1
DEMO MANUAL DC233
SOT-23 SWITCHING REGULATORS
LT1611/LT1613
1.4MHz Switching Regulators
in SOT-23
Burst Mode is a trademark of Linear Technology Corporation.
, LTC and LT are registered trademarks of Linear Technology Corporation.
The LT
®
1611 and LT1613 are 5-lead SOT-23, current
mode DC/DC converters. Intended for small, low power
applications, both operate from inputs as low as 1V and
switch at 1.4MHz, allowing the use of tiny, low cost
capacitors and inductors.
DC233 contains three switching regulator circuits. Two of
these demonstrate the use of the LT1613CS5 in a simple
boost regulator circuit and in an uncoupled SEPIC circuit.
Both circuits produce 3.3V or 5V (jumper selected). The
boost circuit produces 200mA in a typical application and
occupies less than 0.2 square inches of circuit board area.
The SEPIC circuit allows operation from input voltages
either higher or lower than the output, making this circuit
suitable for single Li-Ion cell to 3.3V conversion or four
alkaline cells to 5V conversion. Typical output current for
the SEPIC circuit is 120mA. The third circuit demon-
strates the LT1611CS5 in a low
noise inverting circuit.
This circuit can convert 5V to –5V at 160mA.
PARAMETER CONDITIONS VALUE
Boost
Input Voltage (Note 1) V
OUT
= 3.3V 1V to 3.6V
V
OUT
= 5V 1V to 5.3V
Maximum Load Current (Min) V
OUT
= 3.3V, V
IN
= 1.5V 115mA
V
OUT
= 5V, V
IN
= 3V 190mA
Shutdown Current (Typ) V
IN
= 1.5V, SHDN = 0V 10µA
3.3VOUT Efficiency (Boost) 5VOUT Efficiency (Boost)
DESCRIPTIO
U
PERFOR A CE SU ARY
UWWW
TYPICAL PERFOR A CE CHARACTERISTICS A D BOARD PHOTO
UU
W
LOAD CURRENT (mA)
0
30
EFFICIENCY (%)
40
50
60
70
90
50 100 150 200
DC233 TA01
250 300
80
VIN = 1.5V
VIN = 2.4V
LOAD CURRENT (mA)
0
30
EFFICIENCY (%)
40
50
60
70
90
50 100 150 200
DC233 TA02
250 300
80
VIN = 2.4V
VIN = 3.3V
2
DEMO MANUAL DC233
SOT-23 SWITCHING REGULATORS
D
U
ESCRIPTIO
The LT1611 and LT1613 will find applications in battery-
powered products, such as pagers, digital cameras,
cellular phones, cordless phones and palmtop comput-
ers. The small circuit size and low component count
make these parts suitable for use in PC cards, miniature
disk drives and flash memory products, and for generat-
ing local logic supplies—for example, converting 3.3V to
5V. The LT1611 produces a very low noise negative
output and is suitable for generating negative rails for op
amp circuits and disk drives.
W
ARY
A
U
CE SUPERFOR
WW
TYPICAL PERFOR A CE CHARACTERISTICS
UW
3.3VOUT Efficiency (SEPIC) 5VOUT Efficiency (SEPIC)
LOAD CURRENT (mA)
0
30
EFFICIENCY (%)
40
50
60
70
90
50 100 150 200
DC233 TA03
80
VIN = 4.2V
VIN = 2.7V
LOAD CURRENT (mA)
0
30
EFFICIENCY (%)
40
50
60
70
90
50 100 150 200
DC233 TA04
80 VIN = 6V
VIN = 4V
–5VOUT Efficiency (Inverter)
LOAD CURRENT (mA)
0
30
EFFICIENCY (%)
40
50
60
70
90
50 100 150 200
DC233 TA05
80
VIN = 5V
VIN = 3.3V
PARAMETER CONDITIONS VALUE
SEPIC
Input Voltage (Note 1) 1V to 6V
Maximum Load Current (Min) V
OUT
= 3.3V, V
IN
= 3V 130mA
V
OUT
= 5V, V
IN
= 5V 120mA
Shutdown Current (Typ) V
IN
= 3V, SHDN = 0V 0.5µA
Inverter
Input Voltage (Note 1) 1V to 6V
Maximum Load Current (Min) V
OUT
= –5V, V
IN
= 5V 165mA
Shutdown Current (Typ) V
IN
= 5V, SHDN = 0V 0.5µA
Note 1: This limit is based on the DC233 circuits. The LT1611 and LT1613 can operate from higher supply voltages.
3
DEMO MANUAL DC233
SOT-23 SWITCHING REGULATORS
TYPICAL PERFOR A CE CHARACTERISTICS
UW
Max Load Current vs VIN (SEPIC)
Max Load Current vs VIN (Boost)
Max Load Current vs VIN (Inverter)
INPUT VOLTAGE (V)
1
MAXIMUM OUTPUT CURRENT, MIN (mA)
200
300
5
DC233 TA06
100
0234
500 BOOST
400
V
OUT
= 3.3V V
OUT
= 5V
INPUT VOLTAGE (V)
MAXIMUM OUTPUT CURRENT, MIN (mA)
100
150
8
DC233 TA07
50
026
4
250
200
V
OUT
= 5V
V
OUT
= 3.3V
SEPIC
INPUT VOLTAGE (V)
MAXIMUM OUTPUT CURRENT, MIN (mA)
100
150
8
DC233 TA08
50
026
4
250
200
V
OUT
= 5V
INVERTER
4
DEMO MANUAL DC233
SOT-23 SWITCHING REGULATORS
Boost
+
+
V
IN
U2
LT1613
V
IN
(3.3V
OUT
) 1V TO 3.6V
(5V
OUT
) 1V TO 5.3V
SHDN FB
DC233 F01a
R4
100k
L4
4.7µH
R6
59k
R5
71.5k
JP2
C7
0.1µFC10
1µFC11
0.1µF
C8
15µFC9
15µF
V
OUT
GND
SHDN
GND
SW
D2
+
+
V
IN
U3
LT1611
V
IN
1V TO 6V
SHDN FB
DC233 F01c
R7
30.1k
L5
22µHL6
22µH
R8
10k
C12
0.1µFC16
1µFC17
0.1µF
C13
15µFC15
15µF
C14
1µF
V
OUT
GND
SHDN
GND
SW D3
SEPIC
Inverting
+
+
V
IN
U1
LT1613
V
IN
1V TO 6V
SHDN FB
DC233 F01b
R1
100kL2
10µH
L1
10µH
R3
59k
R2
71.5k
JP1
C1
0.1µFC5
1µFC6
0.1µF
C2
15µFC4
15µF
C3
1µF
V
OUT
GND
SHDN
GND
SW
D1
Figure 1. DC233 Schematics
LT1611CS5 LT1613CS5
SW 1
GND 2
TOP VIEW
S5 PACKAGE
5-LEAD PLASTIC SOT-23
FB 3
5 V
IN
4 SHDN
SW 1
GND 2
TOP VIEW
S5 PACKAGE
5-LEAD PLASTIC SOT-23
NFB 3
5 V
IN
4 SHDN
SCHE ATIC A D CO ECTIO DIAGRA S
WW
UUU U
5
DEMO MANUAL DC233
SOT-23 SWITCHING REGULATORS
PARTS LIST
REFERENCE
DESIGNATOR QUANTITY PART NUMBER DESCRIPTION VENDOR TELEPHONE
Boost
C7, C11 2 0805YC104MAT1A 0.1µF 16V X7R 0805 Capacitor AVX (843) 946-0362
C8, C9 2 TAJA156M010R 15µF 10V 20% Tantalum Capacitor AVX (207) 282-5111
C10 1 0805ZC105MAT1A 1µF 10V X7R 0805 Capacitor AVX (843) 946-0362
D2 1 MBR0520LT1 0.5A 20V SOD123 Schottky Diode ON Semiconductor (602) 244-6600
JP2 1 2802S-2-G1 2-Pin Header, 0.079 Center Comm Con (626) 301-4200
L4 1 LQH3C4R7M24 4.7µH Inductor Murata (770) 436-1300
R4 1 CR16-1003FM 100k 1/10W 1% 0603 Resistor TAD (800) 508-1521
R5 1 CR16-7152FM 71.5k 1/10W 1% 0603 Resistor TAD (800) 508-1521
R6 1 CR16-5902FM 59k 1/10W 1% 0603 Resistor TAD (800) 508-1521
U2 1 LT1613CS5 SOT-23 DC/DC Converter LTC (408) 432-1900
1 CCIJ2MM-138G Shunt, 0.079 Center Comm Con (626) 301-4200
SEPIC
C1, C6 2 0805YC104MAT1A 0.1µF 16V X7R 0805 Capacitor AVX (843) 946-0362
C2, C4 2 TAJA156M010R 15µF 10V 20% Tantalum Capacitor AVX (207) 282-5111
C3, C5 2 0805ZC105MAT1A 1µF 10V X7R 0805 Capacitor AVX (843) 946-0362
D1 1 MBR0520LT1 0.5A 20V SOD123 Schottky Diode ON Semiconductor (602) 244-6600
JP1 1 2802S-2-G1 2-Pin Header, 0.079 Center Comm Con (626) 301-4200
L1, L2 2 LQH3C100K24 10µH Inductor Murata (770) 436-1300
R1 1 CR16-1003FM 100k 1/10W 1% 0603 Resistor TAD (800) 508-1521
R2 1 CR16-7152FM 71.5k 1/10W 1% 0603 Resistor TAD (800) 508-1521
R3 1 CR16-5902FM 59k 1/10W 1% 0603 Resistor TAD (800) 508-1521
U1 1 LT1613CS5 SOT-23 DC/DC Converter LTC (408) 432-1900
1 CCIJ2MM-138G Shunt, 0.079 Center Comm Con (626) 301-4200
Inverting
C12, C17 2 0805YC104MAT1A 0.1µF 16V X7R 0805 Capacitor AVX (843) 946-0362
C13, C15 2 TAJA156M010R 15µF 10V 20% Tantalum Capacitor AVX (207) 282-5111
C14, C16 2 0805ZC105MAT1A 1µF 10V X7R 0805 Capacitor AVX (843) 946-0362
D3 1 MBR0520LT1 0.5A 20V SOD123 Schottky Diode ON Semiconductor (602) 244-6600
L5, L6 2 LQH3C220K34 22µH Inductor Murata (770) 436-1300
R7 1 CR16-3012FM 30.1k 1/10W 1% 0603 Resistor TAD (800) 508-1521
R8 1 CR16-1002FM 10k 1/10W 1% 0603 Resistor TAD (800) 508-1521
U3 1 LT1611CS5 SOT-23 DC/DC Converter LTC (408) 432-1900
6
DEMO MANUAL DC233
SOT-23 SWITCHING REGULATORS
QUICK START GUIDE
DC233 contains three switching regulator circuits. Two
of these demonstrate the use of the LT1613CS5 in a simple
boost regulator circuit and in an uncoupled SEPIC circuit.
Both circuits produce 3.3V or 5V (jumper selected). The
third circuit demonstrates the LT1611CS5 in a low noise
inverting circuit, producing a –5V output. The three cir-
cuits are electrically isolated from each other, and have
their own grounds. Each circuit has a similar set of inputs
and outputs—this quick-start guide applies to all.
1. The output of the boost and SEPIC circuits can be set
to either 3.3V or 5V. The board is shipped with a jumper
in place that programs the output for 5V. Remove the
jumper to program the circuit for 3.3V out.
2. Apply a voltage source to the input of the circuit
between the V
IN
and GND terminals. A benchtop
supply with a 1A current limit is a good choice for this
source. The circuit will operate from an input voltage
between 1V and 6V. Do not apply more than 6V to the
circuit. Note that the boost circuit will regulate the
output only when the input voltage is less than the
desired output voltage.
3. Attach a voltmeter or oscilloscope probe between the
V
OUT
and GND terminals of the circuit in order to
monitor the output. To start the circuit, tie the SHDN
terminal to the V
IN
terminal. The LT1611/LT1613 will
begin regulating the output voltage.
4. Attach a load to the output. The power capability of
these circuits depends on the input voltage. A 100Ω
one-half watt resistor soldered between the V
OUT
and
GND pins of the circuit is a good starting point, and will
allow you to observe the operation of the circuit.
5. The circuit can be placed in shutdown mode by either
floating the SHDN terminal or tying it to ground.
6. Proper hook-up is essential for accurate and meaning-
ful evaluation of efficiency and regulation. Figure 2
shows the appropriate arrangement of the supply,
load, ammeters and voltmeters.
Figure 2. Proper Hook-Up for Evaluating the DC233
VIN
DC233
VOUT
GND
SHDN
VOUT
VIN
LOAD
BENCH
SUPPLY
1V TO 6V/1A
IOUT
+
–
+
–
A
IIN
A
7
DEMO MANUAL DC233
SOT-23 SWITCHING REGULATORS
OPERATIO
U
INTRODUCTION
DC233 contains three switching regulator circuits. Two of
these demonstrate the use of the LT1613CS5 in a simple
boost regulator circuit and in an uncoupled SEPIC
1
circuit.
Both circuits produce 3.3V or 5V (jumper selected). The
SEPIC circuit allows operation from input voltages either
higher or lower than the output, making this circuit suit-
able for single Li-Ion cell to 3.3V conversion or four
alkaline cells to 5V conversion. The third circuit demon-
strates the LT1611CS5 in a low noise inverting circuit. This
circuit can convert 5V to –5V at 160mA.
The three circuits on the DC233 are electrically isolated
from each other and have their own grounds. Because the
three circuits are functionally similar and have the same
input and output connections (V
IN
, V
OUT
, GND and SHDN),
many of the comments that follow will apply to all three.
Each circuit is described in more detail in its individual
section.
This manual describes the operation of these demonstra-
tion circuits, their performance, and variations on the
basic circuits. For a thorough discussion of the LT1611
and LT1613 and their applications, please consult the
parts’ data sheets.
Hook-Up and Initial Tests
DC233 contains fairly simple, low power switching regu-
lators. However, some precautions are necessary in
order to test the circuits safely. Proper hook-up and
accurate measurements are necessary for meaningful
evaluation of efficiency and line and load regulation.
Refer to Figure 2 for proper connections.
The outputs of the boost and SEPIC circuits can be set to
either 3.3V or 5V. The board is shipped with a jumper in
place that programs the output for 5V. Remove the
jumper to program the circuit for 3.3V out. The input can
safely accept a voltage as high as 6V. A good starting
point is to apply 2.5V between the V
IN
and GND terminals
of the DC233, using a benchtop supply with a 1A current
limit. Because the SHDN pin has been left floating, the
LT1611/LT1613 will default to its shutdown mode. Tie
the SHDN terminal of the DC233 to the V
IN
to start the
regulator.
Apply a load between the V
OUT
and GND terminals, using
either a fixed resistor, a decade resistor box (provided it
is rated for the power) or an active load. A simple initial
load might be a one-half watt, 100Ω resistor. Warning:
because the boost circuit contains a DC path between the
input and output (through inductor L4 and diode D2), the
circuit is not protected against a shorted output. It is
recommended that preliminary testing of the circuit be
performed using a current-limited supply on the input.
Figure 3 shows some of the boost circuit’s operating
waveforms. The scope photo shows the output voltage,
the current through the internal power switch (the cur-
rent into the SW pin) and the voltage on the SW pin of the
LT1613. The SEPIC and inverting circuits display similar
waveforms.
Figure 3. Operating Waveforms of the DC233 Boost
Circuit (VIN = 2V, VOUT = 3.3V, IOUT = 80mA)
0.2µs/DIV DC231 F03
V
OUT
50mV/DIV
I
SW
200mA/DIV
V
SW
5V/DIV
1
SEPIC is an acronym for "single-ended primary-inductance converter.”
PERFORMANCE
Efficiency
The efficiency of the DC233 circuits is plotted in the
Typical Performance section of this manual. Efficiency
measurements should be made with care, as there are
plenty of opportunities for errors to creep in.
8
DEMO MANUAL DC233
SOT-23 SWITCHING REGULATORS
OPERATIO
U
The efficiency is defined as the power delivered to the
load divided by the power drawn from the input supply.
Normally, the average input voltage, input current, out-
put voltage and output current are measured under
steady-state conditions and the efficiency is calculated
from these values. Each should be measured with the
highest accuracy and precision possible.
Figure 2 shows connections for the proper measurement
of efficiency and output regulation. The input and output
voltages are measured at the DC233 terminals in order to
avoid including voltage drops across ammeters and
terminal connections. It is best to take all of these
measurements at one time. Be aware that most digital
multimeters drop significant voltage when they are used
as ammeters, so you must measure the input voltage
while the ammeter is in the circuit—the input voltage will
be lower than the voltage at the output of your benchtop
supply.
Testing in Your System
You may want to paste this circuit into your system to test
compatibility. This should be done with care, since long
hook-up wires and ground loops can introduce noise
sources and regulation problems that would not be
present if the DC/DC converter were properly designed
into your PCB.
Treat the DC233 as a 3-terminal device, with V
IN
, V
OUT
and GND terminals. Wire the DC233 to your circuit board
with wires as short as practical, to points on the circuit
board that are close to each other. Also, add high fre-
quency bypass capacitors (0.1µF ceramics) from V
IN
and
V
OUT
to ground on your circuit board.
If you are bringing power directly to the DC233, use two
wires from the input source to the V
IN
and GND terminals
of the DC233. The output power should be applied to your
system as described above and either the input supply or
your circuit should be floating in order to avoid ground
loops.
BOOST
The boost circuit is the simplest LT1613 circuit. It can be
used to convert a low voltage to a higher output voltage,
for example, converting 1- or 2-cell alkaline batteries to
3.3V or 5V or generating a local 5V logic supply from a
3.3V rail.
Input Range and Power Capability
The LT1613 will typically run from inputs down to 0.9V
and is guaranteed to operate from inputs above 1V. The
maximum allowable input voltage to this circuit is 6V,
which is based on the voltage ratings of the input and
output capacitors, C8 and C9. The boost circuit will allow
the LT1613 to regulate the output only when the input
voltage is less than the desired output voltage plus one
diode drop. This means that the practical input range is
0.9V to 3.6V for a 3.3V output and 0.9V to 5.3V for a 5V
output.
The power capability of the DC233 boost circuit is deter-
mined primarily by the input voltage and by the current
limit of the LT1613’s internal power switch and, to a
lesser extent, by the value of the inductor L4. Therefore,
the maximum load current that this circuit can supply
depends on the input voltage. A graph of maximum load
appears in the Typical Performance section of this manual.
This curve is based on the minimum current limit speci-
fication in the LT1613 data sheet. A typical LT1613 will
deliver more current. As load current is increased beyond
this level, the output voltage will sag as the LT1613
reaches its current limit.
Be aware that L4 and D2 provide a direct path between the
input and output, and that this circuit does not limit the
output current. As an increasing load drags the output
voltage below the input, a larger current will flow, limited
only by the impedance of the power source, inductor L4
and diode D2.
9
DEMO MANUAL DC233
SOT-23 SWITCHING REGULATORS
OPERATIO
U
Shutdown Mode
The SHDN pin of the LT1613 is tied directly to the SHDN
terminal of the DC233 and has been left floating. In this
condition, or with this pin grounded, the LT1613 is in its
shutdown mode. In this state, the LT1613 will draw less
than 1µA from the input. However, the inductor L4 and
catch diode D2 provide a path from the input to the output
and the feedback divider (R8, R10 and R11) may draw a
few µA, depending on the input voltage. In addition, the
load can draw power from the input while the LT1613 is
shut down.
SEPIC
The LT1613 SEPIC circuit is slightly more complicated
than the boost circuit, but it can regulate the output over
a wider input voltage range. It might be used, for ex-
ample, to convert a Li-Ion cell input (2.7V to 4.2V) to a
3.3V output.
Input Range and Power Capability
The LT1613 will typically run from inputs down to 0.9V,
and is guaranteed to operate from inputs above 1V. The
maximum allowable input voltage to this circuit is 6V,
which is based on the voltage ratings of the input capaci-
tor, C2. Unlike the boost circuit, the SEPIC can regulate
the output voltage when the input voltage is higher.
As in the boost circuit, the power capability of the DC233
SEPIC circuit is determined primarily by the input voltage
and by the current limit of the LT1613’s internal power
switch and, to a lesser extent, by the value of the
inductors L1 and L2. Therefore, the maximum load
current that this circuit can supply depends on the input
voltage. A graph of maximum load appears in the Typical
Performance section of this manual. This curve is based
on the minimum current-limit specification in the LT1613
data sheet. A typical LT1613 will deliver more current. As
load current is increased beyond this level, the output
voltage will sag as the LT1613 reaches its current limit.
The SEPIC circuit can be implemented with either a pair
of inductors or a 1:1 transformer. Figure 4 shows the
transformer arrangement. The DC233 layout includes
pads for installation of two types of 1:1 surface mount
transformers from Sumida. The Sumida CLS62-100 is a
10µH inductor with two windings that can be used as a
transformer. This coupled inductor reduces the ripple
current in the LT1613, raising the output power capability
of the circuit by 20%. There are also pads that accept the
Sumida CLQ61B-8R2. Use this part to implement a low
profile design. It can be mounted within a routed hole
(not present on the DC233 circuit board), reducing the
inductor height to less than 1.5mm above the top surface
of the printed circuit board.
Figure 4. Transformer Arrangement for the SEPIC
V
IN
V
IN
V
OUT
LT1613
DC233 F04
SHDN FBSHDN
GND
SW
L3
+
+
+
•
•
Shutdown Mode
Float the SHDN terminal of the DC233 or tie it to ground
to shut down the LT1613. The coupling capacitor C3
provides a DC block between the input and output of the
SEPIC circuit. This provides an automatic disconnect
function; when the LT1613 is placed in shutdown mode,
the load cannot draw current from the input source. The
shutdown current consumption is less than 1µA.
10
DEMO MANUAL DC233
SOT-23 SWITCHING REGULATORS
OPERATIO
U
INVERTER
The LT1611 inverter uses a very low noise circuit topol-
ogy. Both the input and output of this circuit are con-
nected to inductors and AC current into the input and
output capacitors is very low. This results in low voltage
ripple at the input and output. This circuit provides lower
noise and better regulation than switched capacitor in-
verters of equivalent power.
Input Range and Power Capability
The LT1611 will typically run from inputs down to 0.9V
and is guaranteed to operate from inputs above 1V. The
maximum allowable input voltage to this circuit is 6V,
which is based on the voltage ratings of the input capaci-
tor C13 and coupling capacitor C14. This inverting circuit
can regulate a negative output voltage whose magnitude
is either greater or less than the input voltage.
The power capability of the DC233 inverter is determined
primarily by the input voltage and by the current limit of
the LT1611’s internal power switch and, to a lesser
extent, by the value of the inductors L5 and L6. Therefore,
the maximum load current that this circuit can supply
depends on the input voltage. A graph of maximum load
appears in the Typical Performance section of this manual.
This curve is based on the minimum current limit speci-
fication in the LT1611 data sheet. A typical LT1611 will
deliver more current. As load current is increased beyond
this level, the output voltage will sag as the LT1611
reaches its current limit.
The inverter can be implemented with either a pair of
inductors or with a 1:1 transformer. Figure 5 shows the
transformer arrangement. The DC233 layout includes
pads for installation of two types of 1:1 surface mount
transformers from Sumida. The Sumida CLS62-100 is a
10µH inductor with two windings that can be used as a
transformer. The Sumida CLS62-220 22µH inductor will
increase power capability by ≈10% and decrease output
ripple at the expense of slightly lower efficiency. There
V
IN
V
IN
V
OUT
LT1611
DC233 F05
SHDN FBSHDN
GND
SW
L7
+
+
+
•
•
Figure 5. Transformer Arrangement for the Inverter
are also pads that accept the Sumida CLQ61B-8R2. Use
this part to implement a low profile design. It can be
mounted within a routed hole (not present on the DC233
circuit board), reducing the inductor height to less than
1.5mm above the top surface of the printed circuit board.
Shutdown Mode
Float the SHDN terminal of the DC233 or tie it to ground
to shut down the LT1611. The coupling capacitor C14
provides a DC block between the input and output of the
inverter. This provides an automatic disconnect function:
when the LT1611 is placed in shutdown mode, the load
cannot draw current from the input source. The shut-
down current consumption is less than 1µA.
DESIGN ALTERNATIVES
Component Selection
The components used for the DC233 emphasize low cost
and small size. Other component choices can provide
improved performance. As described above, for ex-
ample, replacing the inductors in the SEPIC and inverting
circuits results in greater output current capability. This
section will describe some other alternatives.
11
DEMO MANUAL DC233
SOT-23 SWITCHING REGULATORS
OPERATIO
U
Diodes D1, D2 and D3 (Motorola MBR0520LT1) are one-
half amp, 20V Schottky diodes. This is a good choice for
nearly any LT1611/LT1613 application, unless the out-
put voltage or the circuit topology requires a diode rated
for higher reverse voltages. Motorola also offers 30V and
40V versions. Most one-half amp and one amp Schottky
diodes are suitable; these are available from many manu-
facturers. If you use a silicon diode, it must be an ultrafast
recovery type. Efficiency will be lower due to the silicon
diode’s higher forward voltage drop.
Inductors used with the LT1611 and LT1613 should be
rated for approximately 0.5A. The value of the inductor
should be matched to the power requirements and oper-
ating voltages of the application. In most cases a value of
4.7µH or 10µH is suitable. The Murata inductors used on
the DC233 are small and inexpensive and are a good fit
for the LT1611 and LT1613. Alternatives are the CD43
series from Sumida and the DO1608 series from Coil-
craft. These inductors are slightly larger but will result in
slightly higher circuit efficiency.
The voltage rating of the input capacitor limits the input
voltage range of the circuits. The input range to the SEPIC
and inverting circuit can be raised to 10V by replacing the
input capacitor (C2 or C13) with a 16V capacitor and (in
the case of the inverter) the coupling capacitor (C14) with
a 16V part. Note that, in power supply applications, most
tantalum capacitor manufacturers recommend using a
capacitor with a voltage rating higher than the operating
voltage.
The coupling capacitor in the SEPIC and inverting circuits
(C3 or C14) should have a low ESR to ensure good
efficiency and must have an adequate ripple current
rating. It also must have a suitable voltage rating. In the
case of the SEPIC circuit, it should be rated for the
maximum input voltage or higher; in the inverter, its
voltage rating must be higher than the sum of the
magnitudes of the input and output voltages. If a coupled
inductor is used, the value of this ceramic capacitor can
be reduced to 0.22µF from the 1µF used here.
Lower Ripple
The quality of the output capacitor is the greatest deter-
minant of the output voltage ripple. The output capacitor
performs two major functions: it must have enough
capacitance to satisfy the load under transient conditions
and it must shunt the AC component of the current
coming through the diode from the inductor. The ripple
on the output results when this AC current passes
through the finite impedance of the output capacitor. The
capacitor should have low impedance at the 1.4MHz
switching frequency of the LT1611/LT1613. At this fre-
quency, the impedance is usually dominated by the
capacitor’s equivalent series resistance (ESR). Choosing
a capacitor with lower ESR will result in lower output
ripple.
The DC233 uses a combination of two capacitors to
achieve these ends. The 15µF tantalum output capacitor
(C4, C9 or C15) provides the bulk capacitance for good
transient response. A 1µF ceramic capacitor (C5, C10 or
C16) in parallel with the tantalum capacitor provides a
low impedance bypass at the switching frequency. This
results in low output ripple and helps to maintain good
efficiency at high loads by eliminating AC losses in the
main output capacitor.
This combination output capacitor provides good perfor-
mance at low cost. Both capacitors are quite small.
However, low ESR and the required bulk output capaci-
tance can be obtained using a single larger output capaci-
tor. Larger tantalum capacitors, newer capacitor tech-
nologies (for example the POSCAP from Sanyo and
SPCAP from Panasonic) or large value ceramic capaci-
tors will reduce the output ripple. Note, however, that the
stability of the circuit depends on both the value of the
output capacitor and its ESR. When using low value
capacitors or capacitors with very low ESR, circuit stabil-
ity should be evaluated carefully, as described below.
12
DEMO MANUAL DC233
SOT-23 SWITCHING REGULATORS
OPERATIO
U
Loop Compensation
The LT1611 and LT1613 are current mode, PWM switch-
ing regulators. Each uses a linear control loop to regulate
its output. This control loop is compensated internally,
eliminating several external components. However, the
stability of the control loop depends on the value of the
output capacitor and its ESR. A tantalum capacitor’s
combination of capacitance and ESR will result in stable
operation. As the amount of capacitance or ESR is
decreased, the phase margin of the circuit will decrease
and the transient response of the circuit may ring or the
circuit may become unstable. After the power compo-
nents (including the output capacitor) have been chosen,
the circuit should be tested under transient loads for
stable response. Linear Technology’s Application Note
19 provides details of this method.
All-Ceramic Design
Large value ceramic capacitors that are suitable for use
as the main output capacitor of an LT1611/LT1613
regulator are now available. These capacitors have very
low ESR and therefore offer very low output ripple in a
small package. However, you should approach their use
with some caution.
Ceramic capacitors are manufactured using a number of
dielectrics, each with different behavior across tempera-
ture and applied voltage. Y5V is a common dielectric type
used for high value capacitors, but it can lose more than
80% of the original capacitance with applied voltage and
extreme temperatures. The transient behavior and loop
stability of the switching regulator depend on the value of
the output capacitor, so you may not be able to afford this
loss. Other dielectrics (X7R and X5R) result in more
stable characteristics and are suitable for use as the
output capacitor. The X7R type has better stability across
temperature, whereas the X5R is less expensive and is
available in higher values.
The second concern in using ceramic capacitors is that
many switching regulators benefit from the ESR of the
output capacitor because it introduces a zero in the
regulator’s loop gain. This zero may not be effective
because the ceramic capacitor’s ESR is very low. Most
current mode switching regulators can be easily com-
pensated without this zero. Any design should be tested
for stability at the extremes of operating temperatures;
this is particularly so of circuits that use ceramic output
capacitors.
Figure 6 shows a boost design that uses ceramic capaci-
tors at both the input and output, resulting in small circuit
size and very low noise. A capacitor has been added in the
feedback path for phase lead, compensating for the
output capacitor’s low ESR. Figure 7 compares the tran-
sient response and output ripple of the DC233 boost
circuit with those for the all-ceramic design. The lower
trace in each scope photo shows the load current step-
ping from 50mA to150mA. The upper trace shows the
output as it responds to this load step. The output ripple
for the DC233 boost circuit appears when the load current
is high and is approximately 30mV
P-P
. The low ESR, 10µF
ceramic capacitor results in output ripple under 5mV
P-P
.
VIN
LT1613
VIN
SHDN FB
DC233 F06
100k
4.7µH
32.4k
C21
2.2µF220pF C22
10µF
VOUT
5V
SHDN
C21: TAIYO YUDEN LMK212BJ225MF (0805 CASE SIZE)
C22: TAIYO YUDEN JMK316BJ106ML (1206 CASE SIZE)
GND
SW
Figure 6. This Boost Design Uses Ceramic Input and
Output Capacitors for Small Circuit Size and Low Noise
13
DEMO MANUAL DC233
SOT-23 SWITCHING REGULATORS
OPERATIO
U
0.1ms/DIV DC231 F07a
V
OUT
100mV/DIV
I
LOAD
100mA/DIV
0.1ms/DIV DC231 F07b
V
OUT
100mV/DIV
I
LOAD
100mA/DIV
Figure 7. Transient Response of the DC233 Boost Circuit (Top Photo)
and All Ceramic Design in Figure 6. (VIN = 3V, VOUT = 5V)
14
DEMO MANUAL DC233
SOT-23 SWITCHING REGULATORS
W
PCB LAYOUT A
U
D FIL
Component Side Silkscreen Component Side
Component Side Solder Mask Component Side Paste Mask
15
DEMO MANUAL DC233
SOT-23 SWITCHING REGULATORS
W
PCB LAYOUT A
U
D FIL
Solder Side Solder Side Solder Mask
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 represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
16
DEMO MANUAL DC233
SOT-23 SWITCHING REGULATORS
PC FAB DRAWI
U
G
LINEAR TECHNOLOGY CORPORATION 2000
dc233f LT/TP 0400 500 • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
●
www.linear-tech.com
NOTES: UNLESS OTHERWISE SPECIFIED
1. MATERIAL: FR4 OR EQUIVALENT EPOXY,
2 OZ COPPER CLAD, THICKNESS 0.062 ±0.006
TOTAL OF 2 LAYERS
2. FINISH: ALL PLATED HOLES 0.001 MIN/0.0015 MAX
COPPER PLATE, ELECTRODEPOSITED TIN-LEAD COMPOSITION
BEFORE REFLOW, SOLDER MASK OVER BARE COPPER (SMOBC)
3. SOLDER MASK: BOTH SIDES USING SR1020 OR EQUIVALENT
4. SILKSCREEN: USING WHITE NONCONDUCTIVE EPOXY INK
5. ALL DIMENSIONS IN INCHES
SYMBOL
A
B
C
D
DIAMETER
0.020
0.035
0.065
0.072
TOTAL HOLES
NUMBER
OF HOLES
24
4
12
2
42
2.125
2.975
A
A
D
AA
A
A
A
AA
B
B
C
C
CD
233 FAB
