LT3493
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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)
EF
FI
CI
EN
CY
(%
)
70
80
3493 TA01b
60
50
0.4 0.8 1.20.20 0.6 1.0
90
65
75
55
85
VIN = 12V
VOUT = 3.3V
L = 10μH
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输入范围:3.6~36V
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输入范围:3.6V~36V
输出电流:1.2A
固定操作频率:750KHz
输出可调到780mV
有短路保护
···
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较高的操作频率允许了微型低功耗的电感和瓷片电容。这样使得输出的波纹有明显改善。
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cycle-by-cycle current limiting
周期性即时电流限制
LT3493
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PIN CONFIGURATION ABSOLUTE MAXIMUM RATINGS
(Note 1)
TOP VIEW
SHDN
VIN
SW
FB
GND
BOOST
DCB PACKAGE
6-LEAD (2mm s 3mm) PLASTIC DFN
4
57
6
3
2
1
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 l 765 780 795 mV
FB Pin Bias Current VFB = Measured VREF + 10mV (Note 4) l 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 0.007 %/V
Switching Frequency VFB = 0.7V
VFB = 0V
685 750
36
815 kHz
kHz
Maximum Duty Cycle
TA = 25°C
l 88
91
95
95
%
%
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欠压锁定
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 2 μA
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
6
0.01
15
0.1
μA
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)
0
50
EF
FI
CI
EN
CY
(%
)
55
65
70
75
0.8 1.0
95
3493 G01
60
0.2 0.4 0.6 1.2
80
85
90
VIN = 8V
VIN = 12V
VIN = 24V
LOAD CURRENT (A)
0
50
EF
FI
CI
EN
CY
(%
)
55
65
70
75
0.8 1.0
3493 G02
60
0.2 0.4 0.6 1.2
80
85
90
VIN = 8V
VIN = 12V
VIN = 24V
LOAD CURRENT (A)
0
50
EF
FI
CI
EN
CY
(%
)
55
65
70
75
0.8 1.0
3493 G03
60
0.2 0.4 0.6 1.2
80
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)
8
1.60
1.50
1.40
1.30
1.20
1.10
1.00
0.90
20 28
3493 G04
12 16 24
OU
TP
UT
C
UR
RE
NT
(A
)
TYPICAL
MINIMUM
VIN (V)
8
1.60
1.50
1.40
1.30
1.20
1.10
1.00
0.90
20 28
3493 G22
12 16 24
OU
TP
UT
C
UR
RE
NT
(A
)
TYPICAL
MINIMUM
VIN (V)
5
1.40
1.50
1.60
25
3493 G05
1.30
1.20
10 15 20 30
1.10
1.00
0.90
OU
TP
UT
C
UR
RE
NT
(A
) TYPICAL
MINIMUM
VIN (V)
5
1.40
1.50
1.60
25
3493 G21
1.30
1.20
10 15 20 30
1.10
1.00
0.90
OU
TP
UT
C
UR
RE
NT
(A
) TYPICAL
MINIMUM
SWITCH CURRENT (A)
0
V C
E(
SW
) (
m
V)
150
450
500
550
0.4 0.8 1.0
3493 G06
50
350
250
100
400
0
300
200
0.2 0.6 1.41.2 1.6 1.8
TA = 25°C
TA = 85°C
TA = –40°C
TEMPERATURE (°C)
UV
LO
(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 15050
TEMPERATURE (°C)
FR
EQ
UE
NC
Y
(k
Hz
)
720
760
800
125
3493 G09
680
640
700
740
780
660
620
600
–25–50 250 75 100 15050
FEEDBACK VOLTAGE (mV)
0
SW
IT
CH
IN
G
FR
EQ
UE
NC
Y
(k
Hz
)
400
600
800
3493 G11
200
0
200 400 600100 300 500 700
800
300
500
100
700
SHDN PIN VOLTAGE (V)
0
0
SW
IT
CH
C
UR
RE
NT
L
IM
IT
(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)
0
I S
H
D
N
(μ
A) 30
40
50
16
3493 G14
20
10
25
35
45
15
5
0
42 86 12 14 1810 20
IOUT (mA)
1
5.0
V I
N
(V
) 6.5
7.0
7.5
10 100 1000
3493 G15
6.0
5.5
TO START
TO RUN
IOUT (mA)
1
4.3
V I
N
(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
SW
IT
CH
C
UR
RE
NT
L
IM
IT
(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 (%)
0
SW
IT
CH
C
UR
RE
NT
L
IM
IT
(A
)
1.2
1.6
2.0
80
3493 G18
0.8
0.4
1.0
1.4
1.8
0.6
0.2
0
20 40 60 100
VSW
5V/DIV
IL
0.5A/DIV
0
VOUT
20mV/DIV
VIN = 12V
VOUT = 3.3V
IOUT = 0.5A
L = 10μH
COUT = 10μF
1μs/DIV 3493 G19
VSW
5V/DIV
IL
0.5A/DIV
0
VOUT
20mV/DIV
1μs/DIV 3493 G20VIN = 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.
1
3
R
DRIVER Q1
S
OSC
SLOPE
COMP
FREQUENCY
FOLDBACK
INT REG
AND
UVLO
VC
gm
780mV
3493 BD
2
5
6
Q
Q
3
4
BOOST
SW
FB
R2 R1
VOUT
L1
D2
C3
C1D1
VIN
C2
VIN
ON OFF
GND
C4
R3
SHDN
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SHDN模式提供了软启动的功能。
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保护二极管
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.
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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
VOUT
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 – 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:
VIN(MIN) =
VOUT + VD
DCMAX
– VD + 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
– 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
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
IL
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
IL
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
IL
0.5A/DIV
Figure 3
LT3493
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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