LT1933ES6-PBF [Linear]
600mA, 500kHz Step-Down Switching Regulator in SOT-23 and DFN Packages; 600毫安，为500kHz降压型开关稳压器采用SOT -23和DFN封装
| 型号： | LT1933ES6-PBF |
| 厂家： | 
Linear |
| 描述： | 600mA, 500kHz Step-Down Switching Regulator in SOT-23 and DFN Packages |
| 文件： | 总20页 (文件大小：360K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT1933
600mA, 500kHz Step-Down
Switching Regulator in SOT-
23 and DFN Packages
DESCRIPTION
The LT®1933 is a current mode PWM step-down DC/DC
converter with an internal 0.75A power switch, packaged
in a tiny 6-lead SOT-23. The wide input range of 3.6V
to 36V makes the LT1933 suitable 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.
FEATURES
n
Wide Input Range: 3.6V to 36V
n
5V at 600mA from 16V to 36V Input
n
3.3V at 600mA from 12V to 36V Input
n
5V at 500mA from 6.3V to 36V Input
n
3.3V at 500mA from 4.5V to 36V Input
n
Fixed Frequency 500kHz Operation
n
Uses Tiny Capacitors and Inductors
n
Soft-Start
Internally Compensated
n
n
Low Shutdown Current: <2μA
Output Adjustable Down to 1.25V
Low Profile (1mm) SOT-23 (ThinSOT™) and
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
providesoutputdisconnect,enablingeasypowermanage-
ment in battery-powered systems.
n
n
(2mm x 3mm x 0.75mm) 6-Pin DFN Packages
APPLICATIONS
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. ThinSOT is
a trademark of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
n
Automotive Battery Regulation
n
Industrial Control Supplies
n
Wall Transformer Regulation
Distributed Supply Regulation
n
n
Battery-Powered Equipment
TYPICAL APPLICATION
3.3V Step-Down Converter
Efficiency
95
V
IN
= 12V
1N4148
V
IN
V
BOOST
LT1933
IN
90
85
80
75
70
65
4.5V TO 36V
V
= 5V
OUT
0.1μF
22μH
V
OUT
OFF ON
SHDN
SW
FB
3.3V/500mA
V
OUT
= 3.3V
GND
MBRM140
16.5k
2.2μF
22μF
10k
1933 TA01a
100
200
LOAD CURRENT (mA)
500
600
0
300
400
1933 TA01b
1933fd
1
LT1933
ABSOLUTE MAXIMUM RATINGS
(Note 1)
LT1933I ................................................. –40°C to 125°C
LT1933H................................................. –40°C to 150°C
Maximum Junction Temperature
Input Voltage (V ).....................................–0.4V to 36V
IN
BOOST Pin Voltage ..................................................43V
BOOST Pin Above SW Pin.........................................20V
SHDN Pin................................................... –0.4V to 36V
FB Voltage.................................................... –0.4V to 6V
Operating Temperature Range (Note 2)
LT1933E, LT1933I ................................................ 125°C
LT1933H................................................................ 150°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature, S6 Package
LT1933E .................................................. –40°C to 85°C
(Soldering, 10 sec) ........................................... 300°C
PIN CONFIGURATION
TOP VIEW
TOP VIEW
6
5
4
FB
BOOST
1
2
3
BOOST 1
GND 2
FB 3
6 SW
5 V
7
V
IN
GND
SHDN
IN
SW
4 SHDN
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
= 165°C/W, θ = 102°C/W
DCB PACKAGE
6-LEAD (2mm × 3mm) PLASTIC DFN
θ
JA
JC
θ
JA
= 73.5°C/W, θ = 12°C/W
JC
EXPOSED PAD (PIN 7) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT1933IDCB#PBF
LT1933HDCB#PBF
LT1933ES6#PBF
LT1933IS6#PBF
LT1933HS6#PBF
LEAD BASED FINISH
LT1933IDCB
TAPE AND REEL
LT1933IDCB#TRPBF
LT1933HDCB#TRPBF
LT1933ES6#TRPBF
LT1933IS6#TRPBF
LT1933HS6#TRPBF
TAPE AND REEL
LT1933IDCB#TR
LT1933HDCB#TR
LT1933ES6#TR
PART MARKING
LCGM
PACKAGE DESCRIPTION
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 150°C
–40°C to 85°C
6-Lead (2mm × 3mm) Plastic DFN
6-Lead (2mm × 3mm) Plastic DFN
6-Lead Plastic TSOT-23
LCGN
LTAGN
LTAGP
6-Lead Plastic TSOT-23
–40°C to 125°C
–40°C to 150°C
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 150°C
–40°C to 85°C
LTDDQ
6-Lead Plastic TSOT-23
PART MARKING
LCGM
PACKAGE DESCRIPTION
6-Lead (2mm × 3mm) Plastic DFN
6-Lead (2mm × 3mm) Plastic DFN
6-Lead Plastic TSOT-23
LT1933HDCB
LCGN
LT1933ES6
LTAGN
LT1933IS6
LT1933IS6#TR
LTAGP
6-Lead Plastic TSOT-23
–40°C to 125°C
–40°C to 150°C
LT1933HS6
LT1933HS6#TR
LTDDQ
6-Lead Plastic TSOT-23
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/
1933fd
2
LT1933
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
3.35
1.245
40
MAX
3.6
UNITS
V
Undervoltage Lockout
Feedback Voltage
l
l
1.225
1.265
120
2.5
V
FB Pin Bias Current
Quiescent Current
V
FB
= Measured V + 10mV (Note 4)
nA
REF
Not Switching
1.6
mA
μA
Quiescent Current in Shutdown
Reference Line Regulation
Switching Frequency
V
V
V
V
= 0V
0.01
0.01
500
55
2
SHDN
= 5V to 36V
= 1.1V
%/V
kHz
kHz
%
IN
FB
FB
400
600
= 0V
l
Maximum Duty Cycle
Switch Current Limit
88
94
(Note 3)
0.75
1.05
A
Switch V
I
SW
I
SW
= 400mA, S6 Package
= 400mA, DCB6 Package
370
370
500
mV
mV
CESAT
Switch Leakage Current
Minimum Boost Voltage Above Switch
BOOST Pin Current
2
μA
V
I
I
= 400mA
= 400mA
1.9
18
2.3
25
SW
mA
V
SW
SHDN Input Voltage High
SHDN Input Voltage Low
SHDN Bias Current
2.3
0.3
V
V
V
= 2.3V (Note 5)
= 0V
34
0.01
50
0.1
μA
μA
SHDN
SHDN
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.
guaranteed over the –40°C to 125°C temperature range. The LT1933H
specifications are guaranteed over the –40°C to 150°C temperature range.
Note 3: Current limit guaranteed by design and/or correlation to static test.
Slope compensation reduces current limit at higher duty cycle.
Note 2: The LT1933E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LT1933I specifications are
Note 4: Current flows out of pin.
Note 5: Current flows into pin.
1933fd
3
LT1933
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency, VOUT = 5V
Efficiency, VOUT = 3.3V
Switch Current Limit
100
90
80
70
60
100
90
80
70
60
1200
1000
800
600
400
200
0
T
= 25°C
OUT
T = 25°C
A
T
= 25°C
A
A
V
= 5V
V
= 3.3V
OUT
TYPICAL
V
IN
= 12V
V
IN
= 5V
MINIMUM
V
IN
= 12V
V
IN
= 24V
V
IN
= 24V
D1 = MBRM140
L1 = Toko D53LCB 33μH
D1 = MBRM140
L1 = Toko D53LCB 22μH
100
200
LOAD CURRENT (mA)
500
600
100
200
LOAD CURRENT (mA)
500
600
0
300
400
0
300
400
0
20
40
60
80
100
DUTY CYCLE (%)
1933 G01
1933 G02
1933 G03
Maximum Load Current
Maximum Load Current
Switch Voltage Drop
800
700
600
500
400
600
500
400
300
200
100
0
800
700
600
500
400
T
= 25°C
OUT
T
V
= 25°C
= 5V
A
A
OUT
V
= 3.3V
T
= 25°C
A
L = 22μH
L = 33μH
T
= 85°C
A
T
= –40°C
A
L = 15μH
L = 22μH
0
5
10
15
20
25
30
0
0.1
0.2
0.3
0.4
0.5
0.6
0
5
10
15
20
25
30
SWITCH CURRENT (A)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
1933 G05
1933 G06
1933 G04
Feedback Voltage
Undervoltage Lockout
Switching Frequency
1.260
1.255
1.250
1.245
1.240
1.235
1.230
3.8
3.6
3.4
3.2
3.0
600
550
500
450
400
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1933 G07
1933 G08
1933 G09
1933fd
4
LT1933
TYPICAL PERFORMANCE CHARACTERISTICS
Frequency Foldback
Soft-Start
SHDN Pin Current
700
600
500
400
300
200
100
0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
200
150
100
50
T
= 25°C
T
= 25°C
T = 25°C
A
A
A
DC = 30%
0
0.0
0.5
1.0
1.5
0
1
2
3
4
0
4
8
12
16
FB PIN VOLTAGE (V)
SHDN PIN VOLTAGE (V)
SHDN PIN VOLTAGE (V)
1933 G10
1933 G11
1933 G12
Typical Minimum Input Voltage
Typical Minimum Input Voltage
Switch Current Limit
8
7
6
5
4
6.0
5.5
5.0
4.5
4.0
3.5
3.0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
V
T
= 5V
V
T
= 3.3V
OUT
A
OUT
A
= 25°C
= 25°C
L = 33μH
L = 22μH
TO START
TO START
TO RUN
TO RUN
1
10
100
1
10
100
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
1933 G13
1933 G14
1933 G15
Operating Waveforms,
Discontinuous Mode
Operating Waveforms
V
SW
10V/DIV
V
10V/DIV
SW
I
L
200mA/DIV
I
200mA/DIV
L
V
10mV/DIV
V
OUT
10mV/DIV
OUT
1933 G16
1933 G17
V
= 12V, V
= 3.3V, I
= 22μF
= 400mA,
V
= 12V, V
= 3.3V, I
= 22μF
= 20mA,
OUT
IN
OUT
OUT
OUT
IN
OUT
OUT
L = 22μH, C
L = 22μH, C
1933fd
5
LT1933
PIN FUNCTIONS (SOT-23/DFN)
BOOST (Pin 1): The BOOST pin is used to provide a drive
voltage,higherthantheinputvoltage,totheinternalbipolar
NPN power switch.
SHDN (Pin 4): The SHDN pin is used to put the LT1933 in
shutdown mode. Tie to ground to shut down the LT1933.
Tie to 2.3V or more for normal operation. If the shutdown
feature is not used, tie this pin to the V pin. SHDN also
IN
GND (Pin 2/Pin 5 and Exposed Pad, Pin 7): Tie the
GND pin to a local ground plane below the LT1933 and
the circuit components. Return the feedback divider to
this pin.
provides a soft-start function; see the Applications Infor-
mation section.
V
(Pin 5/Pin 2): The V pin supplies current to the
IN
IN
LT1933’s internal regulator and to the internal power
FB (Pin 3/Pin 6): The LT1933 regulates its feedback pin to
1.245V. Connect the feedback resistor divider tap to this
pin. Set the output voltage according to V
(1 + R1/R2). A good value for R2 is 10k.
switch. This pin must be locally bypassed.
= 1.245V
SW (Pin 6): The SW pin is the output of the internal power
switch. Connect this pin to the inductor, catch diode and
boost capacitor.
OUT
BLOCK DIAGRAM
V
IN
V
IN
C2
INT REG
AND
UVLO
D2
BOOST
Σ
ON OFF
SLOPE
COMP
R
S
Q
R3
SHDN
C3
Q
DRIVER
Q1
C4
L1
SW
OSC
V
OUT
C1
D1
FREQUENCY
FOLDBACK
V
C
g
m
1.245V
GND
FB
R2
R1
1933 BD
1933fd
6
LT1933
OPERATION (Refer to Block Diagram)
The LT1933 is a constant frequency, current mode step
down regulator. A 500kHz oscillator enables an RS flip-
flop, turning on the internal 750mA power switch Q1. An
amplifier and comparator monitor the current flowing
An internal regulator provides power to the control cir-
cuitry. This regulator includes an undervoltage lockout
to prevent switching when V is less than ~3.35V. The
IN
SHDN pin is used to place the LT1933 in shutdown, dis-
connecting the output and reducing the input current to
less than 2μA.
between the V and SW pins, turning the switch off when
IN
this current reaches a level determined by the voltage at
V .Anerroramplifiermeasurestheoutputvoltagethrough
C
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 opera-
tion.
an external resistor divider tied to the FB pin and servos
the V node. If the error amplifier’s output increases, more
C
current is delivered to the output; if it decreases, less cur-
rent is delivered. An active clamp (not shown) on the V
C
node provides current limit. The V node is also clamped
C
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.
The oscillator reduces the LT1933’s operating frequency
when the voltage at the FB pin is low. This frequency
foldbackhelpstocontroltheoutputcurrentduringstartup
and overload.
APPLICATIONS INFORMATION
(~0.4V at maximum load). This leads to a minimum input
voltage of:
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:
V
= (V
+ V )/DC
– V + V
MAX D SW
IN(MIN)
OUT
D
with DC
= 0.88
MAX
R1 = R2(V /1.245 – 1)
OUT
The maximum input voltage is determined by the absolute
maximum ratings of the V and BOOST pins and by the
R2 should be 20k or less to avoid bias current errors.
Reference designators refer to the Block Diagram.
IN
MIN
minimum duty cycle DC
= 0.08 (corresponding to a
minimum on time of 130ns):
Input Voltage Range
V
= (V + V )/DC
– V + V
D SW
IN(MAX)
OUT
D
MIN
The input voltage range for LT1933 applications depends
on the output voltage and on the absolute maximum rat-
Notethatthisisarestrictionontheoperatinginputvoltage;
the circuit will tolerate transient inputs up to the absolute
ings of the V and BOOST pins.
IN
maximum ratings of the V and BOOST pins.
IN
The minimum input voltage is determined by either the
LT1933’s minimum operating voltage of ~3.35V, 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:
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L = 5 (V
+ V )
D
OUT
where V is the voltage drop of the catch diode (~0.4V)
DC = (V
+ V )/(V – V + V )
D
OUT
D
IN
SW
D
and L is in μH. With this value the maximum load current
will be above 500mA. The inductor’s RMS current rating
must be greater than your maximum load current and its
where V is the forward voltage drop of the catch diode
D
(~0.4V) and V is the voltage drop of the internal switch
SW
1933fd
7
LT1933
APPLICATIONS INFORMATION
saturation current should be about 30% higher. For robust
operation in fault conditions the saturation current should
be~1A.Tokeepefficiencyhigh,theseriesresistance(DCR)
should be less than 0.2Ω. Table 1 lists several vendors
and types that are suitable.
Catch Diode
A0.5Aor1ASchottkydiodeisrecommendedforthecatch
diode, D1. The diode must have a reverse voltage rating
equal to or greater than the maximum input voltage. The
ON Semiconductor MBR0540 is a good choice; it is rated
for 0.5A forward current and a maximum reverse voltage
of 40V. The MBRM140 provides better efficiency, and will
handle extended overload conditions.
Of course, such a simple design guide will not always re-
sult in the optimum inductor for your application. A larger
value provides a slightly higher maximum load current,
and will reduce the output voltage ripple. If your load is
lower than 500mA, 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 OK, but
further reduces maximum load current. For details of
maximum output current and discontinuous mode opera-
tion, see Linear Technology Application Note 44. Finally,
Input Capacitor
Bypass the input of the LT1933 circuit with a 2.2μF 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 2.2μF ceramic
is adequate to bypass the LT1933 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 sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT1933 and to force this very high frequency
for duty cycles greater than 50% (V /V > 0.5), there
OUT IN
is a minimum inductance required to avoid subharmonic
oscillations. Choosing L greater than 3(V
+ V ) μH
OUT
D
prevents subharmonic oscillations at all duty cycles.
Table 1.Inductor Vendors
Vendor
URL
Part Series
D01608C
MSS5131
MSS6122
CR43
Inductance Range (μH)
10 to 22
Size (mm)
Coilcraft
www.coilcraft.com
2.9 × 4.5 × 6.6
3.1 × 5.1 × 5.1
2.2 × 6.1 × 6.1
3.5 × 4.3 × 4.8
3.0 × 5.0 × 5.0
3.0 × 5.7 × 5.7
2.0 × 5.0 × 5.0
3.0 × 5.0 × 5.0
2.8 × 4.8 × 4.8
2.9 × 4.5 × 6.6
3.2 × 4.0 × 4.5
10 to 22
10 to 33
Sumida
www.sumida.com
10 to 22
CDRH4D28
CDRH5D28
D52LC
10 to 33
22 to 47
Toko
www.toko.com
10 to 22
D53LC
22 to 47
Würth Elektronik
www.we-online.com
WE-TPC MH
WE-PD4 S
WE-PD2 S
10 to 22
10 to 22
10 to 47
1933fd
8
LT1933
APPLICATIONS INFORMATION
switching current into a tight local loop, minimizing EMI.
A 2.2μF capacitor is capable of this task, but only if it is
placed close to the LT1933 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 LT1933. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT1933 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT1933’s
voltage rating. This situation is easily avoided; see the Hot
Plugging Safely section.
Ceramic capacitors have very low equivalent series re-
sistance (ESR) and provide the best ripple performance.
A good value is
C
= 60/V
OUT
OUT
where C
is in μF. Use X5R or X7R types, and keep
OUT
in mind that a ceramic capacitor biased with V
will
OUT
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).
Output Capacitor
High performance electrolytic capacitors can be used for
theoutputcapacitor. LowESRisimportant, sochooseone
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
capacitor must be large to achieve low ESR. Table 2 lists
several capacitor vendors.
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated
by the LT1933 to produce the DC output. In this role it
determines the output ripple, and low impedance at the
switching frequency is important. The second function
is to store energy in order to satisfy transient loads and
stabilize the LT1933’s control loop.
Table 2.Inductor Vendors
Vendor
Phone
URL
Part Series
Comments
Panasonic
(714) 373-7366
www.panasonic.com
Ceramic,
Polymer,
Tantalum
EEF Series
Kemet
Sanyo
(864) 963-6300
(408) 749-9714
www.kemet.com
Ceramic,
Tantalum
T494, T495
POSCAP
www.sanyovideo.com
Ceramic,
Polymer,
Tantalum
Murata
AVX
(404)436-1300
(864)963-6300
www.murata.com
www.avxcorp.com
Ceramic
Ceramic,
Tantalum
TPS Series
Taiyo Yuden
www.taiyo-yuden.com
Ceramic
1933fd
9
LT1933
APPLICATIONS INFORMATION
Figure 1 shows the transient response of the LT1933 with
several output capacitor choices. The output is 3.3V. The
loadcurrentissteppedfrom100mAto400mAandbackto
100mA, 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
leadcapacitor.Thelastphotoshowstheresponsetoahigh
performanceelectrolyticcapacitor. Transientperformance
isimprovedduetothelargeoutputcapacitance,butoutput
ripple(asshownbythebroadtrace)hasincreasedbecause
of the higher ESR of this capacitor.
V
OUT
V
OUT
50mV/DIV
16.5k
FB
22μF
10k
I
OUT
200mA/DIV
1933 F01a
V
OUT
V
OUT
50mV/DIV
470pF
16.5k
FB
22μF
2x
10k
I
OUT
200mA/DIV
1933 F01b
V
OUT
V
OUT
50mV/DIV
16.5k
FB
+
100μF
10k
SANYO
4TPB100M
I
OUT
200mA/DIV
1933 F01c
Figure 1. Transient Load Response of the LT1933 with Different
Output Capacitors as the Load Current is Stepped from 100mA
to 400mA. VIN = 12V, VOUT = 3.3V, L = 22μH.
1933fd
10
LT1933
APPLICATIONS INFORMATION
BOOST Pin Considerations
The minimum operating voltage of an LT1933 application
is limited by the undervoltage lockout (~3.35V) and by
the maximum duty cycle as outlined above. For proper
startup, the minimum input voltage is also limited by the
boost circuit. If the input voltage is ramped slowly, or the
LT1933 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 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 3 shows a plot of minimum load
to start and to run as a function of input voltage. In many
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 2 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 3V and above, the standard circuit (Figure 2a)
is best. For outputs between 2.5V and 3V, use a 0.47μF
capacitorandasmallSchottkydiode(suchastheBAT-54).
For lower output voltages the boost diode can be tied to
the input (Figure 2b). The circuit in Figure 2a is more ef-
ficientbecausetheBOOSTpincurrentcomesfromalower
voltage source. You must also be sure that the maximum
voltage rating of the BOOST pin is not exceeded.
D2
D2
C3
C3
BOOST
LT1933
BOOST
LT1933
V
IN
V
OUT
V
IN
V
OUT
V
IN
SW
V
IN
SW
GND
GND
1933 F02b
1933 F02a
V
– V ≅ V
V
– V ≅ V
BOOST
BOOST
SW
OUT
BOOST
SW
IN
IN
MAX V
≅ V + V
MAX V
≅ 2V
BOOST
IN
OUT
(2a)
(2b)
Figure 2. Two Circuits for Generating the Boost Voltage
Minimum Input Voltage VOUT = 3.3V
Minimum Input Voltage VOUT = 5V
6.0
5.5
5.0
4.5
4.0
3.5
3.0
6.0
5.5
5.0
4.5
4.0
3.5
3.0
V
T
= 5V
V
T
= 3.3V
OUT
A
OUT
A
= 25°C
= 25°C
L = 22μH
L = 22μH
TO START
TO START
TO RUN
TO RUN
1
10
100
1
10
100
LOAD CURRENT (mA)
LOAD CURRENT (mA)
1933 F03b
1933 F03a
Figure 3. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
1933fd
11
LT1933
APPLICATIONS INFORMATION
cases the discharged output capacitor will present a load
Soft-Start
to the switcher which will allow it to start. The plots show
TheSHDNpincanbeusedtosoft-starttheLT1933,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 4 shows the start up waveforms
with and without the soft-start circuit. By choosing a large
RCtimeconstant, thepeakstartupcurrentcanbereduced
to the current that is required to regulate the output, with
no overshoot. Choose the value of the resistor so that it
can supply 60μA when the SHDN pin reaches 2.3V.
theworst-casesituationwhereV isrampingveryslowly.
IN
For lower start-up voltage, the boost diode can be tied to
V ; however, this restricts the input range to one-half of
IN
the absolute maximum rating of the BOOST pin.
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
300mV above V . At higher load currents, the inductor
OUT
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT1933, requiring a higher
input voltage to maintain regulation.
RUN
5V/DIV
RUN
SHDN
GND
I
IN
1933 F04a
100mA/DIV
V
OUT
5V/DIV
50μs/DIV
RUN
15k
RUN
5V/DIV
SHDN
GND
0.1μF
I
IN
100mA/DIV
1933 F04b
V
OUT
5V/DIV
0.5ms/DIV
Figure 4. To Soft-Start the LT1933, Add a Resistor and Capacitor to
the SHDN Pin. VINI = 12V, VOUT = 3.3V, COUT = 22μF, RLOAD = 10Ω
1933fd
12
LT1933
APPLICATIONS INFORMATION
Shorted and Reversed Input Protection
and the V pin. Figure 5 shows a circuit that will run only
IN
whentheinputvoltageispresentandthatprotectsagainst
If the inductor is chosen so that it won’t saturate exces-
sively, an LT1933 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
LT1933 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 LT1933’s
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
bypasscapacitorofLT1933circuits.However,thesecapaci-
tors can cause problems if the LT1933 is plugged into a
live supply (see Linear Technology Application Note 88 for
a complete discussion). The low loss ceramic capacitor
combined with stray inductance in series with the power
sourceformsanunderdampedtankcircuit,andthevoltage
output. If the V pin is allowed to float and the SHDN pin
IN
is held high (either by a logic signal or because it is tied
to V ), then the LT1933’s internal circuitry will pull its
IN
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
at the V pin of the LT1933 can ring to twice the nominal
IN
input voltage, possibly exceeding the LT1933’s rating and
damaging the part. If the input supply is poorly controlled
or the user will be plugging the LT1933 into an energized
supply, the input network should be designed to prevent
this overshoot.
zero. However, if the V pin is grounded while the output
IN
is held high, then parasitic diodes inside the LT1933 can
pull large currents from the output through the SW pin
D4
V
V
IN
BOOST
LT1933
IN
V
OUT
SHDN
SW
FB
GND
BACKUP
1933 F05
D4: MBR0540
Figure 5. 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 LT1933 Rns Only When the Input is Present
1933fd
13
LT1933
APPLICATIONS INFORMATION
CLOSING SWITCH
SIMULATES HOT PLUG
I
IN
V
IN
DANGER!
LT1933
2.2μF
V
IN
20V/DIV
RINGING V MAY EXCEED
IN
ABSOLUTE MAXIMUM
RATING OF THE LT1933
+
I
IN
5A/DIV
LOW
STRAY
IMPEDANCE
ENERGIZED
24V SUPPLY
INDUCTANCE
20μs/DIV
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
(6a)
V
LT1933
2.2μF
IN
20V/DIV
+
+
10μF
35V
AI.EI.
I
IN
5A/DIV
20μs/DIV
(6b)
1ꢀ
V
LT1933
2.2μF
IN
20V/DIV
+
0.1μF
I
IN
5A/DIV
1933 F06
20μs/DIV
(6c)
Figure 6. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT1933 is Connected to a Live Supply
1933fd
14
LT1933
APPLICATIONS INFORMATION
Figure 6 shows the waveforms that result when an LT1933
circuit is connected to a 24V supply through six feet of
24-gauge twisted pair. The first plot is the response with
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 6b
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
likelytobethelargestcomponentinthecircuit. Analterna-
tive solution is shown in Figure 6c. 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.
Figure7showsanequivalentcircuitfortheLT1933control
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 cur-
rent proportional to the voltage at the V node. Note that
C
the output capacitor integrates this current, and that the
capacitor on the V node (C ) integrates the error ampli-
C
C
fier output current, resulting in two poles in the loop. R
C
provides a zero. With the recommended output capacitor,
theloopcrossoveroccursabovetheR C zero.Thissimple
C C
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
phaseleadcapacitor(C )acrossthefeedbackdividermay
PL
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.TheLT1375datasheetcontainsamorethorough
discussion of loop compensation and describes how to
test the stability using a transient load.
Frequency Compensation
The LT1933 uses current mode control to regulate the
output. This simplifies loop compensation. In particular,
the LT1933 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.
CURRENT MODE
POWER STAGE
SW
LT1933
–
+
0.7V
g
m
OUT
C
PL
1.1mho
C
R1
R2
–
FB
g
=
V
m
150μmhos
ESR
+
1.245V
R
C
C1
ERROR
+
100k
AMPLIFIER
C1
C
C
500k
80pF
GND
1933 F07
Figure 7. Model for Loop Response
1933fd
15
LT1933
APPLICATIONS INFORMATION
PCB Layout
unbroken ground plane below these components, and tie
thisgroundplanetosystemgroundatonelocation, 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
traceswillshielditfromtheSWandBOOSTnodes.Include
two vias near the GND pin of the LT1933 to help remove
heat from the LT1933 to the ground plane.
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 8 shows
the recommended component placement with trace,
ground plane and via locations. Note that large, switched
currents flow in the LT1933’s V and SW pins, the catch
IN
diode (D1) and the input capacitor (C2). The loop formed
by these components should be as small as possible and
tiedtosystemgroundinonlyoneplace.Thesecomponents,
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,
Figure 8a shows the layout for the DFN package. Vias
near and under the exposed die attach paddle minimize
the thermal resistance of the LT1933.
V
OUT
V
IN
C1
D1
C2
GND
1933 F08a
VIAS
(8a)
DFN Package
SHUTDOWN
V
IN
C1
V
OUT
SYSTEM
GROUND
C2
D1
1933 F08b
VIAS
OUTLINE OF LOCAL GROUND PLANE
(8b)
SOT-23 Package
Figure 8. A Good PCB Layout Ensures Proper, Low EMI Operation
1933fd
16
LT1933
TYPICAL APPLICATIONS
3.3V Step-Down Converter
D2
V
IN
V
IN
BOOST
LT1933
4.5V TO
36V
C3
0.1μF
L1
22μH
V
OUT
3.3V/
OFF ON
SHDN
SW
FB
500mA
GND
R1
16.5k
D1
C2
2.2μF
C1
22μF
6.3V
R2
10k
1933 TA02b
12V Step-Down Converter
D3, 6V
D2
V
IN
14.5V TO
36V
V
IN
BOOST
LT1933
C3
0.1μF
L1
47μH
V
OUT
OFF ON
SHDN
SW
FB
12V/
450mA
GND
R1
86.6k
D1
C2
2.2μF
C1
10μF
R2
10k
1933 TA02d
1933fd
17
LT1933
PACKAGE DESCRIPTION
DCB Package
6-Lead Plastic DFN (2mm × 3mm)
(Reference LTC DWG # 05-08-1715)
0.70 ±0.05
1.65 ±0.05
3.55 ±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)
0.40 ± 0.10
R = 0.05
TYP
4
6
3.00 ±0.10 1.65 ± 0.10
(2 SIDES)
(2 SIDES)
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
PIN 1 NOTCH
R0.20 OR 0.25
× 45° CHAMFER
(DCB6) DFN 0405
3
1
0.25 ± 0.05
0.50 BSC
0.75 ±0.05
0.200 REF
1.35 ±0.10
(2 SIDES)
BOTTOM VIEW—EXPOSED PAD
0.00 – 0.05
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
1933fd
18
LT1933
PACKAGE DESCRIPTION
S6 Package
6-Lead Plastic SOT-23
(Reference LTC DWG # 05-08-1634)
2.80 – 3.10
(NOTE 4)
0.62
MAX
0.95
REF
1.22 REF
1.50 – 1.75
(NOTE 4)
2.60 – 3.00
1.4 MIN
3.85 MAX 2.62 REF
PIN ONE ID
0.25 – 0.50
TYP 6 PLCS
NOTE 3
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
0.95 BSC
0.90 – 1.30
0.20 BSC
DATUM ‘A’
0.90 – 1.45
0.35 – 0.55 REF
1.90 BSC
0.09 – 0.15
0.09 – 0.20
(NOTE 3)
NOTE 3
NOTE:
S6 SOT-23 0502
1. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
3. DIMENSIONS ARE INCLUSIVE OF PLATING
ATTENTION: ORIGINAL SOT23-6L PACKAGE.
4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR
5. MOLD FLASH SHALL NOT EXCEED 0.254mm
6. PACKAGE EIAJ REFERENCE IS SC-74A (EIAJ)
MOST SOT23-6L PRODUCTS CONVERTED TO THIN SOT23
PACKAGE, DRAWING # 05-08-1636 AFTER APPROXIMATELY
APRIL 2001 SHIP DATE
1933fd
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.
19
LT1933
TYPICAL APPLICATION
2.5V Step-Down Converter
D2
V
IN
V
IN
BOOST
LT1933
3.6V TO 36V
C3
0.47μF
L1
15μH
V
OUT
OFF ON
SHDN
SW
FB
2.5V/500mA
GND
R1
10.5k
D1
C2
2.2μF
C1
22μF
R2
10k
1933 TA03
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
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60V, 1.2A I , 200kHz, High Efficiency Step-Down DC/DC V : 5.5V to 60V, V
= 1.2V, I = 2.5mA, I = 25μA,
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S8, TSSOP16/TSSOP16E Packages
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S8, N8, S8 Packages
25V, Dual 1.4A I , 1.1MHz, High Efficiency Step-Down
V : 3.6V to 25V, V
= 1.25V, I = 3.8mA, I = <30μA,
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60V, Dual 1.2A I , 500kHz, High Efficiency Step-Down
V : 5.5V to 60V, V
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TSSOP16/TSSOP16E Packages
60V, Dual 1.2A I , 200kHz, High Efficiency Step-Down
V : 3.3V to 60V, V = 1.2V, I = 100μA, I = <1μA,
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TSSOP16E Package
80V, 50mA, Low Noise Linear Regulator
V : 1.5V to 80V, V
= 1.28V, I = 30μA, I = <1μA,
IN
OUT(MIN) Q SD
MS8E Package
Dual 600mA I , 1.5MHz, Synchronous Step-Down
V : 2.5V to 5.5V, V
=0.6V, I = 40μA, I = <1μA,
Q SD
OUT
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MS10E Package
2.5A I , 4MHz, Synchronous Step-Down DC/DC
V : 2.5V to 5.5V, V
=0.8V, I = 60μA, I = <1μA,
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TSSOP16E Package
4A I , 4MHz, Synchronous Step-Down DC/DC Converter V : 2.3V to 5.5V, V
=0.8V, I = 64μA, I = <1μA,
Q SD
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TSSOP20E Package
60V, 2.75A I , 200kHz/500kHz, Synchronous Step-Down V : 5.5V to 60V, V
=1.2V, I = 2.5mA, I = 30μA,
Q SD
OUT
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TSSOP16E Package
Burst Mode is a registered trademark of Linear Technology Corporation.
1933fd
LT 0108 REV D • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
20
●
●
© LINEAR TECHNOLOGY CORPORATION 2007
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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