0.90 20 28

3493 G22

12 16 24

OUTPUT CURRENT (A)

TYPICAL

MINIMUM

VIN (V)

1.40

1.50

1.60

3493 G05

1.30

1.20

10 15 20 30

1.10

1.00

0.90

OUTPUT CURRENT (A)

TYPICAL

MINIMUM

VIN (V)

1.40

1.50

1.60

3493 G21

1.30

1.20

10 15 20 30

1.10

1.00

0.90

OUTPUT CURRENT (A)

TYPICAL

MINIMUM

SWITCH CURRENT (A)

VCE(SW) (mV)

150

450

500

550

0.4 0.8 1.0

3493 G06

350

250

100

400

300

200

0.2 0.6 1.4

1.2 1.6 1.8

TA = 25°C

TA = 85°C

TA = –40°C

TEMPERATURE (°C)

UVLO (V)

3.60

3.80

4.00

125

3493 G08

3.40

3.20

3.50

3.70

3.90

3.30

3.10

3.00 –25–50 250 75 100 150

TEMPERATURE (°C)

FREQUENCY (kHz)

720

760

800

125

3493 G09

680

640

700

740

780

660

620

600 –25–50 250 75 100 150

FEEDBACK VOLTAGE (mV)

SWITCHING FREQUENCY (kHz)

400

600

800

3493 G11

200

0200 400 600

100 300 500 700

800

300

500

100

700

SHDN PIN VOLTAGE (V)

SWITCH CURRENT LIMIT (A)

0.2

0.6

0.8

1.0

2.0

1.4

0.50 1 1.25

3493 G13

0.4

1.6

1.8

1.2

0.25 0.75 1.50 1.75 2

LT3493

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SHDN Pin Current

Typical Minimum Input Voltage

(VOUT = 5V)

Typical Minimum Input Voltage

(VOUT = 3.3V)

Switch Current Limit Switch Current Limit

Operating Waveforms

Operating Waveforms,

Discontinuous Mode

TYPICAL PERFORMANCE CHARACTERISTICS

TA = 25°C unless otherwise noted.

VSHDN (V)

ISHDN (μA)

3493 G14

042 86 12 14 18

10 20

IOUT (mA)

5.0

VIN (V)

6.5

7.0

7.5

10 100 1000

3493 G15

6.0

5.5

TO START

TO RUN

IOUT (mA)

4.3

VIN (V)

4.5

4.7

4.9

5.1

10 100 1000

3493 G16

4.1

3.9

3.7

3.5

5.3

5.5

TO START

TO RUN

TEMPERATURE (°C)

–50

1.0

SWITCH CURRENT LIMIT (A)

1.1

1.3

1.4

1.5

2.0

1.7

0 25 100 125 150

3493 G17

1.2

1.8

1.9

1.6

–25 50 75

DUTY CYCLE (%)

SWITCH CURRENT LIMIT (A)

1.2

1.6

2.0

3493 G18

0.8

0.4

1.0

1.4

1.8

0.6

0.2

020 40 60 100

VSW

5V/DIV

0.5A/DIV

VOUT

20mV/DIV

VIN = 12V

VOUT = 3.3V

IOUT = 0.5A

L = 10μH

COUT = 10μF

1μs/DIV 3493 G19

VSW

5V/DIV

0.5A/DIV

VOUT

20mV/DIV

1μs/DIV 3493 G20

VIN = 12V

VOUT = 3.3V

IOUT = 50mA

L = 10μH

COUT = 10μF

LT3493

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BLOCK DIAGRAM

PIN FUNCTIONS

FB (Pin 1): The LT3493 regulates its feedback pin to

780mV. Connect the feedback resistor divider tap to this

pin. Set the output voltage according to VOUT = 0.78V •

(1 + R1/R2). A good value for R2 is 10k.

GND (Pin 2): Tie the GND pin to a local ground plane

below the LT3493 and the circuit components. Return the

feedback divider to this pin.

BOOST (Pin 3): The BOOST pin is used to provide a drive

voltage, higher than the input voltage, to the internal bipolar

NPN power switch.

SW (Pin 4): The SW pin is the output of the internal power

switch. Connect this pin to the inductor, catch diode and

boost capacitor.

VIN (Pin 5): The VIN pin supplies current to the LT3493’s

internal regulator and to the internal power switch. This

pin must be locally bypassed.

SHDN (Pin 6): The SHDN pin is used to put the LT3493 in

shutdown mode. Tie to ground to shut down the LT3493.

Tie to 2.3V or more for normal operation. If the shutdown

feature is not used, tie this pin to the VIN pin. SHDN also

provides a soft-start function; see the Applications Infor-

mation section.

Exposed Pad (Pin 7): The Exposed Pad must be soldered

to the PCB and electrically connected to ground. Use a

large ground plane and thermal vias to optimize thermal

performance.

DRIVER Q1

OSC

SLOPE

COMP

FREQUENCY

FOLDBACK

INT REG

AND

UVLO

VCgm

780mV

3493 BD

BOOST

R2 R1

VOUT

VIN

ON OFF

GND

SHDN

LT3493

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OPERATION

(Refer to Block Diagram)

The LT3493 is a constant frequency, current mode step-

down regulator. A 750kHz oscillator enables an RS ﬂ ip-ﬂ op,

turning on the internal 1.75A power switch Q1. An ampliﬁ er

and comparator monitor the current ﬂ owing between the

VIN and SW pins, turning the switch off when this current

reaches a level determined by the voltage at VC. An error

ampliﬁ er measures the output voltage through an external

resistor divider tied to the FB pin and servos the VC node.

If the error ampliﬁ er’s output increases, more current is

delivered to the output; if it decreases, less current is

delivered. An active clamp (not shown) on the VC node

provides current limit. The VC node is also clamped to

the voltage on the SHDN pin; soft-start is implemented

by generating a voltage ramp at the SHDN pin using an

external resistor and capacitor.

An internal regulator provides power to the control circuitry.

This regulator includes an undervoltage lockout to prevent

switching when VIN is less than ~3.4V. The SHDN pin is

used to place the LT3493 in shutdown, disconnecting the

output and reducing the input current to less than 2μA.

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 LT3493’s operating frequency

when the voltage at the FB pin is low. This frequency

foldback helps to control the output current during start-

up and overload.

LT3493

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APPLICATIONS INFORMATION

FB Resistor Network

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the 1% resis-

tors according to:

R1=R2 V

OUT

0.78V

–1











R2 should be 20k or less to avoid bias current errors.

Reference designators refer to the Block Diagram.

An optional phase lead capacitor of 22pF between VOUT

and FB reduces light-load output ripple.

Input Voltage Range

The input voltage range for LT3493 applications depends

on the output voltage and on the absolute maximum rat-

ings of the VIN and BOOST pins.

The minimum input voltage is determined by either the

LT3493’s minimum operating voltage of 3.6V, or by its

maximum duty cycle. The duty cycle is the fraction of

time that the internal switch is on and is determined by

the input and output voltages:

DC =

VOUT +VD

VIN –V

SW +VD

where VD is the forward voltage drop of the catch diode

(~0.4V) and VSW is the voltage drop of the internal switch

(~0.4V at maximum load). This leads to a minimum input

voltage of:

VIN(MIN) =VOUT +VD

DCMAX

–V

D+VSW

with DCMAX = 0.91 (0.88 over temperature).

The maximum input voltage is determined by the absolute

maximum ratings of the VIN and BOOST pins. For con-

tinuous mode operation, the maximum input voltage is

determined by the minimum duty cycle DCMIN = 0.10:

VIN(MAX) =VOUT +VD

DCMIN

–V

D+VSW

Note that this is a restriction on the operating input voltage

for continuous mode operation; the circuit will tolerate

transient inputs up to the absolute maximum ratings

of the VIN and BOOST pins. The input voltage should be

limited to the VIN operating range (36V) during overload

conditions (short-circuit or start-up).

Minimum On Time

The part will still regulate the output at input voltages that

exceed VIN(MAX) (up to 40V), however, the output voltage

ripple increases as the input voltage is increased. Figure 1

illustrates switching waveforms in continuous mode for a

3V output application near VIN(MAX) = 33V.

As the input voltage is increased, the part is required

to switch for shorter periods of time. Delays associated

with turning off the power switch dictate the minimum

on time of the part. The minimum on time for the LT3493

is ~120ns. Figure 2 illustrates the switching waveforms

when the input voltage is increased to VIN = 35V.

VSW

20V/DIV

VOUT

200mV/DIV

AC COUPLED

COUT = 10μF

VOUT = 3V

VIN = 30V

ILOAD = 0.75A

L = 10μH

2μs/DIV 3493 F01

0.5A/DIV

VSW

20V/DIV

VOUT

200mV/DIV

AC COUPLED

COUT = 10μF

VOUT = 3V

VIN = 35V

ILOAD = 0.75A

L = 10μH

2μs/DIV 3493 F02

0.5A/DIV

Figure 1

Figure 2

LT3493

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APPLICATIONS INFORMATION

Now the required on-time has decreased below the

minimum on time of 120ns. Instead of the switch pulse

width becoming narrower to accommodate the lower duty

cycle requirement, the switch pulse width remains ﬁ xed

at 120ns. In Figure 2 the inductor current ramps up to a

value exceeding the load current and the output ripple

increases to ~200mV. The part then remains off until the

output voltage dips below 100% of the programmed value

before it begins switching again.

Provided that the load can tolerate the increased output

voltage ripple and that the components have been properly

selected, operation above VIN(MAX) is safe and will not

damage the part. Figure 3 illustrates the switching wave-

forms when the input voltage is increased to its absolute

maximum rating of 40V.

As the input voltage increases, the inductor current ramps

up quicker, the number of skipped pulses increases and

the output voltage ripple increases. For operation above

VIN(MAX) the only component requirement is that the com-

ponents be adequately rated for operation at the intended

voltage levels.

The part is robust enough to survive prolonged operation

under these conditions as long as the peak inductor current

does not exceed 2.2A. Inductor current saturation may

further limit performance in this operating regime.

Inductor Selection and Maximum Output Current

A good ﬁ rst choice for the inductor value is:

L = 1.6 (VOUT + VD)

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

L is in μH. With this value there will be no subharmonic

oscillation for applications with 50% or greater duty cycle.

The inductor’s RMS current rating must be greater than

your maximum load current and its saturation current

should be about 30% higher. For robust operation in fault

conditions, the saturation current should be above 2.2A.

To keep efﬁ ciency high, the series resistance (DCR) should

be less than 0.1Ω. Table 1 lists several vendors and types

that are suitable.

Of course, such a simple design guide will not always

result in the optimum inductor for your application. A

larger value provides a higher maximum load current and

reduces output voltage ripple at the expense of slower

transient response. If your load is lower than 1.2A, then

you can decrease the value of the inductor and operate

with higher ripple current. This allows you to use a physi-

cally 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 volt-

ages. Low inductance may result in discontinuous mode

operation, which is okay, but further reduces maximum

load current. For details of the maximum output current

and discontinuous mode operation, see Linear Technology

Application Note 44.

Catch Diode

Depending on load current, a 1A to 2A Schottky diode is

recommended for the catch diode, D1. The diode must

have a reverse voltage rating equal to or greater than the

maximum input voltage. The ON Semiconductor MBRM140

is a good choice; it is rated for 1A continuous forward

current and a maximum reverse voltage of 40V.

VSW

20V/DIV

VOUT

200mV/DIV

AC COUPLED

COUT = 10μF

VOUT = 3V

VIN = 40V

ILOAD = 0.75A

L = 10μH

2μs/DIV 3493 F03

0.5A/DIV

Figure 3

LT3493

3493fb

APPLICATIONS INFORMATION

Input Capacitor

Bypass the input of the LT3493 circuit with a 1μF or

higher value ceramic capacitor of X7R or X5R type. Y5V

types have poor performance over temperature and ap-

plied voltage and should not be used. A 1μF ceramic is

adequate to bypass the LT3493 and will easily handle the

ripple current. However, 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 low performance

electrolytic capacitor.

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 LT3493 and to force this very high frequency

switching current into a tight local loop, minimizing EMI.

A 1μF capacitor is capable of this task, but only if it is

placed close to the LT3493 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 LT3493. A ceramic input capacitor

combined with trace or cable inductance forms a high

quality (underdamped) tank circuit. If the LT3493 circuit

is plugged into a live supply, the input voltage can ring to

twice its nominal value, possibly exceeding the LT3493’s

voltage rating. This situation is easily avoided; see the Hot

Plugging Safely section.

Output Capacitor

The output capacitor has two essential functions. Along

with the inductor, it ﬁ lters the square wave generated

by the LT3493 to produce the DC output. In this role it

determines the output ripple so low impedance at the

switching frequency is important. The second function

is to store energy in order to satisfy transient loads and

stabilize the LT3493’s control loop.

Ceramic capacitors have very low equivalent series re-

sistance (ESR) and provide the best ripple performance.

A good value is:

OUT = 65/VOUT

where COUT is in μF. Use X5R or X7R types and keep in

mind that a ceramic capacitor biased with VOUT will have

less than its nominal capacitance. This choice will provide

low output ripple and good transient response. Transient

performance can be improved with a high value capacitor,

but a phase lead capacitor across the feedback resistor

R1 may be required to get the full beneﬁ t (see the Com-

pensation section).

For small size, the output capacitor can be chosen ac-

cording to:

OUT = 25/VOUT

where COUT is in μF. However, using an output capacitor

this small results in an increased loop crossover frequency

and increased sensitivity to noise. A 22pF capacitor con-

nected between VOUT and the FB pin is required to ﬁ lter

noise at the FB pin and ensure stability.

High performance 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 0.1Ω

or less. Such a capacitor will be larger than a ceramic

capacitor and will have a larger capacitance, because the

Table 1. Inductor Values

VENDOR URL PART SERIES INDUCTANCE RANGE (μH) SIZE (MM)

Sumida www.sumida.com CDRH4D28

CDRH5D28

CDRH8D28

1.2 to 4.7

2.5 to 10

2.5 to 33

4.5 × 4.5

5.5 × 5.5

8.3 × 8.3

Toko www.toko.com A916CY

D585LC

2 to 12

1.1 to 39

6.3 × 6.2

8.1 × 8.0

Würth Elektronik www.we-online.com WE-TPC(M)

WE-PD2(M)

WE-PD(S)

1 to 10

2.2 to 22

1 to 27

4.8 × 4.8

5.2 × 5.8

7.3 × 7.3

LT3493

3493fb

APPLICATIONS INFORMATION

capacitor must be large to achieve low ESR. Table 2 lists

several capacitor vendors.

Figure 4 shows the transient response of the LT3493 with

several output capacitor choices. The output is 3.3V. The

load current is stepped from 250mA to 1A and back to

250mA, and the oscilloscope traces show the output volt-

age. The upper photo shows the recommended value. The

second photo shows the improved response (less voltage

drop) resulting from a larger output capacitor and a phase

lead capacitor. The last photo shows the response to a high

performance electrolytic capacitor. Transient performance

is improved due to the large output capacitance.

BOOST Pin Considerations

Capacitor C3 and diode D2 are used to generate a boost

voltage that is higher than the input voltage. In most cases

a 0.1μF capacitor and fast switching diode (such as the

1N4148 or 1N914) will work well. Figure 5 shows two

ways to arrange the boost circuit. The BOOST pin must

be at least 2.3V above the SW pin for best efﬁ ciency. For

outputs of 3.3V and above, the standard circuit (Figure 5a)

is best. For outputs between 3V and 3.3V, use a 0.22μF

capacitor. For outputs between 2.5V and 3V, use a 0.47μF

capacitor and a small Schottky diode (such as the BAT-

54). For lower output voltages the boost diode can be tied

to the input (Figure 5b). The circuit in Figure 5a is more

efﬁ cient because the BOOST pin current comes from a lower

voltage source. You must also be sure that the maximum

voltage rating of the BOOST pin is not exceeded.

The minimum operating voltage of an LT3493 applica-

tion is limited by the undervoltage lockout (3.6V) and by

the maximum duty cycle as outlined above. For proper

start-up, the minimum input voltage is also limited by

the boost circuit. If the input voltage is ramped slowly,

or the LT3493 is turned on with its SHDN 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 the 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 6 shows a plot of

minimum load to start and to run as a function of input

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 VIN

is ramping verly slowly. For lower start-up voltage, the

boost diode can be tied to VIN; however this restricts the

input range to one-half of the absolute maximum rating

of the BOOST pin.

Table 2. Capacitor Vendors

VENDOR PHONE URL PART SERIES COMMENTS

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

Polymer,

Tantalum

EEF Series

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

Tantalum T494, T495

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

Polymer,

Tantalum

POSCAP

Murata (404) 436-1300 www.murata.com Ceramic

AVX www.avxcorp.com Ceramic,

Tantalum TPS Series

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

LT3493

3493fb

APPLICATIONS INFORMATION

10μFFB

32.4k

ILOAD

2A/DIV

0.5A/DIV

VOUT

0.1V/DIV

AC COUPLED

ILOAD

2A/DIV

0.5A/DIV

VOUT

0.1V/DIV

AC COUPLED

40μs/DIV

ILOAD

2A/DIV

0.5A/DIV

VOUT

0.1V/DIV

AC COUPLED

40μs/DIV

10k

VOUT

3493 F04a

3493 F04b

3493 F04c

VOUT

32.4k

10k

10μF

3.3nF

SANYO

4TPB100M

VOUT

32.4k

10k

100μF

Figure 4. Transient Load Response of the LT3493 With Different Output Capacitors

as the Load Current is Stepped from 250mA to 1A. VIN = 12V, VOUT = 3.3V, L = 10μH

VIN

BOOST

GND

VIN

LT3493

(5a)

VOUT

VBOOST – VSW VOUT

MAX VBOOST VIN + VOUT

3493 F05a

VIN

BOOST

GND

VIN

LT3493

(5b)

3493 F05b

VOUT

VBOOST – VSW VIN

MAX VBOOST 2VIN

Figure 5. Two Circuits for Generating the Boost Voltage

LT3493

3493fb

APPLICATIONS INFORMATION

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

400mV above VOUT

. At higher load currents, the inductor

current is continuous and the duty cycle is limited by the

maximum duty cycle of the LT3493, requiring a higher

input voltage to maintain regulation.

Soft-Start

The SHDN pin can be used to soft-start the LT3493, reducing

the maximum input current during start-up. The SHDN pin

is driven through an external RC ﬁ lter to create a voltage

ramp at this pin. Figure 7 shows the start-up waveforms

with and without the soft-start circuit. 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 SHDN pin reaches 2.3V.

Shorted and Reversed Input Protection

If the inductor is chosen so that it won’t saturate exces-

sively, an LT3493 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

LT3493 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 LT3493’s

output. If the VIN pin is allowed to ﬂ oat and the SHDN pin

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

to VIN), then the LT3493’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 SHDN pin, the SW pin current will drop to essentially

zero. However, if the VIN pin is grounded while the output

is held high, then parasitic diodes inside the LT3493 can

pull large currents from the output through the SW pin

and the VIN pin. Figure 8 shows a circuit that will run only

when the input voltage is present and that protects against

a shorted or reversed input.

Hot Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LT3493 circuits. However, these capaci-

tors can cause problems if the LT3493 is plugged into a

live supply (see Linear Technology Application Note 88 for

IOUT (mA)

5.0

VIN (V)

6.5

7.0

7.5

10 100 1000

3493 G15

6.0

5.5

TO START

TO RUN

IOUT (mA)

4.3

VIN (V)

4.5

4.7

4.9

5.1

10 100 1000

3493 G16

4.1

3.9

3.7

3.5

5.3

5.5

TO START

TO RUN

(6a) Typical Minimum Input Voltage, VOUT = 5V (6b) Typical Minimum Input Voltage, VOUT = 3.3V

Figure 6

LT3493

3493fb

APPLICATIONS INFORMATION

RUN

VSW

10V/DIV

VIN = 12V

VOUT = 3.3V

L = 10μH

COUT = 10μF

VOUT

2V/DIV

20μs/DIV

0.5A/DIV

VSW

10V/DIV

VIN = 12V

VOUT = 3.3V

L = 10μH

COUT = 10μF

VOUT

2V/DIV

20μs/DIV

0.5A/DIV

SHDN

GND

3493 F07a

RUN

15k

0.1μF

SHDN

GND

3493 F07b

Figure 7. To Soft-Start the LT3493, Add a Resistor and Capacitor to the SHDN Pin. VIN = 12V, VOUT = 3.3V, COUT = 10μF, RLOAD = 5Ω

VIN BOOST

GND FB

SHDN SW

VIN

LT3493

3493 F08

VOUT

BACKUP

D4: MBR0540

Figure 8. 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 LT3493 Runs Only When the Input

is Present

a complete discussion). The low loss ceramic capacitor

combined with stray inductance in series with the power

source forms an underdamped tank circuit, and the voltage

at the VIN pin of the LT3493 can ring to twice the nominal

input voltage, possibly exceeding the LT3493’s rating and

damaging the part. If the input supply is poorly controlled

or the user will be plugging the LT3493 into an energized

supply, the input network should be designed to prevent

this overshoot.

Figure 9 shows the waveforms that result when an LT3493

circuit is connected to a 24V supply through six feet of

24-gauge twisted pair. The ﬁ rst plot is the response with

LT3493

3493fb

APPLICATIONS INFORMATION

a 2.2μF ceramic capacitor at the input. The input voltage

rings as high as 35V and the input current peaks at 20A.

One method of damping the tank circuit is to add another

capacitor with a series resistor to the circuit. In Figure 9b

an aluminum electrolytic capacitor has been added. This

capacitor’s high equivalent series resistance damps the

circuit and eliminates the voltage overshoot. The extra

capacitor improves low frequency ripple ﬁ ltering and can

slightly improve the efﬁ ciency of the circuit, though it is

likely to be the largest component in the circuit. An alterna-

tive solution is shown in Figure 9c. A 1Ω 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 ﬁ ltering. This solution is smaller

and less expensive than the electrolytic capacitor. For high

input voltages its impact on efﬁ ciency is minor, reducing

efﬁ ciency less than one half percent for a 5V output at full

load operating from 24V.

Frequency Compensation

The LT3493 uses current mode control to regulate the

output. This simpliﬁ es loop compensation. In particular,

the LT3493 does not require the ESR of the output capaci-

tor for stability allowing the use of ceramic capacitors to

achieve low output ripple and small circuit size.

Figure 10 shows an equivalent circuit for the LT3493 control

loop. The error amp 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 cur-

rent proportional to the voltage at the VC node. Note that

the output capacitor integrates this current, and that the

capacitor on the VC node (CC) integrates the error ampli-

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

provides a zero. With the recommended output capacitor,

the loop crossover occurs above the RCCC zero. This simple

LT3493

2.2μF

VIN

20V/DIV

IIN

5A/DIV

20μs/DIV

VIN

CLOSING SWITCH

SIMULATES HOT PLUG

IIN

(9a)

(9b)

(9c)

LOW

IMPEDANCE

ENERGIZED

24V SUPPLY

STRAY

INDUCTANCE

DUE TO 6 FEET

(2 METERS) OF

TWISTED PAIR

LT3493

2.2μF

10μF

35V

AI.EI.

LT3493

2.2μF0.1μF

1Ω

3493 F09

VIN

20V/DIV

IIN

5A/DIV

20μs/DIV

VIN

20V/DIV

IIN

5A/DIV

20μs/DIV

DANGER!

RINGING VIN MAY EXCEED

ABSOLUTE MAXIMUM

RATING OF THE LT3493

Figure 9. A Well Chosen Input Network Prevents Input Voltage Overshoot and

Ensures Reliable Operation When the LT3493 is Connected to a Live Supply

LT3493

3493fb

model works well as long as the value of the inductor is

not too high and the loop crossover frequency is much

lower than the switching frequency. With a larger ceramic

capacitor (very low ESR), crossover may be lower and a

phase lead capacitor (CPL) across the feedback divider may

improve the phase margin and transient response. Large

electrolytic capacitors may have an ESR large enough to

create an additional zero, and the phase lead may not be

necessary.

If the output capacitor is different than the recommended

capacitor, stability should be checked across all operating

conditions, including load current, input voltage and tem-

perature. The LT1375 data sheet contains a more thorough

discussion of loop compensation and describes how to

test the stability using a transient load.

PCB Layout

For proper operation and minimum EMI, care must be taken

during printed circuit board layout. Figure 11 shows the

recommended component placement with trace, ground

plane and via locations. Note that large, switched currents

ﬂ ow in the LT3493’s VIN and SW pins, the catch diode (D1)

and the input capacitor (C2). The loop formed by these

components should be as small as possible and tied to

system ground in only one place. 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, and tie this ground plane

to system ground at one location, ideally at the ground

terminal of the output capacitor C1. The SW and BOOST

nodes should be as small as possible. Finally, keep the

FB node small so that the ground pin and ground traces

will shield it from the SW and BOOST nodes. Include vias

near the exposed GND pad of the LT3493 to help remove

heat from the LT3493 to the ground plane.

APPLICATIONS INFORMATION

–

780mV

LT3493

GND

3493 F10

OUT

ESR

ERROR

AMPLIFIER

CURRENT MODE

POWER STAGE

60k

100pF

gm =

300μA/V

gm =

1.6A/V

CPL

0.7V

Figure 10. Model for Loop Response

Figure 11. A Good PCB Layout Ensures Proper, Low EMI Operation

C2 D1 C1

SYSTEM

GROUND

: VIAS TO LOCAL GROUND PLANE

: OUTLINE OF LOCAL GROUND PLANE

VOUT

3493 F11

VIN

SHDN

LT3493

3493fb

APPLICATIONS INFORMATION

High Temperature Considerations

The die temperature of the LT3493 must be lower than the

maximum rating of 125°C. This is generally not a concern

unless the ambient temperature is above 85°C. For higher

temperatures, care should be taken in the layout of the

circuit to ensure good heat sinking of the LT3493. The

maximum load current should be derated as the ambient

temperature approaches 125°C. The die temperature is

calculated by multiplying the LT3493 power dissipation by

the thermal resistance from junction to ambient. Power

dissipation within the LT3493 can be estimated by calcu-

lating the total power loss from an efﬁ ciency measure-

ment and subtracting the catch diode loss. The resulting

temperature rise at full load is nearly independent of input

voltage. Thermal resistance depends on the layout of the

circuit board, but 64°C/W is typical for the (2mm × 3mm)

DFN (DCB) package.

Outputs Greater Than 6V

For outputs greater than 6V, add a resistor of 1k to 2.5k

across the inductor to damp the discontinuous ringing

of the SW node, preventing unintended SW current. The

12V Step-Down Converter circuit in the Typical Applica-

tions section shows the location of this resistor. Also note

that for outputs above 6V, the input voltage range will be

limited by the maximum rating of the BOOST pin. The 12V

circuit shows how to overcome this limitation using an

additional zener diode.

Other Linear Technology Publications

Application notes AN19, AN35 and AN44 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 DN100 shows how to generate a bipolar output

supply using a Buck regulator.

TYPICAL APPLICATIONS

VIN

3.6V TO 25V

0.1μF 3.3μH

MBRM140

47μF

3493 TA02

2.2μF

VOUT

0.78V

1.2A

VIN BOOST

GND FB

SHDN SW

LT3493

1N4148

ON OFF

VIN

3.6V TO 25V

0.1μF 5μH

MBRM140 26.1k

22μF

3493 TA03

2.2μF 20k

VOUT

1.8V

1.2A

VIN BOOST

GND FB

SHDN SW

LT3493

1N4148

ON OFF

0.78V Step-Down Converter

1.8V Step-Down Converter

LT3493

3493fb

TYPICAL APPLICATIONS

2.5V Step-Down Converter

VIN

3.6V TO 28V

0.47μF 6.8μH

BAT54

MBRM140 22.1k

22μF

3493 TA04

1μF 10k

VOUT

2.5V

1A, VIN > 5V

1.2A, VIN > 10V

VIN BOOST

GND FB

SHDN SW

LT3493

ON OFF

3.3V Step-Down Converter

VIN

4.2V TO 36V

0.1μF 8.2μH

1N4148

MBRM140 32.4k

10μF

3493 TA05

1μF 10k

VOUT

3.3V

0.9A, VIN > 4.5V

1.2A, VIN > 12V

VIN BOOST

GND FB

SHDN SW

LT3493

ON OFF

5V Step-Down Converter

VIN

6.4V TO 36V

0.1μF 10μH

1N4148

MBRM140 59k

10μF

3493 TA06

1μF 11k

VOUT

0.9A, VIN > 7V

1.1A, VIN > 14V

VIN BOOST

GND FB

SHDN SW

LT3493

ON OFF

LT3493

3493fb

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.

PACKAGE DESCRIPTION

DCB Package

6-Lead Plastic DFN (2mm × 3mm)

(Reference LTC DWG # 05-08-1715)

3.00 ±0.10

(2 SIDES)

2.00 ±0.10

(2 SIDES)

NOTE:

1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (TBD)

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

0.40 ± 0.10

BOTTOM VIEW—EXPOSED PAD

1.65 ± 0.10

(2 SIDES)

0.75 ±0.05

R = 0.115

TYP

R = 0.05

TYP

1.35 ±0.10

(2 SIDES)

PIN 1 BAR

TOP MARK

(SEE NOTE 6)

0.200 REF

0.00 – 0.05

(DCB6) DFN 0405

0.25 ± 0.05

0.50 BSC

PIN 1 NOTCH

R0.20 OR 0.25

s 45° CHAMFER

0.25 ± 0.05

1.35 ±0.05

(2 SIDES)

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

1.65 ±0.05

(2 SIDES)

2.15 ±0.05

0.70 ±0.05

3.55 ±0.05

PACKAGE

OUTLINE

0.50 BSC

LT3493

3493fb

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com

LT 1108 REV B • PRINTED IN USA

RELATED PARTS

TYPICAL APPLICATION

12V Step-Down Converter

VIN

14.5V TO 36V

0.1μF 22μH

1k*

0.25W

* FOR CONTINUOUS OPERATION ABOVE 30V

USE TWO 2k, 0.25Ω RESISTORS IN PARALLEL.

D1: CMDZ5235B

1N4148

MBRM140 71.5k

4.7μF

3493 TA07

1μF 4.99k

VOUT

12V

VIN BOOST

GND FB

SHDN SW

LT3493

ON OFF

PART NUMBER DESCRIPTION COMMENTS

LT1766 60V, 1.2A IOUT

, 200kHz, High Efﬁ ciency Step-Down

DC/DC Converter

VIN: 5.5V to 60V, VOUT(MIN) = 1.2V, IQ = 2.5mA, ISD = 25μA,

TSSOP16/TSSOP16E Packages

LT1933 36V, 600mA IOUT

, 500kHz, High Efﬁ ciency Step-Down

DC/DC Converter

LT1936 36V, 1.4A IOUT

, 500kHz, High Efﬁ ciency Step-Down

DC/DC Converter

LT1940 25V, Dual 1.4A IOUT

, 1.1MHz, High Efﬁ ciency Step-Down

DC/DC Converter

VIN: 3.6V to 25V, VOUT(MIN) = 1.25V, IQ = 3.8mA, ISD < 30μA,

TSSOP16E Package

LT1976 60V, 1.2A IOUT

, 200kHz, High Efﬁ ciency Step-Down

DC/DC Converter with Burst Mode

Operation

VIN: 3.3V to 60V, VOUT(MIN) = 1.2V, IQ = 100μA, ISD < 1μA,

TSSOP16E Package

LT3010 80V, 50mA, Low Noise Linear Regulator VIN: 1.5V to 80V, VOUT(MIN) = 1.28V, IQ = 30μA, ISD < 1μA,

MS8E Package

LTC3407 Dual 600mA IOUT

, 1.5MHz, Synchronous Step-Down

DC/DC Converter

VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40μA, ISD < 1μA,

MS10E Package

LT3430/LT3431 60V, 2.75A IOUT

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

DC/DC Converter

VIN: 5.5V to 60V, VOUT(MIN) = 1.2V, IQ = 2.5mA, ISD = 30μA,

TSSOP16E Package

LT3470 40V, 200mA IOUT

, 26μA IQ, Step-Down DC/DC Converter

Burst Mode is a registered trademark of Linear Technology Corporation.