QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 519
1.1MHZ DUAL STEP-DOWN DC/DC CONVERTER
LT1940
DESCRIPTION
Demonstration circuit 519 is a 1.1MHz dual step-down
DC/DC switching converter using the LT1940. The
LT1940 features fast switching speed, two 1.6A inter-
nal power switches, and a wide input voltage range,
making it a versatile and powerful IC that fits easily
into space-constrained applications and removes the
need for two separate dc/dc converters in dual output
step-down applications. The constant 1.1MHz switch-
ing frequency allows for the use of tiny, surface mount
external components. The current-mode control topol-
ogy yields fast transient response and good loop sta-
bility, requiring a minimum number of external com-
pensation components and allowing the use of ce-
ramic input and output capacitors. The anti-phase
switching and single-IC solution require the use of
only a single input capacitor. The low resistance inter-
nal 2A power switches (0.21
Ω
) maintain high efficien-
cies (as high as 90%) over a wide range of input volt-
ages and loads. Its 30µA shutdown current (activated
via the Run/SS pins) extends battery life. The wide V
IN
range of the LT1940 allows step-down configurations
with up to 25V
input.
The demonstration board has two circuits to demon-
strate both the small space that the monolithic dual
step-down converter can achieve with high currents
and a single IC as well as the versatility and maximum
power output of the IC. The circuit demonstrating
smallest size generates 3.3V at up to 1A and 1.8V at
up to 1A output from a 4.8V–14V input. The larger,
more versatile circuit generates 5V at up to 1.4A and
3.3V at up to 1.4A output from a separate 6.8V to 25V
input. The two circuits are completely isolated from
each other and can be run simultaneously or inde-
pendently. The versatile circuit has options for chang-
ing the boost diodes for highest efficiency under dif-
ferent output voltage conditions. The feedforward ca-
pacitors and noise-reduction capacitors on the Vc pins
further increase the parts count on the versatile board,
yet they improve the transient response and give the
ability to optimize loop compensation under various
conditions.
This board is designed for applications that require
two step-down supplies with up to 1.4A load current in
a small board space with low cost and low parts count
as well as simple circuit design. The high current, on-
board, dual monolithic switches eliminate the need for
external switches. The use of ceramic capacitors in
this circuit not only demonstrates small size and low
cost, but the advantage of current-mode control in
step-down applications with a simple compensation
network and a feedforward capacitor for more rugged
stability and excellent transient response.
Design files for this circuit board are available. Call
the LTC factory.
Table 1.
Typical Performance (T
A
= 25°C) for Small Dual Step-Down Converter (VOUT1=3.3V and VOUT2=1.8V)
PARAMETER VALUE
V
IN
4.8V to 14V
V
OUT1
3.3V
V
OUT2
1.8V
V
OUT1
Load Current 1A(max)
V
OUT2
Load Current 1A(max)
Efficiency 80% at 1A load current (V
OUT1
and V
OUT2
)
1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 519
1.1MHZ DUAL STEP-DOWN DC/DC CONVERTER
Table 2.
Typical Performance (T
A
= 25°C) for Customizable Dual Step-Down Converter (VOUT1=5.0V and VOUT2=3.3V)
Parameter Value
V
IN
6.8V to 25V
VOUT1 5.0V
VOUT2 3.3V
VOUT1 Load Current 1.4A(max)
VOUT2 Load Current 1.4A(max)
Efficiency 84% at 1A load current (VOUT1 and VOUT2)
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 200 400 600 800 1.0k
V OUT1 LOAD CURRENT (mA)
VOUT2 LOAD CURRENT = 1A
EFFICIENCY
VIN = 5V
VIN = 8V
VIN = 12V
Figure 1.
Typical Efficiency of Demo Circuit 519 LT1940, Small
Dual Step-Down VOUT1=3.3V, VOUT2=1.8V
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 200 400 600 800 1.0k
V OUT2 LOAD CURRENT (mA)
VOUT1 LOAD CURRENT = 1A
EFFICIENCY
VIN = 5V
VIN = 8V
VIN = 12V
Figure 2.
Typical Efficiency of Demo Circuit 519 LT1940, Small
Dual Step-Down VOUT1=3.3V, VOUT2=1.8V
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 200 400 600 800 1.0k 1.2k 1.4k
V OUT1 LOAD CURRENT (mA)
VOUT2 LOAD CURRENT = 1A
EFFICIENCY
VIN = 8V
VIN = 12V
VIN = 18V
Figure 3.
Typical Efficiency of Demo Circuit 519 LT1940, Custom-
izable Dual Step-Down VOUT1=5.0V, VOUT2=3.3V
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 200 400 600 800 1.0k 1.2k 1.4k
V OUT2 LOAD CURRENT (mA)
VOUT1 LOAD CURRENT = 1A
EFFICIENCY
VIN = 8V
VIN = 12V
VIN = 18V
Figure 4.
Typical Efficiency of Demo Circuit 519 LT1940, Custom-
izable Dual Step-Down VOUT1=5.0V, VOUT2=3.3V
2
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 519
1.1MHZ DUAL STEP-DOWN DC/DC CONVERTER
QUICK START PROCEDURE
+
–
+
–
+
–
+
–
+
–
+
–
+
–
+
–
+
–
Demonstration circuit 519 is easy to set up to evaluate
the performance of the LT1940. Refer to Figure 5 for
proper measurement equipment setup and follow the
procedure below:
1.
Connect the 4.8V–14V or 6.8V–25V input power sup-
ply to the VIN and GND terminals on the board corre-
sponding to the small, top circuit or the larger, cus-
tomizable, bottom circuit. Each circuit is powered
separately from its own VIN and GND terminals.
2.
After all connections are made, turn on input power
and verify that the output voltage is either 3.3V and
1.8V (small, top circuit) or 5V and 3.3V (larger, bot-
tom circuit) for the circuit to which the input supply is
connected.
3.
The Run/SS and Power Good functions are optional
and their terminals can be left floating (disconnected)
if their functions are not being used. Connecting a
Run/SS terminal to the GND terminal will disable the
corresponding output.
LOAD
+
–
+
–
LOAD
+
–
LOAD
+–
+–
LOAD
+–
+–
+–
+–
Figure 5.
Proper Measurement Equipment Setup
3
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 519
1.1MHZ DUAL STEP-DOWN DC/DC CONVERTER
CUSTOMIZING THE BOARD
The components used in the demonstration circuits are
optimized for wide input voltage ranges. Nevertheless,
the bandwidths can be increased for more specific input
voltages, such as 12V±10% or 5V±10%, by changing
the V
C
components, the noise filter caps, the feedfor-
ward caps, and the output capacitors. The adjustable
feedback resistors allow the output voltages to be cus-
tomized. The boost diodes D8 and D11 on the small cir-
cuit are the only boost diode options on that circuit for
minimal space considerations. The anode of D8 is con-
nected to Vout1 (3.3V). The anode of D11 is connected
to Vin (4.8V to 14V) since Vout2 is 1.8V and is below
3.0V. The minimum boost voltage required for the inter-
nal power switch is 3.0V and that can usually be taken
directly from a 3.3V output or greater. However, if the
output voltage is below 3.3V, in order to maintain high
efficiency, the boost diode should get its voltage from
either the input (which is less efficient than a greater
than 3.3V output) or another source such as the other
output on the board (if it is greater than 3.3V). Since
there is not space to make these changes in boost diode
location on the small circuit, D8 and D11 should always
be used (not customized) and Vout1 should always be
3.3V or greater and Vout 2 can be less than 3.3V. If
Vout2 is 3.3V or greater, the small circuit, with D11 in-
stalled connecting to the input, will not be as efficient as
it could be if D11 was instead connected to Vout2.
The larger, versatile circuit has pad placements built-in
for customized output voltages of all acceptable levels.
D1 and D3 are connected to Vout1 (5V) and Vout2
(3.3V) respectively since they are both 3.3V or greater
and they provide enough boost to saturate the high-side
NPN power switches. However, if either output voltage is
customized to less than 3.3V, D1 can be removed and
replaced by stuffing D2 with the same part. D3 can be
removed and replaced by stuffing D4 with the same part.
Both D2 and D4 connect the boost to Vin. Additionally,
D5 provides a way to replace D3 with a boost diode that
is connected to Vout1 in case Vout1 is 3.3V or greater
and Vout2 is less than 3.3V.
Make sure that the boost capacitors (C27, C15, C1, C2)
have voltage ratings greater than or equal to the output
voltage for applications where the boost diode is con-
nected to the output. However, the boost capacitor must
have a voltage rating greater than the input voltage
whenever the boost diode is connected to the input.
4
5
5
4
4
3
3
2
2
1
1
D D
C C
B B
A A
VOUT1
VOUT2
VIN
NOTES: UNLESS OTHERWISE SPECIFIED,
[1] DO NOT STUFF.
Run/SS2
Run/SS1
PG1
PG2
GND
GND
GND
5V @ 1.4A
3.3V @ 1.4A
6.8V to 25V
3
DC519A
Monday, July 22, 2002 12
Custom
SCH, LT1940EFE 1.1MHz DUAL STEP-DOW N
DC/DC CONVERTER
519A-3.DSNNONE
MEI 05-24-02
SIZE
SCALE:
CAGE CODE DWG NO REV
SHEET OF
FILENAME:
TITLE
CONTRACT NO.
APPROVALS DATE
DRAWN
CHECKED
APPROVED
ENGINEER
DESIGNER
UNLESS OTHERWISE SPECIFIED
DIMENSIONS ARE IN INCHES
TOLERANCE ON ANGLE - - -
2 PLACES - - - 3 PLACES - - -
INTERPRET DIM AND TOL
PER ASME Y14.5M -1994
THIRD ANGLE PROJECTION
DO NOT SCALE DRAWING
TECHNOLOGY
1630 McCarthy Blvd.
Milpitas, CA 95035
Phone: (408)432-1900
Fax: (408)434-0507
PROTO
>305-24-02
REVISION HISTORY
DESCRIPTION DATE APPROVEDECO REV
VIN
VOUT1
VOUT1
VOUT2
D2
[1]
TP6
R4
41.2k
C1 0.1uF
L2
4.1uH
CDRH5D18-4R1
TP7
R1
24.9k
D3
CMDSH-3
SOD323
CIN
4.7uF
25V
X7R
C4
10uF
6.3V
R2
78.7k
D4
[1]
TP3
Cff1
62pF
D6
B130
R7
100K
TP2
TP9
C12
0.1uF
C11
0.1uF
R3
24.9k
C3
10uF
6.3V
TP4
TP8
L1
5.3uH
CDRH5D28-5R3
C6
100pF
R8
100K
D1
CMDSH-3
SOD323
C8
100pF
D5
[1]
C5
1000pF
C7
1000pF
R6 22k
R5 22k
TP1
C2 0.1uF
U1
LT1940EFE
1
2
3
4
5
6
7
8 9
10
11
12
13
14
15
16
17
BOOST1
SW1
VIN0
VIN1
VIN2
VIN3
SW2
BOOST2 FB2
Vc2
PG2
RUN/SS2
RUN/SS1
PG1
Vc1
FB1
GND
TP10
C13
0.1uF
C9 0.1uF
D7
B130
Cff2
62pF
C10
0.1uF
TP5
5
5
4
4
3
3
2
2
1
1
D D
C C
B B
A A
1.8V @ 1A
GND
[1]
VOUT1
3.3V @ 1A
NOTES: UNLESS OTHERWISE SPECIFIED,
DO NOT STUFF.
Run/SS2
GND
VIN Run/SS1
PG1
VOUT2
PG2
GND
4.8V to 14V
3
DC519A
Monday, July 22, 2002 22
Custom
SCH, LT1940EFE 1.1MHz DUAL STEP-DOW N
DC/DC CONVERTER
519A-3.DSNNONE
MEI 05-24-02
SIZE
SCALE:
CAGE CODE DWG NO REV
SHEET OF
FILENAME:
TITLE
CONTRACT NO.
APPROVALS DATE
DRAWN
CHECKED
APPROVED
ENGINEER
DESIGNER
UNLESS OTHERWISE SPECIFIED
DIMENSIONS ARE IN INCHES
TOLERANCE ON ANGLE - - -
2 PLACES - - - 3 PLACES - - -
INTERPRET DIM AND TOL
PER ASME Y14.5M -1994
THIRD ANGLE PROJECTION
DO NOT SCALE DRAWING
TECHNOLOGY
1630 McCarthy Blvd.
Milpitas, CA 95035
Phone: (408)432-1900
Fax: (408)434-0507
PROTO
>305-24-02
REVISION HISTORY
DESCRIPTION DATE APPROVEDECO REV
VOUT1
VOUT2
VIN
C26
0.1uF
C18
1000pF
C23
0.1uF
R1322k
R1422k
TP15
C27 0.1uF
TP16
TP19
D14
UPS120
C17
22uF
6.3V
R15
100K
D11
CMDSH-3
SOD323
D13
UPS120
R10
41.2k
R11
24.9k
C22 0.1uF
U2
LT1940EFE
1
2
3
4
5
6
7
8 9
10
11
12
13
14
15
16
17
BOOST1
SW1
VIN0
VIN1
VIN2
VIN3
SW2
BOOST2 FB2
Vc2
PG2
RUN/SS2
RUN/SS1
PG1
Vc1
FB1
GND
C15 0.1uF
C16
10uF
6.3V
R16
100K
C20
1500pF
L3
2.7uH
1008PS-272M
TP13
C24
0.1uF
TP14
CIN2
4.7uF
16V
X5R
TP12
C25
0.1uF
R9
24.9k
D8
CMDSH-3
SOD323
TP17
L4
1.8uH
1008PS-182M
TP18
TP20
R12
12.4k
TP11
5
5
4
4
3
3
2
2
1
1
D D
C C
B B
A A
VOUT1
VOUT2
VIN
NOTES: UNLESS OTHERWISE SPECIFIED,
[1] DO NOT STUFF.
Run/SS2
Run/SS1
PG1
PG2
GND
GND
GND
5V @ 1.4A
3.3V @ 1.4A
6.8V to 25V
3
DC519A
Thursday, October 31, 2002 12
Custom
SCH, LT1940EFE 1.1MHz DUAL STEP-DOW N
DC/DC CONVERTER
519A-3.DSNNONE
MEI 05-24-02
SIZE
SCALE:
CAGE CODE DWG NO REV
SHEET OF
FILENAME:
TITLE
CONTRACT NO.
APPROVALS DATE
DRAWN
CHECKED
APPROVED
ENGINEER
DESIGNER
UNLESS OTHERWISE SPECIFIED
DIMENSIONS ARE IN INCHES
TOLERANCE ON ANGLE - - -
2 PLACES - - - 3 PLACES - - -
INTERPRET DIM AND TOL
PER ASME Y14.5M -1994
THIRD ANGLE PROJECTION
DO NOT SCALE DRAWING
TECHNOLOGY
1630 McCarthy Blvd.
Milpitas, CA 95035
Phone: (408)432-1900
Fax: (408)434-0507
RELEASE
>310/31/02
REVISION HISTORY
DESCRIPTION DATE APPROVEDECO REV
VIN
VOUT1
VOUT1
VOUT2
TP6
D2
[1]
R4
41.2k
C1 0.1uF
TP7
L2
4.1uH
CDRH5D18-4R1
R1
24.9k
C4
10uF
6.3V
CIN
4.7uF
25V
X7R
D3
CMDSH-3
SOD323
R2
75.0k
D4
[1]
TP3
Cff1
62pF
D6
B130
R7
100K
TP2
TP9
C12
0.1uF
C11
0.1uF
R3
24.9k
TP4
C3
10uF
6.3V
TP8
L1
5.3uH
CDRH5D28-5R3
R8
100K
C6
100pF
D1
CMDSH-3
SOD323
C8
100pF
D5
[1]
C5
1000pF
C7
1000pF
R6 22k
R5 22k
TP1
C2 0.1uF
U1
LT1940EFE
1
2
3
4
5
6
7
8 9
10
11
12
13
14
15
16
17
BOOST1
SW1
VIN0
VIN1
VIN2
VIN3
SW2
BOOST2 FB2
Vc2
PG2
RUN/SS2
RUN/SS1
PG1
Vc1
FB1
GND
C13
0.1uF
TP10
C9 0.1uF
Cff2
62pF
D7
B130
C10
0.1uF
TP5
5
5
4
4
3
3
2
2
1
1
D D
C C
B B
A A
1.8V @ 1A
GND
[1]
VOUT1
3.3V @ 1A
NOTES: UNLESS OTHERWISE SPECIFIED,
DO NOT STUFF.
Run/SS2
GND
VIN Run/SS1
PG1
VOUT2
PG2
GND
4.8V to 14V
3
DC519A
Thursday, October 31, 2002 22
Custom
SCH, LT1940EFE 1.1MHz DUAL STEP-DOW N
DC/DC CONVERTER
519A-3.DSNNONE
MEI 05-24-02
SIZE
SCALE:
CAGE CODE DWG NO REV
SHEET OF
FILENAME:
TITLE
CONTRACT NO.
APPROVALS DATE
DRAWN
CHECKED
APPROVED
ENGINEER
DESIGNER
UNLESS OTHERWISE SPECIFIED
DIMENSIONS ARE IN INCHES
TOLERANCE ON ANGLE - - -
2 PLACES - - - 3 PLACES - - -
INTERPRET DIM AND TOL
PER ASME Y14.5M -1994
THIRD ANGLE PROJECTION
DO NOT SCALE DRAWING
TECHNOLOGY
1630 McCarthy Blvd.
Milpitas, CA 95035
Phone: (408)432-1900
Fax: (408)434-0507
RELEASE
>310/31/02
REVISION HISTORY
DESCRIPTION DATE APPROVEDECO REV
VOUT1
VOUT2
VIN
C26
0.1uF
C18
1000pF
C23
0.1uF
R1322k
R1422k
TP15
C27 0.1uF
TP16
C17
22uF
6.3V
D14
UPS120
TP19
R15
100K
D11
CMDSH-3
SOD323
D13
UPS120
R11
24.9k
R10
41.2k
C22 0.1uF
C15 0.1uF
U2
LT1940EFE
1
2
3
4
5
6
7
8 9
10
11
12
13
14
15
16
17
BOOST1
SW1
VIN0
VIN1
VIN2
VIN3
SW2
BOOST2 FB2
Vc2
PG2
RUN/SS2
RUN/SS1
PG1
Vc1
FB1
GND
R16
100K
C16
10uF
6.3V
C20
1500pF
L3
2.7uH
1008PS-272M
TP13
C24
0.1uF
TP14
C25
0.1uF
TP12
CIN2
4.7uF
16V
X5R
R9
24.9k
TP17
D8
CMDSH-3
SOD323
L4
1.8uH
1008PS-182M
TP18
R12
11.0k
TP20
TP11
Linear Technology Corporation
LT1940EFE Parts List DC519A Rev 3
7/25/2002
Item
Qty
Ref
Desc
Part Number
1 1 CIN CAP, X7R 4.7uF 25V 20% 1210 TDK C3225X7R1E475M
2 1 CIN2 CAP, X5R 4.7uF 16V 10% 1206 TAIYO YUDEN EMK316BJ475KL
3 2 Cff1,Cff2 CAP, NPO 62pF 25V 5% 0603 AVX 06033A620JAT
4 8 C1,C2,C9,C12,C13,C15,C22, CAP, X7R 0.1uF 16V 10% 0603 AVX 0603YC104KAT
C27
5 2 C4,C3 CAP, X5R 10uF 6.3V 20% 1206 TAIYO YUDEN JMK316BJ106ML-T
6 2 C5,C7 CAP, X7R 1000pF 50V 5% 0603 AVX 06035C102JAT2A
7 2 C6,C8 CAP, NPO 100pF 50V 5% 0603 AVX 06035A101JAT
8 0 C10,C11,C23,C24 CAP, X7R 0.1uF 16V 10% 0603 AVX 0603YC104KAT OPTION
9 1 C16 CAP, X5R 10uF 6.3V 10% 0805 TDK C2012X5R0J106M
10 1 C17 CAP, X5R 22uF 6.3V 10% 1210 TDK C3225X5R0J226M
11 1 C18 CAP, X7R 1000pF 50V 5% 0402 AVX 04025C102JAT
12 1 C20 CAP, X7R 1500pF 50V 5% 0402 AVX 04025C152KAT
13 2 C26,C25 CAP, X5R 0.1uF 10V 10% 0402 AVX 0402ZD104KAT
14 4 D1,D3,D8,D11 DIODE, CMDSH-3 SOD323 CENTRAL CMDSH-3
15 0 D2,D4,D5 DO NOT STUFF OPTION
16 2 D6,D7 DIODE, B130 SCHOTTKY RECTIFIER 1A DIODES INC. B130
17 2 D14,D13 DIODE, SCHOTTKY RECTIFIER 1A UPS120 MICROSEMI UPS120
18 1 L1 IND, 5.3uH SUMIDA CDRH5D28-5R3
19 1 L2 IND, 4.1uH SUMIDA CDRH5D18-4R1NC
20 1 L3 IND, 2.7uH COILCRAFT 1008PS-272M
21 1 L4 IND, 1.8uH COILCRAFT 1008PS-182M
22 2 R1,R3 RES, 24.9k OHMS 1% 1/16W 0603 AAC CR16-2492FM
23 1 R2 RES, 78.7k OHMS 1% 1/16W 0603 AAC CR16-7872FM
24 1 R4 RES, 41.2k OHMS 1% 1/16W 0603 AAC CR16-4122FM
25 2 R6,R5 RES, 22K OHMS 5% 1/16W 0603 AAC CR16-223JM
26 2 R7,R8 RES, 100K OHMS 5% 1/16W 0603 AAC CR16-104JM
27 2 R11,R9 RES, 24.9k OHMS 1% 1/16W 0402 AAC CR05-2492FM
28 1 R10 RES, 41.2k OHMS 1% 1/16W 0402 AAC CR05-4122FM
29 1 R12 RES, 12.4k OHMS 1% 1/16W 0402 AAC CR05-1242FM
30 2 R13,R14 RES, 22K OHMS 5% 1/16W 0402 AAC CR05-223JM
31 2 R15,R16 RES, 100K OHMS 5% 1/16W 0402 AAC CR05-104JM
32 20 TP1-TP20 TURRETS MILL-MAX-2501-2
33 2 U1,U2 IC, LT1940EFE DUAL STEP-DOWN DC/DC CONVERTER LINEAR TECH LT1940EFE
1 of 1
