LT1933 - 600mA, 500kHz Step-Down Switching Regulator in SOT-23

LT1933
600mA, 500kHz Step-Down
Switching Regulator in
SOT-23 and DFN Packages
FEATURES
DESCRIPTION
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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.
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Wide Input Range: 3.6V to 36V
5V at 600mA from 16V to 36V Input
3.3V at 600mA from 12V to 36V Input
5V at 500mA from 6.3V to 36V Input
3.3V at 500mA from 4.5V to 36V Input
Fixed Frequency 500kHz Operation
Uses Tiny Capacitors and Inductors
Soft-Start
Internally Compensated
Low Shutdown Current: <2μA
Output Adjustable Down to 1.25V
Low Profile (1mm) SOT-23 (ThinSOT™) and
(2mm × 3mm × 0.75mm) 6-Pin DFN Packages
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
provides output disconnect, enabling easy power management in battery-powered systems.
APPLICATIONS
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L, LT, LTC, LTM, Linear Technology and the Linear logo 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.
Automotive Battery Regulation
Industrial Control Supplies
Wall Transformer Regulation
Distributed Supply Regulation
Battery-Powered Equipment
TYPICAL APPLICATION
Efficiency
95
3.3V Step-Down Converter
SHDN
GND
0.1μF
22μH
SW
FB
16.5k
2.2μF
VOUT = 5V
BOOST
LT1933
OFF ON
90
1N4148
VIN
10k
VOUT
3.3V/500mA
MBRM140
EFFICIENCY (%)
VIN
4.5V TO 36V
VIN = 12V
85
VOUT = 3.3V
80
75
22μF
70
1933 TA01a
65
0
100
200
300
400
LOAD CURRENT (mA)
500
600
1933 TA01b
1933fe
1
LT1933
ABSOLUTE MAXIMUM RATINGS
(Note 1)
Input Voltage (VIN) ..................................... –0.4V to 36V
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 ...................................................–40°C to 85°C
LT1933I .................................................. –40°C to 125°C
LT1933H ................................................ –40°C to 150°C
Maximum Junction Temperature
LT1933E, LT1933I ................................................. 125°C
LT1933H ............................................................... 150°C
Storage Temperature Range .................. –65°C to 150°C
Lead Temperature, S6 Package
(Soldering, 10 sec) ............................................ 300°C
PIN CONFIGURATION
TOP VIEW
TOP VIEW
6 FB
BOOST 1
VIN 2
7
5 GND
4 SHDN
SW 3
BOOST 1
6 SW
GND 2
5 VIN
FB 3
4 SHDN
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
θJA = 165°C/W, θJC = 102°C/W
DCB PACKAGE
6-LEAD (2mm × 3mm) PLASTIC DFN
θJA = 73.5°C/W, θJC = 12°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
LT1933IDCB#PBF
LT1933IDCB#TRPBF
LCGM
6-Lead (2mm × 3mm) Plastic DFN
–40°C to 125°C
LT1933HDCB#PBF
LT1933HDCB#TRPBF
LCGN
6-Lead (2mm × 3mm) Plastic DFN
–40°C to 150°C
LT1933ES6#PBF
LT1933ES6#TRPBF
LTAGN
6-Lead Plastic TSOT-23
–40°C to 85°C
LT1933IS6#PBF
LT1933IS6#TRPBF
LTAGP
6-Lead Plastic TSOT-23
–40°C to 125°C
LT1933HS6#PBF
LT1933HS6#TRPBF
LTDDQ
6-Lead Plastic TSOT-23
–40°C to 150°C
LEAD BASED FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT1933IDCB
LT1933IDCB#TR
LCGM
6-Lead (2mm × 3mm) Plastic DFN
–40°C to 125°C
LT1933HDCB
LT1933HDCB#TR
LCGN
6-Lead (2mm × 3mm) Plastic DFN
–40°C to 150°C
LT1933ES6
LT1933ES6#TR
LTAGN
6-Lead Plastic TSOT-23
–40°C to 85°C
LT1933IS6
LT1933IS6#TR
LTAGP
6-Lead Plastic TSOT-23
–40°C to 125°C
LT1933HS6
LT1933HS6#TR
LTDDQ
6-Lead Plastic TSOT-23
–40°C to 150°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/
1933fe
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
3.6
V
1.225
1.245
1.265
V
40
120
nA
Undervoltage Lockout
l
Feedback Voltage
l
MAX
UNITS
FB Pin Bias Current
VFB = Measured VREF + 10mV (Note 4)
Quiescent Current
Not Switching
1.6
2.5
mA
Quiescent Current in Shutdown
VSHDN = 0V
0.01
2
μA
Reference Line Regulation
VIN = 5V to 36V
Switching Frequency
VFB = 1.1V
0.01
400
VFB = 0V
l
Maximum Duty Cycle
Switch Current Limit
(Note 3)
Switch VCESAT
ISW = 400mA, S6 Package
ISW = 400mA, DCB6 Package
500
%/V
600
kHz
55
kHz
88
94
%
0.75
1.05
A
370
370
Switch Leakage Current
500
mV
mV
2
μA
Minimum Boost Voltage Above Switch
ISW = 400mA
1.9
2.3
V
BOOST Pin Current
ISW = 400mA
18
25
mA
SHDN Input Voltage High
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 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
34
0.01
0.3
V
50
0.1
μA
μA
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 4: Current flows out of pin.
Note 5: Current flows into pin.
1933fe
3
LT1933
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C
VOUT = 5V
90
VIN = 24V
80
70
Switch Current Limit
1200
TA = 25°C
VOUT = 3.3V
90
VIN = 12V
EFFICIENCY (%)
EFFICIENCY (%)
Efficiency, VOUT = 3.3V
100
VIN = 5V
VIN = 12V
80
VIN = 24V
70
D1 = MBRM140
L1 = Toko D53LCB 33μH
60
0
100
200
300
400
LOAD CURRENT (mA)
500
0
100
200
300
400
LOAD CURRENT (mA)
500
1933 G01
800
600
400
200
0
600
MINIMUM
20
0
60
40
DUTY CYCLE (%)
TA = 25°C
VOUT = 5V
Switch Voltage Drop
600
TA = 25°C
VOUT = 3.3V
600
L = 22μH
500
700
SWITCH VOLTAGE (mV)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
500
L = 33μH
100
1933 G03
Maximum Load Current
800
700
80
1933 G02
Maximum Load Current
800
1000
D1 = MBRM140
L1 = Toko D53LCB 22μH
60
600
TA = 25°C
TYPICAL
SWITCH CURRENT LIMIT (mA)
Efficiency, VOUT = 5V
100
L = 22μH
600
L = 15μH
500
TA = 25°C
400
TA = 85°C
300
TA = –40°C
200
100
400
400
0
5
15
20
10
INPUT VOLTAGE (V)
25
30
0
5
15
20
10
INPUT VOLTAGE (V)
25
Feedback Voltage
SWITCHING FREQUENCY (kHz)
UVLO (V)
FEEDBACK VOLTAGE (V)
3.6
1.250
3.4
1.240
3.2
1.235
1933 G07
3.0
–50 –25
0.5
0.6
Switching Frequency
1.255
25 50 75 100 125 150
TEMPERATURE (°C)
0.3
0.2
0.4
SWITCH CURRENT (A)
600
3.8
0
0.1
1933 G06
Undervoltage Lockout
1.260
1.245
0
1933 G05
1933 G04
1.230
–50 –25
0
30
0
25 50 75 100 125 150
TEMPERATURE (°C)
1933 G08
550
500
450
400
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
1933 G09
1933fe
4
LT1933
TYPICAL PERFORMANCE CHARACTERISTICS
Frequency Foldback
SHDN Pin Current
Soft-Start
1.4
TA = 25°C
400
300
200
100
SHDN PIN CURRENT (μA)
500
200
TA = 25°C
DC = 30%
1.2
SWITCH CURRENT LIMIT (A)
600
1.0
0.8
0.6
0.4
TA = 25°C
150
100
50
0.2
0
0
1.0
0.5
FB PIN VOLTAGE (V)
0
1.5
0
2
3
1
SHDN PIN VOLTAGE (V)
1933 G10
VOUT = 3.3V
TA = 25°C
L = 22μH
5.5
7
TO START
INPUT VOLTAGE (V)
6
TO RUN
5.0
4.5
4.0
5
TO RUN
3.5
4
10
100
LOAD CURRENT (mA)
1
1.2
1.0
0.8
0.6
0.4
0
25 50 75 100 125 150
TEMPERATURE (°C)
1933 G14
1933 G15
Operating Waveforms,
Discontinuous Mode
Operating Waveforms
VOUT1
1.8V
VOUT2
1.2V
IL 200mA/DIV
Switch Current Limit
1.4
0
–50 –25
10
100
LOAD CURRENT (mA)
1933 G13
VSW 10V/DIV
16
0.2
3.0
1
8
12
4
SHDN PIN VOLTAGE (V)
1933 G12
Typical Minimum Input Voltage
6.0
VOUT = 5V
TA = 25°C
L = 33μH
TO START
0
1933 G11
Typical Minimum Input Voltage
8
4
SWITCH CURRENT LIMIT (A)
0.0
INPUT VOLTAGE (V)
SWITCHING FREQUENCY (kHz)
700
VOUT 10mV/DIV
VOUT1
1.8V
VSW 10V/DIV
VOUT2
1.2V
IL 200mA/DIV
VOUT 10mV/DIV
VIN = 12V, VOUT = 3.3V, IOUT = 400mA,
L = 22μH, COUT = 22μF
1933 G16
VIN = 12V, VOUT = 3.3V, IOUT = 20mA,
L = 22μH, COUT = 22μF
1933 G17
1933fe
5
LT1933
PIN FUNCTIONS
(SOT-23/DFN)
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 VIN pin. SHDN also
provides a soft-start function; see the Applications Information section.
BOOST (Pin 1): The BOOST pin is used to provide a drive
voltage, higher than the input voltage, to the internal bipolar
NPN power switch.
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.
VIN (Pin 5/Pin 2): The VIN pin supplies current to the
LT1933’s internal regulator and to the internal power
switch. This pin must be locally bypassed.
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 VOUT = 1.245V
(1 + R1/R2). A good value for R2 is 10k.
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.
BLOCK DIAGRAM
VIN
VIN
C2
INT REG
AND
UVLO
ON OFF
SLOPE
COMP
R3
R
Q
S
Q
SHDN
C3
DRIVER
C4
Q1
L1
SW
OSC
FREQUENCY
FOLDBACK
D2
BOOST
Σ
VOUT
D1
VC
C1
gm
1.245V
GND
FB
R2
R1
1933 BD
1933fe
6
LT1933
OPERATION
(Refer to Block Diagram)
The LT1933 is a constant frequency, current mode step
down regulator. A 500kHz oscillator enables an RS flipflop, turning on the internal 750mA 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 ~3.35V. The
SHDN pin is used to place the LT1933 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 LT1933’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.
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:
R1 = R2(VOUT/1.245 – 1)
R2 should be 20k or less to avoid bias current errors.
Reference designators refer to the Block Diagram.
Input Voltage Range
The input voltage range for LT1933 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
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:
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 = 0.88
The maximum input voltage is determined by the absolute
maximum ratings of the VIN and BOOST pins and by the
minimum duty cycle DCMIN = 0.08 (corresponding to a
minimum on time of 130ns):
VIN(MAX) = (VOUT + VD)/DCMIN – VD + VSW
Note that this is a restriction on the operating input voltage;
the circuit will tolerate transient inputs up to the absolute
maximum ratings of the VIN and BOOST pins.
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L = 5 (VOUT + VD)
where VD is the voltage drop of the catch diode (~0.4V)
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
1933fe
7
LT1933
APPLICATIONS INFORMATION
saturation current should be about 30% higher. For robust
operation in fault conditions the saturation current should
be ~1A. To keep efficiency high, the series resistance (DCR)
should be less than 0.2Ω. 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 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 operation, see Linear Technology Application Note 44. Finally,
for duty cycles greater than 50% (VOUT/VIN > 0.5), there
is a minimum inductance required to avoid subharmonic
oscillations. Choosing L greater than 3(VOUT + VD) μH
prevents subharmonic oscillations at all duty cycles.
Catch Diode
A 0.5A or 1A 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 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.
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 applied 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 supply 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
Table 1.Inductor Vendors
Vendor
URL
Part Series
Inductance Range (μH)
Size (mm)
Coilcraft
www.coilcraft.com
D01608C
10 to 22
2.9 × 4.5 × 6.6
MSS5131
10 to 22
3.1 × 5.1 × 5.1
MSS6122
10 to 33
2.2 × 6.1 × 6.1
CR43
10 to 22
3.5 × 4.3 × 4.8
CDRH4D28
10 to 33
3.0 × 5.0 × 5.0
CDRH5D28
22 to 47
3.0 × 5.7 × 5.7
Sumida
Toko
Würth Elektronik
www.sumida.com
www.toko.com
www.we-online.com
D52LC
10 to 22
2.0 × 5.0 × 5.0
D53LC
22 to 47
3.0 × 5.0 × 5.0
WE-TPC MH
10 to 22
2.8 × 4.8 × 4.8
WE-PD4 S
10 to 22
2.9 × 4.5 × 6.6
WE-PD2 S
10 to 47
3.2 × 4.0 × 4.5
1933fe
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.
Output Capacitor
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.
Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance.
A good value is
COUT = 60/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).
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
capacitor must be large to achieve low ESR. Table 2 lists
several capacitor vendors.
Table 2.Inductor Vendors
Vendor
Phone
URL
Part Series
Comments
Panasonic
(714) 373-7366
www.panasonic.com
Ceramic,
Polymer,
Tantalum
EEF Series
Kemet
(864) 963-6300
www.kemet.com
Ceramic,
Tantalum
Sanyo
(408) 749-9714
www.sanyovideo.com
Ceramic,
Polymer,
Tantalum
Murata
(404)436-1300
AVX
Taiyo Yuden
(864)963-6300
www.murata.com
Ceramic
www.avxcorp.com
Ceramic,
Tantalum
www.taiyo-yuden.com
T494, T495
POSCAP
TPS Series
Ceramic
1933fe
9
LT1933
APPLICATIONS INFORMATION
Figure 1 shows the transient response of the LT1933 with
several output capacitor choices. The output is 3.3V. The
load current is stepped from 100mA to 400mA and back to
100mA, and the oscilloscope traces show the output voltage. The upper photo shows the recommended value. The
second photo shows the improved response (less voltage
VOUT
16.5k
FB
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, but output
ripple (as shown by the broad trace) has increased because
of the higher ESR of this capacitor.
VOUT
50mV/DIV
22μF
10k
IOUT
200mA/DIV
1933 F01a
VOUT
16.5k
470pF
FB
VOUT
50mV/DIV
22μF
2x
10k
IOUT
200mA/DIV
1933 F01b
VOUT
16.5k
VOUT
50mV/DIV
+
FB
10k
100μF
SANYO
4TPB100M
IOUT
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.
1933fe
10
LT1933
APPLICATIONS INFORMATION
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 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
capacitor and a small Schottky diode (such as the BAT-54).
For lower output voltages the boost diode can be tied to
the input (Figure 2b). The circuit in Figure 2a is more 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 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
D2
D2
C3
BOOST
VIN
VIN
C3
BOOST
LT1933
LT1933
VOUT
SW
VIN
VIN
VOUT
SW
GND
GND
1933 F02a
VBOOST – VSW ≅ VOUT
MAX VBOOST ≅ VIN + VOUT
1933 F02b
VBOOST – VSW ≅ VIN
MAX VBOOST ≅ 2VIN
(2b)
(2a)
Figure 2. Two Circuits for Generating the Boost Voltage
Minimum Input Voltage VOUT = 3.3V
6.0
8
VOUT = 3.3V
TA = 25°C
L = 22μH
5.5
VOUT = 5V
TA = 25°C
L = 33μH
7
INPUT VOLTAGE (V)
TO START
INPUT VOLTAGE (V)
Minimum Input Voltage VOUT = 5V
5.0
4.5
4.0
TO START
6
TO RUN
5
TO RUN
3.5
4
3.0
1
1
10
100
LOAD CURRENT (mA)
1933 F03a
10
100
LOAD CURRENT (mA)
1933 F03b
Figure 3. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
1933fe
11
LT1933
APPLICATIONS INFORMATION
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 very 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.
At light loads, the inductor current becomes discontinuous and the effective duty cycle can be very high. This
reduces the minimum input voltage to approximately
300mV above VOUT. At higher load currents, the inductor
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.
Soft-Start
The SHDN pin can be used to soft-start the LT1933, 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
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 60μA when the SHDN pin reaches 2.3V.
RUN
5V/DIV
RUN
SHDN
GND
1933 F04a
IIN
100mA/DIV
VOUT
5V/DIV
50μs/DIV
RUN
RUN
5V/DIV
15k
SHDN
0.1μF
GND
1933 F04b
IIN
100mA/DIV
VOUT
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
1933fe
12
LT1933
APPLICATIONS INFORMATION
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate excessively, 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 applications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT1933’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 LT1933’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 LT1933 can
pull large currents from the output through the SW pin
and the VIN pin. Figure 5 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 LT1933 circuits. However, these capacitors 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
source forms an under damped tank circuit, and the voltage
at the VIN pin of the LT1933 can ring to twice the nominal
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.
D4
VIN
VIN
BOOST
LT1933
SHDN
GND
VOUT
SW
FB
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
1933fe
13
LT1933
APPLICATIONS INFORMATION
CLOSING SWITCH
SIMULATES HOT PLUG
IIN
VIN
DANGER!
LT1933
+
VIN
20V/DIV
2.2μF
LOW
IMPEDANCE
ENERGIZED
24V SUPPLY
RINGING VIN MAY EXCEED
ABSOLUTE MAXIMUM
RATING OF THE LT1933
IIN
5A/DIV
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
20μs/DIV
(6a)
LT1933
+
10μF
35V
AI.EI.
+
VIN
20V/DIV
2.2μF
IIN
5A/DIV
20μs/DIV
(6b)
1Ω
LT1933
+
0.1μF
VIN
20V/DIV
2.2μF
IIN
5A/DIV
20μs/DIV
1933 F06
(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
1933fe
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
likely to be the largest component in the circuit. An alternative 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.
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 capacitor for stability allowing the use of ceramic capacitors to
achieve low output ripple and small circuit size.
LT1933
–
0.7V
+
Figure 7 shows an equivalent circuit for the LT1933 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
model works well as long as the value of the inductor is
not too high and the loop crossover frequency is much
lower than the switching frequency. With a larger ceramic
capacitor (very low ESR), crossover may be lower and a
phase lead capacitor (CPL) across the feedback divider may
improve the phase margin and transient response. Large
electrolytic capacitors may have an ESR large enough to
create an additional zero, and the phase lead may not be
necessary.
If the output capacitor is different than the recommended
capacitor, stability should be checked across all operating
conditions, including load current, input voltage and temperature. The LT1375 data sheet contains a more thorough
discussion of loop compensation and describes how to
test the stability using a transient load.
CURRENT MODE
POWER STAGE
SW
gm
1.1mho
CC
80pF
GND
FB
gm =
150μmhos
ERROR
AMPLIFIER
500k
+
RC
100k
R1
–
VC
OUT
CPL
ESR
1.245V
+
C1
C1
R2
1933 F07
Figure 7. Model for Loop Response
1933fe
15
LT1933
APPLICATIONS INFORMATION
PCB Layout
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
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 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,
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.
VOUT
VIN
C1
D1
C2
GND
VIAS
1933 F08a
(8a)
DFN Package
SHUTDOWN
VIN
C1
C2
VOUT
SYSTEM
GROUND
D1
VIAS
OUTLINE OF LOCAL GROUND PLANE
1933 F08b
(8b)
SOT-23 Package
Figure 8. A Good PCB Layout Ensures Proper, Low EMI Operation
1933fe
16
LT1933
TYPICAL APPLICATIONS
3.3V Step-Down Converter
VIN
4.5V TO
36V
OFF ON
D2
VIN
BOOST
C3
0.1μF
LT1933
SHDN
L1
22μH
SW
GND
FB
C2
2.2μF
R1
16.5k
VOUT
3.3V/
500mA
D1
C1
22μF
6.3V
R2
10k
1933 TA02b
12V Step-Down Converter
VIN
14.5V TO
36V
D3, 6V
VIN
C3
0.1μF
LT1933
OFF ON
SHDN
GND
C2
2.2μF
D2
BOOST
SW
FB
R1
86.6k
R2
10k
L1
47μH
VOUT
12V/
450mA
D1
C1
10μF
1933 TA02d
1933fe
17
LT1933
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
× 45° CHAMFER
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
3
0.200 REF
0.75 ±0.05
1
(DCB6) DFN 0405
0.25 ± 0.05
0.50 BSC
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
1933fe
18
LT1933
PACKAGE DESCRIPTION
S6 Package
6-Lead Plastic SOT-23
(Reference LTC DWG # 05-08-1634)
0.62
MAX
2.80 – 3.10
(NOTE 4)
0.95
REF
1.22 REF
1.4 MIN
3.85 MAX 2.62 REF
2.60 – 3.00 1.50 – 1.75
(NOTE 4)
PIN ONE ID
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
0.25 – 0.50
TYP 6 PLCS
NOTE 3
0.95 BSC
0.90 – 1.30
0.20 BSC
0.90 – 1.45
DATUM ‘A’
0.35 – 0.55 REF
0.09 – 0.20
(NOTE 3)
NOTE:
1. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
3. DIMENSIONS ARE INCLUSIVE OF PLATING
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)
1.90 BSC
0.09 – 0.15
NOTE 3
S6 SOT-23 0502
ATTENTION: ORIGINAL SOT23-6L PACKAGE.
MOST SOT23-6L PRODUCTS CONVERTED TO THIN SOT23
PACKAGE, DRAWING # 05-08-1636 AFTER APPROXIMATELY
APRIL 2001 SHIP DATE
1933fe
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
LT1933
TYPICAL APPLICATION
2.5V Step-Down Converter
D2
VIN
3.6V TO 36V
VIN
BOOST
C3
L1
0.47μF 15μH
LT1933
SHDN
OFF ON
GND
SW
FB
C2
2.2μF
R1
10.5k
VOUT
2.5V/500mA
D1
C1
22μF
R2
10k
1933 TA03
RELATED PARTS
PART NUMBER
DESCRIPTION
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LT1676
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S8 Package
LT1765
25V, 2.75mA IOUT, 1.25MHz, High Efficiency Step-Down
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S8, TSSOP16E Packages
LT1766
60V, 1.2A IOUT, 200kHz, High Efficiency Step-Down DC/DC VIN: 5.5V to 60V, VOUT(MIN) = 1.2V, IQ = 2.5mA, ISD = 25μA,
Converter
S8, TSSOP16/TSSOP16E Packages
LT1767
25V, 1.2A IOUT, 1.25MHz, High Efficiency Step-Down
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VIN: 3V to 25V, VOUT(MIN) = 1.2V, IQ = 1mA, ISD = 6μA,
S8, MS8/MS8E Packages
LT1776
40V, 550mA IOUT, 200kHz, High Efficiency Step-Down
DC/DC Converter
VIN: 7.4V to 40V, VOUT(MIN) = 1.24V, IQ = 3.2mA, ISD = 30μA,
S8, N8, S8 Packages
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
LT1956
60V, Dual 1.2A IOUT, 500kHz, High Efficiency Step-Down
DC/DC Converter
VIN: 5.5V to 60V, VOUT(MIN) = 1.2V, IQ = 2.5mA, ISD = 25μA,
TSSOP16/TSSOP16E Packages
LT1976
60V, Dual 1.2A IOUT, 200kHz, High Efficiency Step-Down
DC/DC Converter with Burst Mode®
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
LT3407
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
LT3412
2.5A IOUT, 4MHz, Synchronous Step-Down DC/DC
Converter
VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60μA, ISD < 1μA,
TSSOP16E Package
LTC3414
4A IOUT, 4MHz, Synchronous Step-Down DC/DC Converter VIN: 2.3V to 5.5V, VOUT(MIN) = 0.8V, IQ = 64μA, ISD < 1μA,
TSSOP20E Package
LT3430/LT3431
60V, 2.75A IOUT, 200kHz/500kHz, Synchronous Step-Down VIN: 5.5V to 60V, VOUT(MIN) = 1.2V, IQ = 2.5mA, ISD = 30μA,
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TSSOP16E Package
Burst Mode is a registered trademark of Linear Technology Corporation.
1933fe
20 Linear Technology Corporation
LT 0409 REV E • PRINTED IN USA
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