DEMO MANUAL DC1736A
LT3975
42V, 2.5A, 2.7µA Micropower
Buck Regulator
DESCRIPTION
Demonstration circuit 1736A is a monolithic step-down
DC/DC switching regulator featuring the LT
®
3975. The
demo circuit is designed for 3.3V, 2.5A output from a 4.3V
to 42V input. The wide input range of the LT3975 allows
a variety of input sources, such as automotive batteries
and industrial supplies. The switching frequency can be
programmed either via oscillator resistor or external clock
up to 2MHz. When the circuit is synchronized to an external
clock connected to the SYNC terminal, the R
T
resistor (R5)
should be chosen to set the LT3975 internal switching
frequency at least 20% below the lowest synchroniza-
tion input frequency. Low ripple Burst Mode
®
operation
increases the efficiency at the light load while keeping the
output ripple below 15mV.
The circuit consumes only 2.7μA of quiescent current.
The LT3975 internal boost diode and loop compensation
reduce the components count and solution size. The cur-
rent mode control scheme creates fast transient response
and good loop stability. The LT3975 is in shutdown when
the EN pin is low and active when the pin is high. The
threshold of the EN pin is accurate at 1.02V with 60mV
of hysteresis. Users can populate R7 and R8 to provide a
programmable undervoltage lockout. During a short-circuit
fault, the LT3975 has current limit foldback to limit the
power dissipation.
The LT3975 data sheet gives a complete description of
the part, operation and application information. The data
sheet must be read in conjunction with this demo manual
for demo circuit 1736A. The LT3975 is assembled in a
16-lead plastic MSOP package with an exposed pad for
low thermal resistance. Proper board layout is essential
for both proper operation and maximum thermal perfor-
mance. See the data sheet section PCB Layout and High
Temperature Considerations.
Design files for this circuit board are available at
http://www.linear.com/demo
L,
LT, LTC, LTM, μModule, Linear Technology, the Linear logo and Burst Mode are registered
trademarks of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
PERFORMANCE SUMMARY
PARAMETER
Minimum Input Voltage
Maximum Input Voltage
Output Voltage, V
OUT
Switching Frequency
Maximum Output Current, I
OUT
Typical Efficiency
Typical Output Voltage Ripple
T
A
= 25°C
VALUE
4.3V
42V
CONDITIONS
V
IN
= 4.3V ~ 42V
R
T
= 78.7k
V
IN
= 4.3V ~ 42V
V
IN
= 12V, I
OUT
= 2.5A
V
IN
= 12V, I
OUT
= 2.5A, 25MHz BW
3.3V ± 3%
600kHz ±20%
2.5A
82.1%
20mV
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DEMO MANUAL DC1736A
QUICK START PROCEDURE
Demonstration circuit 1736A is easy to set up to evalu-
ate the performance of the LT3975. Refer to Figure 2 for
proper measurement equipment setup and follow the
procedure below:
NOTE. When measuring the input or output voltage ripple,
care must be taken to avoid a long ground lead on the
oscilloscope probe. Measure the input or output voltage
ripple by touching the probe tip directly across the V
IN
or V
OUT
and GND terminals. See Figure 3 for the proper
scope technique.
1. Place JP1 on the ON position.
2. Place JP2 on the RT FREQ position.
3. With power off, connect the input power supply to V
IN
and GND.
4. With power off, connect load from V
OUT
to GND.
5. Turn on the power at the input.
NOTE. Make sure that the input voltages do not exceed 42V.
6. Check for the proper output voltage (3.3V).
NOTE. If there is no output, temporarily disconnect the load
to make sure that the load is not set too high or is shorted.
7. Once the proper output voltages are established, adjust
the load within the operating ranges and observe the
output voltage regulation, ripple voltage, efficiency and
other parameters.
8. An external clock can be added to the SYNC terminal
when SYNC function is used (JP2 on the SYNC posi-
tion). Please make sure that the SYNC frequency is at
least 20% higher than the set switching frequency. See
the data sheet section Synchronization.
90
85
80
EFFICIENCY (%)
75
70
65
60
55
50
0
0.5
1
V
OUT
= 3.3V
V
IN
= 12V
f
SW
= 600kHz
1.5
LOAD CURRENT (A)
2
2.5
DC1735A F01
Figure 1. Efficiency vs Load Current
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DEMO MANUAL DC1736A
QUICK START PROCEDURE
–
V
V
OUT
–
LOAD
+
–
V
IN
V
+
A
– +
–
+
–
+
A
+
Figure 2. Proper Measurement Equipment Setup
Figure 3. Measuring Output Ripple
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DEMO MANUAL DC1736A
PARTS LIST
ITEM
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
QTY
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
7
2
2
REFERENCE
C1
C2
C3
C5
C6
D1
L1
R1
R2
R5
R6
U1
C4
C7
R7, R8
E1, E2, E3, E4, E5, E6, E7
JP1, JP2
Shunt
PART DESCRIPTION
Cap., X5R, 47μF 10V 20% 1210
Cap., X5R, 0.47μF 25V 10% 0603
Cap., X5R, 10μF 50V 10%, 1210
Cap., C0G, 10pF 50V 10%, 0402
Cap., X7R, 10nF 25V 10%, 0402
Diode, Scholtky, PowerDi5
IND, 3.3μH
Res., Chip, 1.00M 1/16W 1% 0402
Res., Chip, 576k 1/16W 1% 0402
Res., Chip, 78.7k 1/16W 1% 0402
Res., Chip, 200k 1/16W 1% 0402
I.C., MSE16
Cap., Elect.,100μF 50V 10%
Cap., X5R, 1μF 16V 20% 0603
Res., 0402
Turret, 0.094"
JMP, 1X3-079
Shunt, 2mm Ctrs.
MANUFACTURER/PART NUMBER
TDK, C3225X5R1A476M
TDK, C1608X5R1E474K
TDK, C3225X5R1H106K
TDK, C1005C0G1H100D
TDK, C1005X7R1E103K
DIODE INC, PDS360-13
VISHAY, IHLP2020CZER3R3M01
VISHAY, CRCW04021M00FKED
VISHAY, CRCW0402576KFKED
VISHAY, CRCW040278K7FKED
VISHAY, CRCW0402200KFKED
LINEAR TECH., LT3975EMSE
SUN ELECT, 50CE100BS
AVX, 0603YD105MAT
OPT
MILL-MAX, 2501-2-00-80-00-00-07-0
SAMTEC, TMM-103-02-L-S
SAMTEC, 2SN-BK-G
Required Circuit Components
Additional Circuits
Hardware—For Demo Board Only
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DEMO MANUAL DC1736A
SCHEMATIC DIAGRAM
dc1736af
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.
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