Linear Technology LT1933 User manual

LT1933
1
1933fe
TYPICAL APPLICATION
FEATURES
APPLICATIONS
DESCRIPTION
600mA, 500kHz Step-Down
Switching Regulator in
SOT-23 and DFN Packages
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.
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 manage-
ment in battery-powered systems.
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.
3.3V Step-Down Converter
nWide Input Range: 3.6V to 36V
n5V at 600mA from 16V to 36V Input
n3.3V at 600mA from 12V to 36V Input
n5V at 500mA from 6.3V to 36V Input
n3.3V at 500mA from 4.5V to 36V Input
nFixed Frequency 500kHz Operation
nUses Tiny Capacitors and Inductors
nSoft-Start
nInternally Compensated
nLow Shutdown Current: <2µA
nOutput Adjustable Down to 1.25V
nLow Profile (1mm) SOT-23 (ThinSOT™) and
(2mm ×3mm ×0.75mm) 6-Pin DFN Packages
nAutomotive Battery Regulation
nIndustrial Control Supplies
nWall Transformer Regulation
nDistributed Supply Regulation
nBattery-Powered Equipment
Efficiency
VIN
4.5V TO 36V
OFF ON
0.1µF 22µH
1N4148
MBRM140
10k
16.5k
22µF
1933 TA01a
2.2µF
VOUT
3.3V/500mA
VIN BOOST
GND FB
SHDN SW
LT1933
LOAD CURRENT (mA)
0
90
95
300 400
1933 TA01b
85
80
100 200 600500
75
70
65
EFFICIENCY (%)
VIN = 12V
VOUT = 5V
VOUT = 3.3V

LT1933
2
1933fe
ABSOLUTE MAXIMUM RATINGS
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
(Note 1)
TOP VIEW
FB
GND
SHDN
BOOST
VIN
SW
DCB PACKAGE
6-LEAD (2mm ×3mm) PLASTIC DFN
4
5
7
6
3
2
1
θJA = 73.5°C/W, θJC = 12°C/W
EXPOSED PAD (PIN 7) IS GND, MUST BE SOLDERED TO PCB
BOOST 1
GND 2
FB 3
6 SW
5 VIN
4SHDN
TOP VIEW
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
θJA = 165°C/W, θJC = 102°C/W
PIN CONFIGURATION
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/
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

LT1933
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1933fe
ELECTRICAL CHARACTERISTICS
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
PARAMETER CONDITIONS MIN TYP MAX UNITS
Undervoltage Lockout 3.35 3.6 V
Feedback Voltage l1.225 1.245 1.265 V
FB Pin Bias Current VFB = Measured VREF + 10mV (Note 4) l40 120 nA
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 0.01 %/V
Switching Frequency VFB = 1.1V 400 500 600 kHz
VFB = 0V 55 kHz
Maximum Duty Cycle l88 94 %
Switch Current Limit (Note 3) 0.75 1.05 A
Switch VCESAT ISW = 400mA, S6 Package
ISW = 400mA, DCB6 Package
370
370
500 mV
mV
Switch Leakage Current 2µA
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 0.3 V
SHDN Bias Current VSHDN = 2.3V (Note 5)
VSHDN = 0V
34
0.01
50
0.1
µA
µA
The ldenotes 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)
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.

LT1933
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1933fe
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency, VOUT = 5V Efficiency, VOUT = 3.3V Switch Current Limit
Maximum Load Current Maximum Load Current Switch Voltage Drop
Feedback Voltage Undervoltage Lockout Switching Frequency
LOAD CURRENT (mA)
0
80
90
100
300 400
1933 G01
70
100 200 600500
60
EFFICIENCY (%)
VIN = 24V
VIN = 12V
TA= 25°C
VOUT = 5V
D1 = MBRM140
L1 = Toko D53LCB 33µH
LOAD CURRENT (mA)
0
80
90
100
300 400
1933 G02
70
100 200 600500
60
EFFICIENCY (%)
VIN = 24V
VIN = 12V
VIN = 5V
TA= 25°C
VOUT = 3.3V
D1 = MBRM140
L1 = Toko D53LCB 22µH
DUTY CYCLE (%)
0
SWITCH CURRENT LIMIT (mA)
600
800
1200
1000
80
1933 G03
400
200
020 40 60 100
MINIMUM
TYPICAL
TA= 25°C
INPUT VOLTAGE (V)
0
LOAD CURRENT (mA)
600
700
20
1933 G04
500
400 510 15 3025
800
L = 22µH
L = 33µH
TA= 25°C
VOUT = 5V
INPUT VOLTAGE (V)
0
LOAD CURRENT (mA)
600
700
20
1933 G05
500
400 510 15 3025
800
L = 22µH
L = 15µH
TA= 25°C
VOUT = 3.3V
SWITCH CURRENT (A)
0
0
SWITCH VOLTAGE (mV)
100
200
300
400
600
500
0.2 0.4
1933 G06
0.60.1 0.3 0.5
TA= 85°C
TA= 25°C
TA= –40°C
TEMPERATURE (°C)
FEEDBACK VOLTAGE (V)
1.260
1.255
1.250
1.240
1.230
1.245
1.235
1933 G07
–50 –25 0 25 50 75 100 150125
TEMPERATURE (°C)
UVLO (V)
3.8
3.6
3.4
3.2
3.0
1933 G08
–50 –25 0 25 50 75 100 150125
TEMPERATURE (°C)
SWITCHING FREQUENCY (kHz)
600
550
500
450
400
1933 G09
–50 –25 0 25 50 75 100 150125

LT1933
5
1933fe
TYPICAL PERFORMANCE CHARACTERISTICS
Frequency Foldback Soft-Start SHDN Pin Current
Typical Minimum Input Voltage Typical Minimum Input Voltage Switch Current Limit
Operating Waveforms
Operating Waveforms,
Discontinuous Mode
FB PIN VOLTAGE (V)
0.0
SWITCHING FREQUENCY (kHz)
700
600
300
100
400
500
200
0
0.5
1933 G10
1.0 1.5
TA= 25°C
SHDN PIN VOLTAGE (V)
0
SWITCH CURRENT LIMIT (A)
1.4
1.2
0.6
0.2
0.8
1.0
0.4
0
1
1933 G11
234
TA= 25°C
DC = 30%
0
SHDN PIN CURRENT (µA)
200
150
50
100
0
4
1933 G12
81216
SHDN PIN VOLTAGE (V)
TA= 25°C
LOAD CURRENT (mA)
5
INPUT VOLTAGE (V)
6
7
8
1933 G13
4
VOUT = 5V
TA= 25°C
L = 33µH
TO START
TO RUN
1 10 100
LOAD CURRENT (mA)
4.5
INPUT VOLTAGE (V)
5.0
5.5
6.0
1933 G14
3.0
3.5
4.0
VOUT = 3.3V
TA= 25°C
L = 22µH
TO START
TO RUN
1 10 100
TEMPERATURE (°C)
–50
SWITCH CURRENT LIMIT (A)
1.4
1.2
1.0
0.6
0
0.2
0.8
0.4
–25 0 25
1933 G15
50 75 100 150125
VIN = 12V, VOUT = 3.3V, IOUT = 400mA,
L = 22µH, COUT = 22µF
1933 G16
VOUT 10mV/DIV
IL200mA/DIV
VOUT1
1.8V
VOUT2
1.2V
VSW 10V/DIV
VIN = 12V, VOUT = 3.3V, IOUT = 20mA,
L = 22µH, COUT = 22µF
1933 G17
VOUT 10mV/DIV
IL200mA/DIV
VOUT1
1.8V
VOUT2
1.2V
VSW 10V/DIV

LT1933
6
1933fe
PIN FUNCTIONS
BOOST (Pin 1): The BOOST pin is used to provide a drive
voltage,higher thanthe input voltage,to theinternal 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.
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.
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 Infor-
mation section.
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.
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.
(SOT-23/DFN)
BLOCK DIAGRAM
Σ
R
DRIVER Q1
S
OSC
SLOPE
COMP
FREQUENCY
FOLDBACK
INT REG
AND
UVLO
VCgm
1.245V
1933 BD
Q
Q
BOOST
SW
FB
R2 R1
VOUT
L1
D2
C3
C1
D1
VIN
C2
VIN
ON OFF
GND
C4
R3
SHDN

LT1933
7
1933fe
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
between the VIN and SW pins, turning the switch off when
this current reaches a level determined by the voltage at
VC.An erroramplifier measuresthe output voltage through
an external resistor divider tied to the FB pin and servos
the VCnode. If the error amplifier’s output increases, more
current is delivered to the output; if it decreases, less cur-
rent is delivered. An active clamp (not shown) on the VC
node provides current limit. The VCnode 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 cir-
cuitry. 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, dis-
connecting 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 opera-
tion.
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% resis-
tors 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 rat-
ings 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 VDis 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:
V
IN(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):
V
IN(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 VDis 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

LT1933
8
1933fe
APPLICATIONS INFORMATION
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
Sumida www.sumida.com 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
Toko www.toko.com D52LC 10 to 22 2.0 × 5.0 × 5.0
D53LC 22 to 47 3.0 × 5.0 × 5.0
Würth Elektronik www.we-online.com 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
saturation current should be about 30% higher. For robust
operation in fault conditions the saturation current should
be~1A. To keep efficiency high,the seriesresistance (DCR)
should be less than 0.2. Table 1 lists several vendors
and types that are suitable.
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,
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 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

LT1933
9
1933fe
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 re-
sistance (ESR) and provide the best ripple performance.
A good value is
C
OUT = 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 0.1
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 [phone] www.panasonic.com Ceramic,
Polymer,
Tantalum
EEF Series
Kemet [phone] www.kemet.com Ceramic,
Tantalum T494, T495
Sanyo [phone] www.sanyovideo.com Ceramic,
Polymer,
Tantalum
POSCAP
Murata [phone] www.murata.com Ceramic
AVX www.avxcorp.com Ceramic,
Tantalum TPS Series
Taiyo Yuden [phone] www.taiyo-yuden.com Ceramic

LT1933
10
1933fe
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 volt-
age. The upper photo shows the recommended value. The
second photo shows the improved response (less voltage
drop) resulting from a larger output capacitor and a phase
lead capacitor. The last photo shows the response to a high
performance electrolytic capacitor. Transient performance
is improved due to the large output capacitance, but output
ripple(as shownby thebroad trace)has increased because
of the higher ESR of this capacitor.
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.
22µFFB
16.5k
10k
VOUT
1933 F01a
VOUT
50mV/DIV
IOUT
200mA/DIV
VOUT
50mV/DIV
IOUT
200mA/DIV
FB
VOUT
1933 F01b
16.5k
10k
22µF
2x
470pF
VOUT
50mV/DIV
IOUT
200mA/DIV
SANYO
4TPB100M
FB
1933 F01c
VOUT
+
16.5k
10k
100µF

LT1933
11
1933fe
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 ef-
ficient 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
Figure 2. Two Circuits for Generating the Boost Voltage
Figure 3. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
VIN
BOOST
GND
SW
VIN
LT1933
(2a)
D2
1933 F02a
VOUT
C3
VBOOST – VSW ≅VOUT
MAX VBOOST ≅VIN + VOUT
VIN
BOOST
GND
SW
VIN
LT1933
(2b)
D2
1933 F02b
VOUT
C3
VBOOST – VSW ≅VIN
MAX VBOOST ≅2VIN
LOAD CURRENT (mA)
4.5
INPUT VOLTAGE (V)
5.0
5.5
6.0
1933 F03a
3.0
3.5
4.0
VOUT = 3.3V
TA= 25°C
L = 22µH
TO START
TO RUN
1 10 100
LOAD CURRENT (mA)
5
INPUT VOLTAGE (V)
6
7
8
1933 F03b
4
VOUT = 5V
TA= 25°C
L = 33µH
TO START
TO RUN
1 10 100
Minimum Input Voltage VOUT = 3.3V Minimum Input Voltage VOUT = 5V

LT1933
12
1933fe
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 discontinu-
ous 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
TheSHDN pin can beused 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.
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
RUN
5V/DIV
VOUT
5V/DIV
IIN
100mA/DIV
RUN SHDN
GND
1933 F04a
50µs/DIV
RUN
5V/DIV
VOUT
5V/DIV
IIN
100mA/DIV
RUN
15k
0.1µF
SHDN
GND
1933 F04b
0.5ms/DIV

LT1933
13
1933fe
APPLICATIONS INFORMATION
Shorted and Reversed Input Protection
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
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
bypasscapacitor of LT1933circuits.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
sourceformsan under dampedtank circuit, andthe 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.
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
VIN BOOST
GND FB
SHDN SW
D4
VIN
LT1933
1933 F05
VOUT
BACKUP
D4: MBR0540

LT1933
14
1933fe
APPLICATIONS INFORMATION
Figure 6. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT1933 is Connected to a Live Supply
+
+
LT1933
2.2µF
VIN
20V/DIV
IIN
5A/DIV
20µs/DIV
VIN
CLOSING SWITCH
SIMULATES HOT PLUG
IIN
(6a)
(6b)
(6c)
LOW
IMPEDANCE
ENERGIZED
24V SUPPLY
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
+
+
LT1933
2.2µF
10µF
35V
AI.EI.
LT1933
2.2µF0.1µF
1Ω
1933 F06
VIN
20V/DIV
IIN
5A/DIV
20µs/DIV
VIN
20V/DIV
IIN
5A/DIV
20µs/DIV
DANGER!
RINGING VIN MAY EXCEED
ABSOLUTE MAXIMUM
RATING OF THE LT1933

LT1933
15
1933fe
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 alterna-
tive solution is shown in Figure 6c. A 1 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 capaci-
tor for stability allowing the use of ceramic capacitors to
achieve low output ripple and small circuit size.
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 cur-
rent proportional to the voltage at the VCnode. Note that
the output capacitor integrates this current, and that the
capacitor on the VCnode (CC) integrates the error ampli-
fier output current, resulting in two poles in the loop. RC
provides a zero. With the recommended output capacitor,
theloop crossoveroccursabove theRCCCzero. 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
phaselead capacitor(CPL) acrossthe feedback divider may
improve the phase margin and transient response. Large
electrolytic capacitors may have an ESR large enough to
create an additional zero, and the phase lead may not be
necessary.
If the output capacitor is different than the recommended
capacitor, stability should be checked across all operating
conditions, including load current, input voltage and tem-
perature. The LT1375 data sheet contains a more thorough
discussion of loop compensation and describes how to
test the stability using a transient load.
Figure 7. Model for Loop Response
–
+
–
+
1.245V
SW
VC
LT1933
GND
1933 F07
R1
OUT
ESR
ERROR
AMPLIFIER
CURRENT MODE
POWER STAGE
FB
R2
500k
RC
100k
CC
80pF
C1
C1
gm=
150µmhos
gm
+
CPL
0.7V
1.1mho

LT1933
16
1933fe
APPLICATIONS INFORMATION
PCB Layout
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 systemground in only one place.These components,
along with the inductor and output capacitor, should be
placed on the same side of the circuit board, and their
connections should be made on that layer. Place a local,
unbroken ground plane below these components, and tie
this ground plane to system ground at one location, ideally
at the ground terminal of the output capacitor C1. The SW
and BOOST nodes should be as small as possible. Finally,
keep the FB node small so that the ground pin and ground
traces will shield it from the SW and BOOST nodes. Include
two vias near the GND pin of the LT1933 to help remove
heat from the LT1933 to the ground plane.
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.
Figure 8. A Good PCB Layout Ensures Proper, Low EMI Operation
GND
VOUT
VIAS
VIN
1933 F08a
D1C1 C2
SHUTDOWN
VIAS
OUTLINE OF LOCAL GROUND PLANE
VIN
VOUT
1933 F08b
SYSTEM
GROUND
C2 D1
C1
(8a)
DFN Package
(8b)
SOT-23 Package

LT1933
17
1933fe
TYPICAL APPLICATIONS
3.3V Step-Down Converter
12V Step-Down Converter
VIN
4.5V TO
36V
OFF ON
C3
0.1µF
D2
L1
22µH
R2
10k
R1
16.5k
C1
22µF
6.3V
1933 TA02b
C2
2.2µF
VOUT
3.3V/
500mA
VIN BOOST
GND FB
SHDN SW
LT1933
D1
VIN
14.5V TO
36V
OFF ON
C3
0.1µF
D3, 6V
D1
L1
47µH
R2
10k
R1
86.6k
C1
10µF
1933 TA02d
C2
2.2µF
VOUT
12V/
450mA
D2
VIN BOOST
GND FB
SHDN SW
LT1933

LT1933
18
1933fe
PACKAGE DESCRIPTION
DCB Package
6-Lead Plastic DFN (2mm × 3mm)
(Reference LTC DWG # 05-08-1715)
3.00 ±0.10
(2 SIDES)
2.00 ±0.10
(2 SIDES)
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (TBD)
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
0.40 ±0.10
BOTTOM VIEW—EXPOSED PAD
1.65 ±0.10
(2 SIDES)
0.75 ±0.05
R = 0.115
TYP
R = 0.05
TYP
1.35 ±0.10
(2 SIDES)
1
3
64
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
0.200 REF
0.00 – 0.05
(DCB6) DFN 0405
0.25 ±0.05
0.50 BSC
PIN 1 NOTCH
R0.20 OR 0.25
×45°CHAMFER
0.25 ±0.05
1.35 ±0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
1.65 ±0.05
(2 SIDES)
2.15 ±0.05
0.70 ±0.05
3.55 ±0.05
PACKAGE
OUTLINE
0.50 BSC

LT1933
19
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 representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
PACKAGE DESCRIPTION
S6 Package
6-Lead Plastic SOT-23
(Reference LTC DWG # 05-08-1634)
1.50 – 1.75
(NOTE 4)
2.60 – 3.00
0.25 – 0.50
TYP 6 PLCS
NOTE 3
DATUM ‘A’
0.09 – 0.20
(NOTE 3)
S6 SOT-23 0502
2.80 – 3.10
(NOTE 4)
0.95 BSC
1.90 BSC
0.90 – 1.30
0.90 – 1.45
0.09 – 0.15
NOTE 3
0.20 BSC
0.35 – 0.55 REF
PIN ONE ID
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)
ATTENTION: ORIGINAL SOT23-6L PACKAGE.
MOST SOT23-6L PRODUCTS CONVERTED TO THIN SOT23
PACKAGE, DRAWING # 05-08-1636 AFTER APPROXIMATELY
APRIL 2001 SHIP DATE
3.85 MAX
0.62
MAX
0.95
REF
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
1.4 MIN
2.62 REF
1.22 REF

LT1933
20
1933fe
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
[phone] ●FAX: [phone] ●www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2007
LT 0409 REV E • PRINTED IN USA
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Burst Mode is a registered trademark of Linear Technology Corporation.
2.5V Step-Down Converter
VIN
3.6V TO 36V
OFF ON
C3
0.47µF
D2
L1
15µH
R2
10k
R1
10.5k
C1
22µF
1933 TA03
C2
2.2µF
VOUT
2.5V/500mA
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SHDN SW
LT1933
D1