LT3493
1.2A, 750kHz Step-Down
Switching Regulator in
2mm × 3mm DFN
FEATURES
DESCRIPTION
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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 transformers, 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.
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Wide Input Range: 3.6V to 36V Operating,
40V Maximum
1.2A Output Current
Fixed Frequency Operation: 750kHz
Output Adjustable Down to 780mV
Short-Circuit Robust
Uses Tiny Capacitors and Inductors
Soft-Start
Internally Compensated
Low Shutdown Current: 5V
SW
SHDN
32.4k
GND
1μF
FB
22pF
10μF
10k
80
EFFICIENCY (%)
VIN
4.2V TO 36V
75
70
65
60
VIN = 12V
VOUT = 3.3V
L = 10μH
55
3493 TA01a
50
0
0.2
0.8
0.4
0.6
LOAD CURRENT (A)
1.0
1.2
3493 TA01b
3493fb
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LT3493
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
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
TOP VIEW
6 SHDN
FB 1
7
GND 2
5 VIN
4 SW
BOOST 3
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
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 specified 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 specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VBOOST = 17V, unless otherwise noted. (Note 2)
PARAMETER
CONDITIONS
MIN
VIN Operating Range
TYP
3.6
Undervoltage Lockout
l
Feedback Voltage
MAX
36
UNITS
V
3.1
3.4
3.6
V
765
780
795
mV
l
FB Pin Bias Current
VFB = Measured VREF + 10mV (Note 4)
50
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
Switching Frequency
VFB = 0.7V
VFB = 0V
0.007
l
Maximum Duty Cycle
TA = 25°C
685
750
36
88
91
95
95
%/V
815
kHz
kHz
%
%
3493fb
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LT3493
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, VBOOST = 17V, unless otherwise noted. (Note 2)
PARAMETER
CONDITIONS
MIN
TYP
MAX
Switch Current Limit
(Note 3)
1.4
1.75
2.2
Switch VCESAT
ISW = 1A
ISW = 1A
BOOST Pin Current
ISW = 1A
SHDN Input Voltage High
mV
2
μA
1.85
2.2
V
30
50
mA
2.3
V
SHDN Input Voltage Low
SHDN Bias Current
VSHDN = 2.3V (Note 5)
VSHDN = 0V
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 specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LT3493I specifications are
guaranteed over the –40°C to 125°C temperature range.
6
0.01
Efficiency (VOUT = 3.3V, L = 10μH)
90
90
85
85
80
EFFICIENCY (%)
80
75
70
65
V
15
0.1
μA
μA
TA = 25°C unless otherwise noted.
Efficiency (VOUT = 1.8V, L = 4.7μH)
80
75
EFFICIENCY (%)
Efficiency (VOUT = 5V, L = 10μH)
95
0.3
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 flows out of pin.
Note 5: Current flows into pin.
TYPICAL PERFORMANCE CHARACTERISTICS
EFFICIENCY (%)
A
330
Switch Leakage Current
Minimum Boost Voltage Above Switch
UNITS
75
70
65
70
65
60
60
60
VIN = 8V
VIN = 12V
VIN = 24V
55
50
0
0.2
0.4
0.6
0.8
LOAD CURRENT (A)
1.0
1.2
3493 G01
VIN = 8V
VIN = 12V
VIN = 24V
55
50
0
0.2
0.4
0.6
0.8
LOAD CURRENT (A)
1.0
1.2
3493 G02
55
VIN = 5V
VIN = 12V
50
0
0.2
0.4
0.6
0.8
LOAD CURRENT (A)
1.0
1.2
3493 G03
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LT3493
TYPICAL PERFORMANCE CHARACTERISTICS
Maximum Load Current,
VOUT = 5V, L = 8.2μH
Maximum Load Current,
VOUT = 5V, L = 33μH
1.60
1.60
1.50
1.50
1.40
1.30
1.20
MINIMUM
1.10
1.00
OUTPUT CURRENT (A)
TYPICAL
OUTPUT CURRENT (A)
1.40
1.30
MINIMUM
1.20
1.10
0.90
8
12
16
24
20
VIN (V)
8
12
16
20
VIN (V)
24
1.50
TYPICAL
VCE(SW) (mV)
MINIMUM
1.10
1.00
500
3.90
3.70
TA = 25°C
350
300
TA = –40°C
250
0.90
25
200
3.40
100
3.20
50
3.10
0
3.00
–50 –25
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
SWITCH CURRENT (A)
Switching Frequency
Soft-Start
2.0
800
740
720
700
680
660
640
1.8
700
SWITCH CURRENT LIMIT (A)
SWITCHING FREQUENCY (kHz)
780
600
500
400
300
200
100
620
25 50 75 100 125 150
TEMPERATURE (°C)
3493 G09
0
25 50 75 100 125 150
TEMPERATURE (°C)
3493 G08
Frequency Foldback
800
760
0
3493 G06
3493 G21
0
3.50
3.30
VIN (V)
600
–50 –25
3.60
150
30
30
25
3.80
TA = 85°C
0
20
20
Undervoltage Lockout
4.00
400
1.40
1.20
15
3493 G05
550
450
1.30
10
5
28
Switch Voltage Drop
1.60
15
MINIMUM
1.10
3493 G22
Maximum Load Current,
VOUT = 3.3V, L = 10μH
10
1.20
VIN (V)
3493 G04
5
1.30
0.90
0.90
28
TYPICAL
1.40
1.00
1.00
UVLO (V)
OUTPUT CURRENT (A)
TYPICAL
OUTPUT CURRENT (A)
Maximum Load Current,
VOUT = 3.3V, L = 4.7μH
1.60
1.50
FREQUENCY (kHz)
TA = 25°C unless otherwise noted.
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0
100 200 300 400 500 600 700 800
FEEDBACK VOLTAGE (mV)
3493 G11
0
0.25 0.50 0.75 1 1.25 1.50 1.75
SHDN PIN VOLTAGE (V)
2
3493 G13
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LT3493
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C unless otherwise noted.
Typical Minimum Input Voltage
(VOUT = 5V)
SHDN Pin Current
50
Typical Minimum Input Voltage
(VOUT = 3.3V)
5.5
7.5
45
5.3
TO START
40
5.1
7.0
4.9
25
20
6.5
VIN (V)
VIN (V)
30
TO RUN
15
10
4.5
4.1
TO RUN
3.9
5.5
3.7
5
0
2
4
6
8 10 12 14 16 18 20
VSHDN (V)
3.5
5.0
1
10
100
1000
1
10
100
1000
IOUT (mA)
IOUT (mA)
3493 G14
3493 G16
3493 G15
Switch Current Limit
Switch Current Limit
2.0
2.0
1.9
1.8
1.8
1.6
SWITCH CURRENT LIMIT (A)
0
TO START
4.7
4.3
6.0
SWITCH CURRENT LIMIT (A)
ISHDN (μA)
35
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.4
1.2
1.0
0.8
0.6
0.4
0.2
1.0
–50 –25
0
0
25 50 75 100 125 150
TEMPERATURE (°C)
0
20
60
40
DUTY CYCLE (%)
Operating Waveforms,
Discontinuous Mode
Operating Waveforms
VSW
5V/DIV
VSW
5V/DIV
IL
0.5A/DIV
0
VOUT
20mV/DIV
IL
0.5A/DIV
0
VOUT
20mV/DIV
1μs/DIV
100
3493 G18
3493 G17
VIN = 12V
VOUT = 3.3V
IOUT = 0.5A
L = 10μH
COUT = 10μF
80
3493 G19
VIN = 12V
VOUT = 3.3V
IOUT = 50mA
L = 10μH
COUT = 10μF
1μs/DIV
3493 G20
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LT3493
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 Information 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.
BLOCK DIAGRAM
5
VIN
VIN
C2
INT REG
AND
UVLO
ON OFF
SLOPE
COMP
R3
6
BOOST
3
R
Q
S
Q
D2
3
SHDN
C3
DRIVER
C4
Q1
SW
OSC
L1
VOUT
4
D1
FREQUENCY
FOLDBACK
VC
C1
gm
780mV
2
GND
1
R2
FB
R1
3493 BD
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LT3493
OPERATION
(Refer to Block Diagram)
The LT3493 is a constant frequency, current mode stepdown regulator. A 750kHz oscillator enables an RS flip-flop,
turning on the internal 1.75A power switch Q1. An amplifier
and comparator monitor the current flowing between the
VIN and SW pins, turning the switch off when this current
reaches a level determined by the voltage at VC. An error
amplifier measures the output voltage through an external
resistor divider tied to the FB pin and servos the VC node.
If the error amplifier’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 efficient 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 startup and overload.
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LT3493
APPLICATIONS INFORMATION
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:
V
R1= R2 OUT – 1
0.78V
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 ratings 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 =
VIN – VSW + 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:
VOUT + VD
DCMAX
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
IL
0.5A/DIV
VOUT + VD
VIN(MIN) =
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).
VOUT
200mV/DIV
AC COUPLED
COUT = 10μF
VOUT = 3V
VIN = 30V
ILOAD = 0.75A
L = 10μH
Figure 1
VSW
20V/DIV
The maximum input voltage is determined by the absolute
maximum ratings of the VIN and BOOST pins. For continuous mode operation, the maximum input voltage is
determined by the minimum duty cycle DCMIN = 0.10:
IL
0.5A/DIV
VIN(MAX) =
DCMIN
3493 F01
– VD + VSW
with DCMAX = 0.91 (0.88 over temperature).
VOUT + VD
2μs/DIV
– VD + 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
VOUT
200mV/DIV
AC COUPLED
COUT = 10μF
VOUT = 3V
VIN = 35V
ILOAD = 0.75A
L = 10μH
2μs/DIV
3493 F02
Figure 2
3493fb
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LT3493
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 fixed
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 waveforms 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 components 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.
VSW
20V/DIV
IL
0.5A/DIV
Inductor Selection and Maximum Output Current
A good first 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 efficiency 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 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 the maximum output current
and discontinuous mode operation, see Linear Technology
Application Note 44.
Catch Diode
VOUT
200mV/DIV
AC COUPLED
COUT = 10μF
VOUT = 3V
VIN = 40V
ILOAD = 0.75A
L = 10μH
2μs/DIV
Figure 3
3493 F03
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.
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LT3493
APPLICATIONS INFORMATION
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
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 applied 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 significant 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 supply 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 filters 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 resistance (ESR) and provide the best ripple performance.
A good value is:
COUT = 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 benefit (see the Compensation section).
For small size, the output capacitor can be chosen according to:
COUT = 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 connected between VOUT and the FB pin is required to filter
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 specified 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
3493fb
10
LT3493
APPLICATIONS INFORMATION
Table 2. Capacitor Vendors
VENDOR
PHONE
URL
PART SERIES COMMENTS
Panasonic
(714) 373-7366
www.panasonic.com
Ceramic,
Polymer,
Tantalum
Kemet
Sanyo
Murata
(864) 963-6300
(408) 749-9714
(404) 436-1300
AVX
Taiyo Yuden
(864) 963-6300
www.kemet.com
www.sanyovideo.com
Ceramic,
Tantalum
Ceramic,
Polymer,
Tantalum
www.murata.com
Ceramic
www.avxcorp.com
Ceramic,
Tantalum
www.taiyo-yuden.com
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 voltage. 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 efficiency. 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 BAT54). For lower output voltages the boost diode can be tied
to the input (Figure 5b). The circuit in Figure 5a is more
EEF Series
T494, T495
POSCAP
TPS Series
Ceramic
efficient 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 application 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.
3493fb
11
LT3493
APPLICATIONS INFORMATION
ILOAD
2A/DIV
VOUT
IL
0.5A/DIV
32.4k
FB
10μF
10k
VOUT
0.1V/DIV
AC COUPLED
40μs/DIV
3493 F04a
40μs/DIV
3493 F04b
40μs/DIV
3493 F04c
ILOAD
2A/DIV
VOUT
32.4k
IL
0.5A/DIV
3.3nF
10μF
s2
FB
10k
VOUT
0.1V/DIV
AC COUPLED
ILOAD
2A/DIV
VOUT
32.4k
IL
0.5A/DIV
+
FB
100μF
10k
SANYO
4TPB100M
VOUT
0.1V/DIV
AC COUPLED
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
D2
D2
C3
BOOST
VIN
VIN
C3
BOOST
LT3493
LT3493
SW
VOUT
GND
VIN
VIN
SW
VOUT
GND
3493 F05b
3493 F05a
VBOOST – VSW VOUT
MAX VBOOST VIN + VOUT
VBOOST – VSW VIN
MAX VBOOST 2VIN
(5a)
(5b)
Figure 5. Two Circuits for Generating the Boost Voltage
3493fb
12
LT3493
APPLICATIONS INFORMATION
5.5
7.5
5.3
TO START
7.0
5.1
VIN (V)
VIN (V)
4.9
6.5
TO START
4.7
4.5
4.3
6.0
TO RUN
4.1
TO RUN
3.9
5.5
3.7
3.5
5.0
1
10
100
1000
1
10
100
1000
IOUT (mA)
IOUT (mA)
3493 G16
3493 G15
(6a) Typical Minimum Input Voltage, VOUT = 5V
(6b) Typical Minimum Input Voltage, VOUT = 3.3V
Figure 6
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
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 filter 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 excessively, 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 applications 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 float 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 fine 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 capacitors can cause problems if the LT3493 is plugged into a
live supply (see Linear Technology Application Note 88 for
3493fb
13
LT3493
APPLICATIONS INFORMATION
VSW
10V/DIV
SHDN
RUN
IL
0.5A/DIV
GND
3493 F07a
VOUT
2V/DIV
VIN = 12V
VOUT = 3.3V
L = 10μH
COUT = 10μF
20μs/DIV
VIN = 12V
VOUT = 3.3V
L = 10μH
COUT = 10μF
20μs/DIV
VSW
10V/DIV
RUN
15k
SHDN
0.1μF
IL
0.5A/DIV
GND
3493 F07b
VOUT
2V/DIV
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
D4
VIN
VIN
BOOST
LT3493
SHDN
GND
VOUT
SW
FB
BACKUP
D4: MBR0540
3493 F08
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 first plot is the response with
3493fb
14
LT3493
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 filtering and can
slightly improve the efficiency of the circuit, though it is
likely to be the largest component in the circuit. An alternative 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 filtering. This solution is smaller
and less expensive than the electrolytic capacitor. For high
input voltages its impact on efficiency is minor, reducing
efficiency 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 simplifies loop compensation. In particular,
the LT3493 does not require the ESR of the output capacitor 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 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 current 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 amplifier 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
CLOSING SWITCH
SIMULATES HOT PLUG
IIN
VIN
DANGER!
LT3493
+
VIN
20V/DIV
2.2μF
LOW
IMPEDANCE
ENERGIZED
24V SUPPLY
IIN
5A/DIV
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
20μs/DIV
(9a)
LT3493
+
10μF
35V
AI.EI.
RINGING VIN MAY EXCEED
ABSOLUTE MAXIMUM
RATING OF THE LT3493
+
VIN
20V/DIV
2.2μF
IIN
5A/DIV
(9b)
20μs/DIV
1Ω
LT3493
+
0.1μF
VIN
20V/DIV
2.2μF
IIN
5A/DIV
(9c)
20μs/DIV
3493 F09
Figure 9. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT3493 is Connected to a Live Supply
3493fb
15
LT3493
APPLICATIONS INFORMATION
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
CURRENT MODE
POWER STAGE
SW
gm =
+1.6A/V
LT3493
–
0.7V
OUT
R1
–
gm =
300μA/V
RC
60k
CC
100pF
ERROR
AMPLIFIER
+
VC
CPL
FB
ESR
780mV
C1
+
C1
1M
R2
GND
3493 F10
Figure 10. Model for Loop Response
C2
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.
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
flow 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.
D1
SYSTEM
GROUND
VOUT
C1
VIN
SHDN
3493 F11
: VIAS TO LOCAL GROUND PLANE
: OUTLINE OF LOCAL GROUND PLANE
Figure 11. A Good PCB Layout Ensures Proper, Low EMI Operation
3493fb
16
LT3493
APPLICATIONS INFORMATION
High Temperature Considerations
Outputs Greater Than 6V
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 calculating the total power loss from an efficiency measurement 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.
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 Applications 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
0.78V Step-Down Converter
1N4148
VIN
3.6V TO 25V
VIN
BOOST
0.1μF 3.3μH
LT3493
ON OFF
SHDN
VOUT
0.78V
1.2A
SW
MBRM140
GND
FB
47μF
2.2μF
3493 TA02
1.8V Step-Down Converter
1N4148
VIN
3.6V TO 25V
VIN
BOOST
0.1μF
LT3493
ON OFF
SHDN
5μH
SW
MBRM140
GND
2.2μF
26.1k
FB
VOUT
1.8V
1.2A
22μF
20k
3493 TA03
3493fb
17
LT3493
TYPICAL APPLICATIONS
2.5V Step-Down Converter
BAT54
VIN
3.6V TO 28V
VIN
0.47μF 6.8μH
LT3493
ON OFF
VOUT
2.5V
1A, VIN > 5V
1.2A, VIN > 10V
BOOST
SHDN
SW
MBRM140
GND
22.1k
FB
22μF
10k
1μF
3493 TA04
3.3V Step-Down Converter
VIN
4.2V TO 36V
1N4148
VIN
0.1μF 8.2μH
LT3493
ON OFF
VOUT
3.3V
0.9A, VIN > 4.5V
1.2A, VIN > 12V
BOOST
SHDN
SW
MBRM140
GND
32.4k
FB
10μF
10k
1μF
3493 TA05
5V Step-Down Converter
VIN
6.4V TO 36V
1N4148
VIN
0.1μF 10μH
LT3493
ON OFF
SHDN
SW
MBRM140
GND
1μF
VOUT
5V
0.9A, VIN > 7V
1.1A, VIN > 14V
BOOST
59k
FB
10μF
11k
3493 TA06
3493fb
18
LT3493
PACKAGE DESCRIPTION
DCB Package
6-Lead Plastic DFN (2mm × 3mm)
(Reference LTC DWG # 05-08-1715)
0.70 ±0.05
3.55 ±0.05
1.65 ±0.05
(2 SIDES)
2.15 ±0.05
PACKAGE
OUTLINE
0.25 ± 0.05
0.50 BSC
1.35 ±0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
R = 0.115
TYP
2.00 ±0.10
(2 SIDES)
R = 0.05
TYP
3.00 ±0.10
(2 SIDES)
0.40 ± 0.10
4
6
1.65 ± 0.10
(2 SIDES)
PIN 1 NOTCH
R0.20 OR 0.25
s 45° CHAMFER
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
3
0.200 REF
1
(DCB6) DFN 0405
0.25 ± 0.05
0.50 BSC
0.75 ±0.05
1.35 ±0.10
(2 SIDES)
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
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
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
19
LT3493
TYPICAL APPLICATION
12V Step-Down Converter
D1
6V
1N4148
1k*
0.25W
VIN
14.5V TO 36V
VIN
BOOST
0.1μF
LT3493
SHDN
ON OFF
22μH
SW
MBRM140
GND
71.5k
FB
VOUT
12V
1A
4.7μF
4.99k
1μF
*FOR CONTINUOUS OPERATION ABOVE 30V
USE TWO 2k, 0.25Ω RESISTORS IN PARALLEL.
D1: CMDZ5235B
3493 TA07
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1766
60V, 1.2A IOUT , 200kHz, High Efficiency 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 Efficiency Step-Down
DC/DC Converter
LT1936
36V, 1.4A IOUT , 500kHz, High Efficiency Step-Down
DC/DC Converter
LT1940
25V, Dual 1.4A IOUT , 1.1MHz, High Efficiency 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 Efficiency 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 Efficiency 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.
3493fb
20 Linear Technology Corporation
LT 1108 REV B • PRINTED IN USA
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