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DEMO MANUAL DC1694B
DESCRIPTION
LT3748
100V Isolated
Flyback Converter
Demonstration circuit 1694B is an isolated flyback con-
verter featuring the LT
®
3748. The demo circuit is designed
for a 12V output from a 22V to 75V DC input. The output
current is up to 2.5A. The part senses the isolated output
voltage directly from the primary side flyback waveform
during the off time of the power switch. No third winding,
opto-coupler, or signal transformer is required for regula-
tion. A minimum load of approximately 2% of maximum
load is required on the output to maintain regulation.
The demo circuit can be easily modified for applications
of different input, and output voltage/current, with some
pre-designed transformers from vendors such as Würth
Electronics, Sumida, Pulse Engineering and Coilcraft. Both
EP13 and EFD20 core transformers can be assembled on
the board. A two-stage filter is optional for applications
requiring very small output ripple.
PERFORMANCE SUMMARY
The LT3748 is a high input voltage isolated flyback con-
troller operating with input supply voltages from 5V to
100V, and 1.9A average gate drive source and sink current.
The LT3748 utilizes boundary mode operation to provide
a high efficiency, small size solution with improved load
regulation of ±5% over the full line, load and temperature
ranges. The LT3748 can be used in a wide variety of indus-
trial, automotive, medical, telecom, data-com applications,
where regulated isolated output is required.
The LT3748 data sheet gives a complete description of
the part, operation and application information. The data
sheet must be read in conjunction with this quick start
guide for Demo Circuit 1694B.
Design files for this circuit board are available at
http://www.linear.com/demo/DC1694B
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Input Voltage Range 22 48 75 V
Output Voltage VIN = 22V to 75V
IOUT = 50mA to 2.5A
11.4 12 12.6 V
Maximum Output Current VIN = 22V to 75V 2.5 A
Output Voltage Ripple (Peak-to-Peak) VIN = 22V to 75V, IOUT = 2.5A 420 mV
Switching Frequency VIN = 48V, IOUT = 2.5A 289 kHz
Efficiency VIN = 48V, IOUT = 2.5A 86.5 %
Specifications are at TA = 25°C
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DEMO MANUAL DC1694B
QUICK START PROCEDURE
Demo circuit 1694B is easy to set up to evaluate the
performance of the LT3748. Refer to Figure 1 for proper
measurement equipment setup and follow the procedure
below.
1. With power off, connect the input power supply to VIN
and GND.
2. Connect the load to the terminals VOUT+ and VOUT– on
the board. Apply a load between 100mA to 2.5A to the
output.
3. Turn on the power at the input. Increase VIN to 22V.
NOTE: Make sure that the input voltage does not exceed
75V.
4. Check for the proper output voltages. The output should
be regulated at 12V (±5%).
NOTE: If there is no output, temporarily disconnect
the power supply. Make sure that the load is properly
connected. If the output voltage is out of spec, make
sure minimum load current is applied to the output,
the load is not set too high, and the DC power supply
does not hit the current limit.
NOTE: If higher operating voltage is required, power
components with higher voltage ratings should be used.
NOTE: The LT3748 requires minimum load to maintain
good regulation. A Zener diode D4 is placed between
VOUT+ and VOUT– to clamp the output voltage to ~13V
if minimum load requirement is not met. Depending on
input voltage and output regulation, a 100mA minimum
load is usually sufficient.
5. Once the proper output voltage is established, adjust the
input and load within the operating range and observe
the output voltage regulation, ripple voltage, efficiency
and other parameters.
NOTE: When measuring the input or output voltage
ripples, 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 VIN and GND, or VOUT+ and VOUT– terminals. See
Figure 2 for proper scope probe technique.
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DEMO MANUAL DC1694B
QUICK START PROCEDURE
Figure 1. Proper Measurement Equipment Setup
Figure 2. Proper Scope Probe Placement for Measuring Input or Output Ripple
INPUT OR OUTPUT CAPACITOR
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DEMO MANUAL DC1694B
PARTS LIST
ITEM QTY REFERENCE PART DESCRIPTION MANUFACTURER/PART #
Required Circuit Components
1 1 C1 CAP., ELECTRO., 100µF, 100V CHEMI-CON, EMVY101ARA101MKE0S
2 2 C2, C3 CAP., X7R, 2.2µF, 100V, 20% 1210 TDK C3225X7R2A225M
3 1 C6 CAP., X7R, 1.0µF, 100V, 20% 1206 AVX, 12061C105MAT2A
4 1 C7 CAP., X7R, 0.1µF, 16V, 10% 0402 TDK, C1005X7R1C104K
5 1 C8 CAP., C0G, 470pF, 50V, 5% 0402 TDK, C1005COG1H471J
6 1 C9 CAP., X5R, 4.7µF, 16V, 20% 0805 AVX, 0805YD475MAT2A
7 1 C10 CAP., C0G, 220pF, 100V, 5% 0805 AVX, 08051A221JAT2A
8 1 C11 CAP., X7R, 22nF, 25V, 10% 0402 MURATA, GRM155R71E223KA61D
9 1 C12 CAP., X7R, 2200pF, 250V, 10% 1812 MURATA, GA343QR7GD222KW01L
10 3 C16, C17, C18 CAP., X7R, 22µF,16V, 20% 1210 TDK, C3225X7R1C226M
11 1 C20 CAP., COG, 270pF, 100V, 5% 0805 MURATA, GRM2165C2A271JA01D
12 1 D1 DIODE., PDS5100, Power DI-5 DIODE INC., PDS5100-13
13 1 D3 Schottky Barrier Rec., BAT41, SOD523 ST, BAT41KFILM
14 1 D4 ZENER DIODE, DFLZ13, 13V, PowerDI-123 DIODE INC., DFLZ13-7
15 1 Q1 N-MOSFET, SI7450DP, PowerPAK VISHAY, SI7450DP-T1-GE3
16 1 R1 RES., CHIP, 12.1k, 1% 0402 VISHAY, CRCW040212K1FKED
17 1 R3 RES., CHIP, 402k, 1% 0402 VISHAY, CRCW0402402KFKED
18 1 R5 RES., CHIP, 6.04k, 1% 0402 NIC, NRC04F6041TRF
19 1 R6 RES., CHIP, 32.4k, 1% 0402 VISHAY, CRCW040232K4FKED
20 1 R7 RES., CHIP, 150k, 1% 0603 VISHAY, CRCW0603150KFKED
21 2 R8, R13 RES., CHIP, 0, 1% 0603 VISHAY, CRCW06030000Z0EA
22 1 R9 RES., CHIP, 63.4k, 1% 0402 VISHAY, CRCW040263K4FKED
23 1 R10 RES., CHIP, 0.015Ω, 1/8W, 1% 0805 VISHAY, WSL0805R015F
24 2 R11, R12 RES., CHIP, 43Ω, 1/4W, 5% 1206 VISHAY, CRCW120643R0JNEA
25 1 R14 RES., CHIP, 10, 1% 0603 VISHAY, CRCW060310R0FKED
26 1 R15 RES., CHIP, 22, 1/8W, 5% 0805 VISHAY, CRCW080522R0JNEA
27 1 T1 TRANSFORMER Midcom, Inc. 750311607
28 1 U1 I.C. LT3748EMS, MSOP-16 LINEAR TECH., LT3748EMS#TRPBF
Additional Demo Board Circuit Components
1 0 C4, C5, C13, C14, C15 ( OPT.) CAP., 1210
2 0 C19 (OPT.) CAP., 0603
3 0 D5 (OPT.) Schottky Barrier Rec., SOD-323
4 0 L1 (OPT.) INDUCTOR
5 0 ROUT (OPT.) RES., 1210
Hardware – For Demo Board Only
1 4 E1, E2, E5, E6 TESTPOINT, TURRET, .094” pbf MILL-MAX, 2501-2-00-80-00-00-07-0
2 1 JP1 3 PIN 0.079 SINGLE ROW HEADER SAMTEC, TMM103-02-L-S
3 1 XJP1 SHUNT, .079” CENTER SAMTEC, 2SN-BK-G
4 4 MTG 4 CORNERS STAND-OFF, NYLON (SNAP ON), 0.25” TALL KEYSTONE, 8831
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DEMO MANUAL DC1694B
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.
Figure 3. DC1694B Demo Circuit Schematic
SCHEMATIC DIAGRAM
NOTE: UNLESS OTHERWISE SPECIFIED
1. ALL RESISTORS ARE 0402.
ALL CAPACITORS ARE 0402.
VOUT-
VOUT+
22V - 75V
12V / 2.5A
750311607
1210
VIN
GND
E1E1
U1
LT3748EMS
U1
LT3748EMS
VIN1
EN/UVLO
3
SS
10
VC
11
SENSE 7
INTVCC 5
GATE 6
RFB 16
RREF
14
TC
12
GND
9
GND
8
0.1uF
C7
0.1uF
C7
C16
16V
22uF
C16
16V
22uF
C17C17
C13
1210
C13
1210
C18C18
R5
6.04K
R5
6.04K
22nF
C11
22nF
C11
D3
BAT41KFILM
SOD523
D3
BAT41KFILM
SOD523
D1
PDS5100
D1
PDS5100
L1
0.47uH
OPT.
L1
0.47uH
OPT.
C4
1210
OPT
C4
1210
OPT
JP1
SHDN
EN
SHDN
JP1
SHDN
EN
SHDN
1
3
2
E5E5
E2E2
C2
100V
2.2uF
1210
C2
100V
2.2uF
1210
470pF
C8
470pF
C8
C15
OPT
C15
OPT
Q1
SI7450DP
Q1
SI7450DP
4
5
23
67 8
1
C3C3
D5
OPT.
D5
OPT.
R14
10
0603
R14
10
0603
270pF
C20
100V
0805
270pF
C20
100V
0805
C5C5
R7
150K
0603
R7
150K
0603
T1
Midcom
T1
Midcom
1
6
5
3
12
9
11
8
2
4
7
10
VOPTVOPT
4.7uF
C9
16V
0805
4.7uF
C9
16V
0805
ROUT
OPT
1210
ROUT
OPT
1210
R12
43 1206
R12
43 1206
R11
43 1206
R11
43 1206
R10
0.015
0805
R10
0.015
0805
R9
63.4K
R9
63.4K
C12
1812
250V
2200pF
C12
1812
250V
2200pF
C19
OPT.
0603
C19
OPT.
0603
C10
220pF 0805
100V
C10
220pF 0805
100V
R8 0
0603
R8 0
0603
R1
12.1k
R1
12.1k
D4
DFLZ13
13V
D4
DFLZ13
13V
C14C14
+
C1
100uF
100V
+
C1
100uF
100V
R6
32.4k
R6
32.4k
R3
402k
R3
402k
E6E6
1uF
C6
1206
100V
1uF
C6
1206
100V
R15
22 0805
R15
22 0805
R13 0
0603
R13 0
0603
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dc1694bfa
DEMO MANUAL DC1694B
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com
LINEAR TECHNOLOGY CORPORATION 2012
LT 0914 • PRINTED IN USA
DEMONSTRATION BOARD IMPORTANT NOTICE
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This demonstration board (DEMO BOARD) kit being sold or provided by Linear Technology is intended for use for ENGINEERING DEVELOPMENT
OR EVALUATION PURPOSES ONLY and is not provided by LT C for commercial use. As such, the DEMO BOARD herein may not be complete
in terms of required design-, marketing-, and/or manufacturing-related protective considerations, including but not limited to product safety
measures typically found in finished commercial goods. As a prototype, this product does not fall within the scope of the European Union
directive on electromagnetic compatibility and therefore may or may not meet the technical requirements of the directive, or other regulations.
If this evaluation kit does not meet the specifications recited in the DEMO BOARD manual the kit may be returned within 30 days from the date
of delivery for a full refund. THE FOREGOING WARRANTY IS THE EXCLUSIVE WARRANTY MADE BY THE SELLER TO BUYER AND IS IN LIEU
OF ALL OTHER WARRANTIES, EXPRESSED, IMPLIED, OR STATUTORY, INCLUDING ANY WARRANTY OF MERCHANTABILITY OR FITNESS
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arising from the handling or use of the goods. Due to the open construction of the product, it is the user’s responsibility to take any and all
appropriate precautions with regard to electrostatic discharge. Also be aware that the products herein may not be regulatory compliant or
agency certified (FCC, UL, CE, etc.).
No License is granted under any patent right or other intellectual property whatsoever. LT C assumes no liability for applications assistance,
customer product design, software performance, or infringement of patents or any other intellectual property rights of any kind.
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Please read the DEMO BOARD manual prior to handling the product. Persons handling this product must have electronics training and
observe good laboratory practice standards. Common sense is encouraged.
This notice contains important safety information about temperatures and voltages. For further safety concerns, please contact a LT C applica-
tion engineer.
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