LT1933
600mA, 500kHz Step-Down
Switching Regulator in
SOT-23 and DFN Packages
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
n
Internally Compensated
n
Low Shutdown Current: <2µA
n
Output Adjustable Down to 1.25V
n
Low Profi le (1mm) SOT-23 (ThinSOT™) and
(2mm × 3mm × 0.75mm) 6-Pin DFN Packages
APPLICATIONS
n
Automotive Battery Regulation
n
Industrial Control Supplies
n
Wall Transformer Regulation
n
Distributed Supply Regulation
n
Battery-Powered Equipment
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.
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.
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.
TYPICAL APPLICATION
3.3V Step-Down Converter
V
4.5V TO 36V
OFF ON
IN
V
IN
LT1933
SHDN SW
GND FB
2.2µF
BOOST
16.5k
10k
1N4148
0.1µF
MBRM140
22µH
V
OUT
3.3V/500mA
22µF
1933 TA01a
95
VIN = 12V
90
85
80
EFFICIENCY (%)
75
70
65
100 200 600500
0
Effi ciency
V
= 5V
OUT
V
= 3.3V
OUT
300 400
LOAD CURRENT (mA)
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
PIN CONFIGURATION
TOP VIEW
6
BOOST
V
SW
1
2
IN
3
7
FB
5
GND
SHDN
4
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
TOP VIEW
BOOST 1
GND 2
FB 3
6 SW
5 V
IN
4 SHDN
6-LEAD (2mm × 3mm) PLASTIC DFN
DCB PACKAGE
θJA = 73.5°C/W, θJC = 12°C/W
EXPOSED PAD (PIN 7) IS GND, MUST BE SOLDERED TO PCB
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
θJA = 165°C/W, θJC = 102°C/W
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE
LT1933IDCB#PBF LT1933IDCB#TRPBF LCGM
LT1933HDCB#PBF LT1933HDCB#TRPBF LCGN
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
LT1933HDCB LT1933HDCB#TR LCGN
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 specifi ed with wider operating temperature ranges.
For more information on lead free part marking, go to:
For more information on tape and reel specifi cations, go to:
http://www.linear.com/leadfree/
http://www.linear.com/tapeandreel/
6-Lead (2mm × 3mm) Plastic DFN
6-Lead (2mm × 3mm) Plastic DFN
6-Lead (2mm × 3mm) Plastic DFN
6-Lead (2mm × 3mm) Plastic DFN
–40°C to 125°C
–40°C to 150°C
–40°C to 125°C
–40°C to 150°C
2
1933fe
LT1933
ELECTRICAL CHARACTERISTICS
The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at T
PARAMETER CONDITIONS MIN TYP MAX UNITS
Undervoltage Lockout 3.35 3.6 V
Feedback Voltage
FB Pin Bias Current V
Quiescent Current Not Switching 1.6 2.5 mA
Quiescent Current in Shutdown V
Reference Line Regulation V
Switching Frequency V
Maximum Duty Cycle
Switch Current Limit (Note 3) 0.75 1.05 A
Switch V
CESAT
Switch Leakage Current 2µA
Minimum Boost Voltage Above Switch I
BOOST Pin Current I
SHDN Input Voltage High 2.3 V
SHDN Input Voltage Low 0.3 V
SHDN Bias Current V
FB
SHDN
IN
FB
V
FB
ISW = 400mA, S6 Package
I
SW
SW
SW
SHDN
V
SHDN
= 25°C. VIN = 12V, V
A
= Measured V
= 0V 0.01 2 µA
= 5V to 36V 0.01 %/V
= 1.1V 400 500 600 kHz
= 0V 55 kHz
= 400mA, DCB6 Package
= 400mA 1.9 2.3 V
= 400mA 18 25 mA
= 2.3V (Note 5)
= 0V
+ 10mV (Note 4)
REF
= 17V, unless otherwise noted. (Note 2)
BOOST
l
1.225 1.245 1.265 V
l
l
88 94 %
40 120 nA
370
370
34
0.01
500 mV
50
0.1
mV
µA
µA
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 specifi cations
from 0°C to 70°C. Specifi cations over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LT1933I specifi cations are
guaranteed over the –40°C to 125°C temperature range. The LT1933H
specifi cations 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 fl ows out of pin.
Note 5: Current fl ows into pin.
1933fe
3
LT1933
TYPICAL PERFORMANCE CHARACTERISTICS
Effi ciency, V
100
TA = 25°C
= 5V
V
OUT
90
80
EFFICIENCY (%)
70
60
100 200 600500
0
= 5V Effi ciency, V
OUT
VIN = 12V
VIN = 24V
D1 = MBRM140
L1 = Toko D53LCB 33
300 400
LOAD CURRENT (mA)
µH
1933 G01
100
EFFICIENCY (%)
OUT
TA = 25°C
= 3.3V
V
OUT
90
80
70
60
0
VIN = 5V
VIN = 12V
100 200 600500
LOAD CURRENT (mA)
= 3.3V Switch Current Limit
1200
TA = 25°C
1000
800
VIN = 24V
D1 = MBRM140
L1 = Toko D53LCB 22
300 400
µH
1933 G02
600
400
SWITCH CURRENT LIMIT (mA)
200
0
20
0
Maximum Load Current Maximum Load Current Switch Voltage Drop
800
TA = 25°C
= 5V
V
OUT
700
600
LOAD CURRENT (mA)
500
L = 33µH
L = 22µH
800
TA = 25°C
= 3.3V
V
OUT
700
600
LOAD CURRENT (mA)
500
L = 22µH
L = 15µH
600
500
400
TA= 85°C
300
200
SWITCH VOLTAGE (mV)
100
TYPICAL
MINIMUM
40
DUTY CYCLE (%)
TA= 25°C
60
80
1933 G03
TA= –40°C
100
400
5
0
15
10
INPUT VOLTAGE (V)
20
3025
1933 G04
400
5
0
15
10
INPUT VOLTAGE (V)
20
3025
1933 G05
0
0
Feedback Voltage Undervoltage Lockout Switching Frequency
1.260
1.255
1.250
1.245
1.240
FEEDBACK VOLTAGE (V)
1.235
1.230
–50 –25 0 25 50 75 100 150125
TEMPERATURE (°C)
1933 G07
3.8
3.6
3.4
UVLO (V)
3.2
3.0
–50 –25 0 25 50 75 100 150125
TEMPERATURE (°C)
1933 G08
600
550
500
450
SWITCHING FREQUENCY (kHz)
400
–50 –25 0 25 50 75 100 150125
4
0.2 0.4
SWITCH CURRENT (A)
TEMPERATURE (°C)
0.60.1 0.3 0.5
1933 G06
1933 G09
1933fe
TYPICAL PERFORMANCE CHARACTERISTICS
Frequency Foldback Soft-Start SHDN Pin Current
700
600
500
TA = 25°C
1.4
1.2
1.0
TA = 25°C
DC = 30%
200
150
TA = 25°C
LT1933
400
300
200
SWITCHING FREQUENCY (kHz)
100
0
0.0
0.5
FB PIN VOLTAGE (V)
1.0 1.5
1933 G10
0.8
0.6
0.4
SWITCH CURRENT LIMIT (A)
0.2
0
0
1
SHDN PIN VOLTAGE (V)
234
1933 G11
100
50
SHDN PIN CURRENT (µA)
0
0
4
SHDN PIN VOLTAGE (V)
Typical Minimum Input Voltage Typical Minimum Input Voltage Switch Current Limit
8
7
TO START
6
INPUT VOLTAGE (V)
TO RUN
5
4
1 10 100
LOAD CURRENT (mA)
V
OUT
= 25°C
T
A
L = 33
= 5V
µH
1933 G13
6.0
5.5
TO START
5.0
4.5
4.0
INPUT VOLTAGE (V)
3.5
3.0
TO RUN
1 10 100
LOAD CURRENT (mA)
V
OUT
= 25°C
T
A
L = 22
= 3.3V
µH
1933 G14
1.4
1.2
1.0
0.8
0.6
0.4
SWITCH CURRENT LIMIT (A)
0.2
0
–25 0 25
–50
81216
1933 G12
50 75 100 150125
TEMPERATURE (°C)
1933 G15
VSW10V/DIV
I
200mA/DIV
L
V
10mV/DIV
OUT
Operating Waveforms
VIN = 12V, V
L = 22
µH, C
OUT
OUT
= 3.3V, I
= 22µF
OUT
= 400mA,
V
OUT1
1.8V
V
OUT2
1.2V
1933 G16
VSW10V/DIV
I
200mA/DIV
L
V
10mV/DIV
OUT
Operating Waveforms,
Discontinuous Mode
= 12V, V
V
IN
L = 22
µH, C
OUT
OUT
= 3.3V, I
= 22µF
OUT
= 20mA,
V
1.8V
V
OUT2
1.2V
1933 G17
OUT1
1933fe
5
LT1933
PIN FUNCTIONS
(SOT-23/DFN)
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.
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
= 1.245V
OUT
(1 + R1/R2). A good value for R2 is 10k.
BLOCK DIAGRAM
V
V
IN
IN
C2
INT REG
AND
UVLO
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
provides a soft-start function; see the Applications Information section.
(Pin 5/Pin 2): The VIN pin supplies current to the
V
IN
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.
ON OFF
R3
SHDN
C4
SLOPE
COMP
OSC
FREQUENCY
FOLDBACK
Σ
R
Q
S
V
GND
Q
C
g
m
1.245V
FB
R2 R1
DRIVER
BOOST
Q1
SW
D2
C3
L1
D1
1933 BD
V
OUT
C1
6
1933fe
LT1933
OPERATION
(Refer to Block Diagram)
The LT1933 is a constant frequency, current mode step
down regulator. A 500kHz oscillator enables an RS fl ipfl op, turning on the internal 750mA power switch Q1. An
amplifi er and comparator monitor the current fl owing
between the V
and SW pins, turning the switch off when
IN
this current reaches a level determined by the voltage at
. An error amplifi er measures the output voltage through
V
C
an external resistor divider tied to the FB pin and servos
node. If the error amplifi er’s output increases, more
the V
C
current is delivered to the output; if it decreases, less current is delivered. An active clamp (not shown) on the V
node provides current limit. The V
node is also clamped
C
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.
APPLICATIONS INFORMATION
An internal regulator provides power to the control circuitry. 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, disconnecting the output and reducing the input current to
less than 2µA.
The switch driver operates from either the input or from
the BOOST pin. An external capacitor and diode are used
to generate a voltage at the BOOST pin that is higher than
the input supply. This allows the driver to fully saturate
the internal bipolar NPN power switch for effi cient operation.
The oscillator reduces the 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.
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(V
/1.245 – 1)
OUT
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 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:
DC = (V
where V
(~0.4V) and V
+ VD)/(VIN – VSW + VD)
OUT
is the forward voltage drop of the catch diode
D
is the voltage drop of the internal switch
SW
(~0.4V at maximum load). This leads to a minimum input
voltage of:
V
IN(MIN)
with DC
MAX
= (V
= 0.88
+ VD)/DC
OUT
MAX
– VD + V
SW
The maximum input voltage is determined by the absolute
maximum ratings of the V
minimum duty cycle DC
and BOOST pins and by the
IN
= 0.08 (corresponding to a
MIN
minimum on time of 130ns):
V
IN(MAX)
= (V
+ VD)/DC
OUT
– VD + V
MIN
SW
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 V
and BOOST pins.
IN
Inductor Selection and Maximum Output Current
A good fi rst choice for the inductor value is:
L = 5 (V
where V
+ VD)
OUT
is the voltage drop of the catch diode (~0.4V)
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
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 effi ciency high, the series resistance (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 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 effi ciency. There are
several graphs in the Typical Performance Characteristics
section of this data sheet that show the maximum load
current as a function of input voltage and inductor value
for several popular output 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% (V
OUT/VIN
> 0.5), there
is a minimum inductance required to avoid subharmonic
oscillations. Choosing L greater than 3(V
+ VD) µH
OUT
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 effi ciency, 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 signifi cant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor.
Step-down regulators draw current from the input 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
8
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
1933fe
APPLICATIONS INFORMATION
LT1933
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 fi lters 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
OUT
= 60/V
OUT
is in µF. Use X5R or X7R types, and keep
OUT
OUT
will
C
where C
in mind that a ceramic capacitor biased with V
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 benefi t (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 specifi ed 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 (714) 373-7366 www.panasonic.com Ceramic,
Kemet (864) 963-6300 www.kemet.com Ceramic,
Sanyo (408) 749-9714 www.sanyovideo.com Ceramic,
Murata (404)436-1300 www.murata.com Ceramic
AVX www.avxcorp.com Ceramic,
Taiyo Yuden (864)963-6300 www.taiyo-yuden.com Ceramic
Polymer,
Tantalum
Tantalum T494, T495
Polymer,
Tantalum
Tantalum TPS Series
EEF Series
POSCAP
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
V
16.5k
16.5k
10k
470pF
22µFFB
1933 F01a
V
OUT
OUT
V
OUT
50mV/DIV
I
OUT
200mA/DIV
V
OUT
50mV/DIV
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.
FB
10k
16.5k
FB
10k
22µF
2x
I
OUT
200mA/DIV
V
OUT
50mV/DIV
I
OUT
200mA/DIV
+
1933 F01b
V
OUT
100µF
SANYO
4TPB100M
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, V
= 3.3V, L = 22μH.
OUT
10
1933fe
APPLICATIONS INFORMATION
LT1933
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 effi ciency. 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 effi cient 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.
D2
BOOST
V
IN
LT1933
V
IN
SW
C3
V
OUT
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
BOOST
V
IN
LT1933
V
IN
SW
C3
V
OUT
V
– VSW≅ V
BOOST
MAX V
BOOST
Minimum Input Voltage V
6.0
5.5
TO START
5.0
4.5
4.0
INPUT VOLTAGE (V)
3.5
3.0
TO RUN
1 10 100
LOAD CURRENT (mA)
Figure 3. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
GND
OUT
≅ VIN + V
1933 F02a
OUT
(2a)
V
MAX V
Figure 2. Two Circuits for Generating the Boost Voltage
= 3.3V Minimum Input Voltage V
OUT
V
= 3.3V
OUT
= 25°C
T
A
L = 22µH
1933 F03a
GND
– VSW≅ V
BOOST
BOOST
8
7
6
INPUT VOLTAGE (V)
5
4
IN
≅ 2V
IN
(2b)
TO START
TO RUN
1 10 100
LOAD CURRENT (mA)
1933 F02b
OUT
V
OUT
= 25°C
T
A
L = 33
= 5V
= 5V
µH
1933 F03b
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 V
is ramping very slowly.
IN
For lower start-up voltage, the boost diode can be tied to
; however, this restricts the input range to one-half of
V
IN
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 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
1933 F04a
100mA/DIV
V
OUT
5V/DIV
IN
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 fi lter 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
0.1µF
50µs/DIV
RUN
15k
SHDN
GND
1933 F04b
5V/DIV
100mA/DIV
V
OUT
5V/DIV
I
IN
0.5ms/DIV
Figure 4. To Soft-Start the LT1933, Add a Resistor and Capacitor to
the SHDN Pin. V
= 12V, V
INI
= 3.3V, C
OUT
= 22μF, R
OUT
LOAD
= 10Ω
1933fe
12
APPLICATIONS INFORMATION
LT1933
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 V
pin is allowed to fl oat and the SHDN pin
IN
is held high (either by a logic signal or because it is tied
), then the LT1933’s internal circuitry will pull its
to V
IN
quiescent current through its SW pin. This is fi ne 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 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
IN
V
BOOST
IN
and the V
pin. Figure 5 shows a circuit that will run only
IN
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 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.
LT1933
1933 F05
V
OUT
BACKUP
SHDN SW
GND FB
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
+
LOW
IMPEDANCE
ENERGIZED
24V SUPPLY
I
IN
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
V
IN
2.2µF
LT1933
(6a)
V
20V/DIV
5A/DIV
IN
I
IN
DANGER!
RINGING VIN MAY EXCEED
ABSOLUTE MAXIMUM
RATING OF THE LT1933
20µs/DIV
LT1933
+
10µF
35V
AI.EI.
+
2.2µF
V
20V/DIV
5A/DIV
IN
I
IN
20µs/DIV
(6b)
1Ω
LT1933
+
2.2µF0.1µF
V
20V/DIV
5A/DIV
IN
I
IN
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
1933 F06
14
1933fe
APPLICATIONS INFORMATION
LT1933
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 fi rst 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 fi ltering and can
slightly improve the effi ciency of the circuit, though it is
likely to be the largest component in the circuit. An alternative 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 fi ltering. This solution is smaller
and less expensive than the electrolytic capacitor. For high
input voltages its impact on effi ciency is minor, reducing
effi ciency 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 simplifi es 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.
Figure 7 shows an equivalent circuit for the LT1933 control
loop. The error amp is a transconductance amplifi er with
fi nite output impedance. The power section, consisting of
the modulator, power switch and inductor, is modeled as
a transconductance amplifi er generating an output current proportional to the voltage at the V
node. Note that
C
the output capacitor integrates this current, and that the
capacitor on the V
fi er output current, resulting in two poles in the loop. R
node (CC) integrates the error ampli-
C
C
provides a zero. With the recommended output capacitor,
the loop crossover occurs above the R
zero. This simple
CCC
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 (C
) across the feedback divider may
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 temperature. The LT1375 data sheet contains a more thorough
discussion of loop compensation and describes how to
test the stability using a transient load.
LT1933
R
100k
C
C
80pF
GND
CURRENT MODE
–
0.7V
1.1mho
V
C
C
500k
POWER STAGE
g
m
+
gm =
150µmhos
ERROR
AMPLIFIER
Figure 7. Model for Loop Response
SW
C
R1
–
FB
+
1.245V
R2
PL
ESR
+
C1
OUT
C1
1933 F07
1933fe
15
LT1933
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 fl ow 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
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,
V
OUT
D1C1 C2
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.
V
IN
GND
VIAS
(8a)
1933 F08a
DFN Package
SHUTDOWN
V
IN
C2 D1
VIAS
OUTLINE OF LOCAL GROUND PLANE
C1
1933 F08b
V
OUT
SYSTEM
GROUND
(8b)
SOT-23 Package
Figure 8. A Good PCB Layout Ensures Proper, Low EMI Operation
16
1933fe
TYPICAL APPLICATIONS
LT1933
3.3V Step-Down Converter
V
4.5V TO
36V
OFF ON
V
14.5V TO
36V
OFF ON
C3
0.1µF
D1
D2
L1
22µH
V
OUT
3.3V/
500mA
C1
22µF
6.3V
1933 TA02b
IN
V
IN
LT1933
SHDN SW
GND FB
C2
2.2µF
BOOST
R1
16.5k
R2
10k
12V Step-Down Converter
D2
IN
V
IN
LT1933
SHDN SW
GND FB
C2
2.2µF
BOOST
R1
86.6k
R2
10k
D3, 6V
C3
0.1µF
D1
L1
47µH
V
OUT
12V/
450mA
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
2.15 ±0.05
1.65 ±0.05
(2 SIDES)
PACKAGE
OUTLINE
0.25 ± 0.05
0.50 BSC
1.35 ±0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
2.00 ±0.10
(2 SIDES)
3.00 ±0.10
(2 SIDES)
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
0.200 REF
0.75 ±0.05
0.00 – 0.05
R = 0.115
R = 0.05
1.65 ± 0.10
(2 SIDES)
TYP
TYP
BOTTOM VIEW—EXPOSED PAD
3
1.35 ±0.10
(2 SIDES)
0.40 ± 0.10
64
1
0.50 BSC
PIN 1 NOTCH
R0.20 OR 0.25
× 45° CHAMFER
(DCB6) DFN 0405
0.25 ± 0.05
18
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
PACKAGE DESCRIPTION
LT1933
S6 Package
6-Lead Plastic SOT-23
(Reference LTC DWG # 05-08-1634)
0.62
MAX
3.85 MAX
0.20 BSC
2.62 REF
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
DATUM ‘A’
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)
0.95
REF
0.35 – 0.55 REF
1.22 REF
1.4 MIN
0.09 – 0.20
(NOTE 3)
2.60 – 3.00
1.50 – 1.75
(NOTE 4)
0.90 – 1.45
2.80 – 3.10
(NOTE 4)
PIN ONE ID
0.95 BSC
0.90 – 1.30
1.90 BSC
ATTENTION: ORIGINAL SOT23-6L PACKAGE.
MOST SOT23-6L PRODUCTS CONVERTED TO THIN SOT23
PACKAGE, DRAWING # 05-08-1636 AFTER APPROXIMATELY
APRIL 2001 SHIP DATE
0.25 – 0.50
TYP 6 PLCS
NOTE 3
0.09 – 0.15
NOTE 3
S6 SOT-23 0502
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.
1933fe
19
LT1933
TYPICAL APPLICATION
2.5V Step-Down Converter
C3
0.47µF
D1
D2
L1
15µH
V
3.6V TO 36V
OFF ON
IN
V
IN
LT1933
SHDN SW
GND FB
C2
2.2µF
BOOST
R1
10.5k
R2
10k
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V
OUT
2.5V/500mA
C1
22µF
1933 TA03
= 2.21V, IQ = 8.5mA, ISD = 10µA,
OUT(MIN)
= 2.21V, IQ = 8.5mA, ISD = 10µA,
OUT(MIN)
= 1.24V, IQ = 3.2mA, ISD = 2.5µA,
OUT(MIN)
= 1.2V, IQ = 1mA, ISD = 15µA,
OUT(MIN)
= 1.2V, IQ = 2.5mA, ISD = 25µA,
OUT(MIN)
= 1.2V, IQ = 1mA, ISD = 6µA,
OUT(MIN)
= 1.24V, IQ = 3.2mA, ISD = 30µA,
OUT(MIN)
= 1.25V, IQ = 3.8mA, ISD < 30µA,
OUT(MIN)
= 1.2V, IQ = 2.5mA, ISD = 25µA,
OUT(MIN)
= 1.2V, IQ = 100µA, ISD < 1µA,
OUT(MIN)
= 1.28V, IQ = 30µA, ISD < 1µA,
OUT(MIN)
= 0.6V, IQ = 40µA, ISD < 1µA,
OUT(MIN)
= 0.8V, IQ = 60µA, ISD < 1µA,
OUT(MIN)
= 0.8V, IQ = 64µA, ISD < 1µA,
OUT(MIN)
= 1.2V, IQ = 2.5mA, ISD = 30µA,
OUT(MIN)
20
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
1933fe
LT 0409 REV E • PRINTED IN USA
© LINEAR TECHNOLOGY CORPORATION 2007
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