QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1171A
SYNCHRONOUS BUCK CONVERTER
LTC3851EGN
DESCRIPTION
Demonstration circuit 1171A is a single output
synchronous buck converter featuring the
LTC3851EGN. The circuit is designed with all
components on the top side. The package style for
the LTC3851EGN is a 16-lead narrow SSOP.
Two versions of the board are available. DC-
1171A-A has an on-board sense resistor for cur-
rent feedback, while the DC1171A-B is configured
with a DCR sense circuit that allows the converter
to use the inductor’s DCR as the sense element
instead of the on-board sense resistor to save
cost, board space while improving full load effi-
ciency over 2%.
The main features of the board include an internal
5V linear regulator for bias and a Mode selector
that allows the converter to run in CCM, pulse skip
or Burst Mode operation (requires some minor
modification). Synchronization to an external clock
is also possible.
The input voltage range is 4.5V to 14V. The
LTC3851 datasheet gives a complete description
of the part, operation and application information
and must be read in conjunction with this quick
start guide for demo circuit 1171A.
Design files for this circuit board are available.
Call the LTC factory.
Burst Mode is a trademark of Linear Technology Corporation
Table 1.
Performance Summary (T
A
= 25°C)
CONDITION
VALUE
4.5V - 14V
V
IN
= 4.5V to 14V, I
OUT
= 0A to 15A
1.5V ±2%
400kHz
V
OUT
= 1.5V, I
OUT1
= 15A; V
IN
= 12V
87.3% Typical (A-A)
90.0% Typical (A-B)
PARAMETER
Input Voltage Range
Output Voltage V
OUT
Nominal Switching Frequency
Efficiency
See Figure 3 and 4 for efficiency curves
QUICK START PROCEDURE
Demonstration circuit 1171A is easy to set up to
evaluate the performance of the LTC3851EGN.
Refer to Figure 1 for proper measurement equip-
ment setup and follow the procedure below:
NOTE:
When
1.
Place jumpers in the following positions:
JP1
JP2
JP3
400k
On
CCM
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 Vin or Vout and GND terminals.
See Figure 2 for proper scope probe technique.
2.
3.
With power off, connect the input power supply
to Vin and GND.
Turn on the power at the input.
NOTE:
Make
sure that the input voltage does not
exceed 14V.
1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1171A
SYNCHRONOUS BUCK CONVERTER
4.
Check for the proper output voltage.
Vout = 1.470V to 1.530V
NOTE:
If there is no output, temporarily discon-
nect the load to make sure that the load is not
set too high.
Once the proper output voltages are estab-
lished, adjust the loads within the operating
range and observe the output voltage regula-
tion, ripple voltage, efficiency and other parame-
ters.
6.
5.
Different operating modes can be evaluated by
changing the position of jumper JP3 and are
discussed in the next section.
Iin
A
+
+
V
Vin
-
Vin supply
-
Vout
+
Iout
V
-
A
Vout
load
+
-
Figure 1. Proper Measurement Equipment Setup
2
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1171A
SYNCHRONOUS BUCK CONVERTER
Figure 2. Measuring Input or Output Ripple Across Terminals or Directly Across Bulk Capacitor
FREQUENCY SYNCHRONIZATION AND MODE SELECTION
Demonstration circuit 1171A’s Mode selector al-
lows the converter to run in CCM or pulse skip
operation by changing position of jumper JP3.
For synchronizing to an external clock source
Table 2.
Mode Selection and Synchronized Operation Options
BOARD MODIFICATION
MODE SELECTOR
JP1
-
-
100k resistor between JP4 pin1 and pin2
-
‘400K’
‘400K’
‘400K’
‘EXTCLK’
JP4
‘CCM’
‘P.S.’
Remove Jumper
‘BURST’
VIN
GND
both JP1 and JP3 needs to be adjusted, while
for Burst Mode operation some bench modifica-
tion is needed. Refer to Table 2 and to the data
sheet for more details.
CONFIGURATION
CCM operation
Pulse skip operation
Burst Mode operation
Synchronized to ext. clock applied to MODE/PLLIN pin
RAIL TRACKING
Demonstration circuit 1171A is configured for an
on board soft start circuit. The soft start ramp
rate can be adjusted by changing the value of
C24. Demonstration circuit 1171A can also be
Table 3.
Output Tracking Options
TRK/SS RESISTORS AND CAPACITOR
CONFIGURATION
Soft Start Without Tracking (Default)
Vout equals External Ramp
Vout tracking scaled external ramp
R32
0Ω
0Ω
R35
Open
Open
C24
0.1µF
Open
Open
RAIL TRACKING
TRK/SS PIN
Open
External reference applied
External reference applied
modified to track an external reference. Refer to
Table 3 for tracking options and to the data
sheet for more details.
Resistor divider
3
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1171A
SYNCHRONOUS BUCK CONVERTER
Figure 3. Typical Efficiency vs. Load Current for DC1171A-A
Figure 4. Typical Efficiency vs. Load Current for DC1171A-B
Figure 5. Thermal image of DC1171A-A, 14Vin, 1.5V@15Aout
4
5
4
3
2
1
This circuit is proprietary to Linear Technology and supplied
for use with Linear Technology parts.
Customer Notice:
Linear Technology has made a best effort to
design a circuit that meets customer-supplied specifications;
however, it remains the customer's responsibility to verify proper
and reliable operation in the actual application, Component
substitution and printed circuit board layout may significantly
affect circuit performance or reliability. Contact Linear
Applications Engineering for assistance.
D
REVISION HISTORY
ECO
REV
3
3.1
DESCRIPTION
PROTO
CHANGED C21, C22, R31, U1
DATE
09/25/07
12/02/10
DING L.
APPROVED
D
E4
E5
MODE/PLLIN
FREQ/PLLFLTR
C20
0.1uF 10V
RUN
VIN
400K
1
2
JP1
EXTCLK
3
R31
1K
C21
J1
VIN
1nF
C22
10nF
Q1
RJK0305DPB
+
CIN1
150uF
35V
J2
4.5V-14V
ON
1
2
JP2
E1
OFF
3
R30
82.5K 1%
5
5
RUN
C23
OPT
INTVCC
MODE/PLLIN
CIN4
10uF
25V
CIN3
10uF
25V
GND
E6
4
4
Q2
OPT
GND
VOUT
E7
CCM BURST P.S.
1
2
4
3
C19
1
2
3
JP3
C
1uF
U1 LTC3851AEGN
1
2
3
E2
R32
0
C24
VOUT
L1
TRK/SS
1
0.1uF 10V
R35
OPT
MODE/PLLIN
FREQ/PLLFLTR
RUN
TK/SS
ITH
FB
SENSE-
SENSE+
SW
TG
16
15
14
13
R36
R37
C25
0.1uF
10V
0
1
INTVCC
D2
CMDSH-3
SW
Q3
RJK0301DPB
1
5
0.68uH
D1
B340A
IN DASHED BOX
KEEP PARTS CLOSE
2
RSNSE
J3
C
VOUT
[1]
COUT3
47uF
6.3V
+
COUT1
330uF
4V
+
COUT2
330uF
4V
J4
5
1.5V/15A
2
3
4
5
4
4
Q4
OPT
BOOST
VIN
INTVCC
BG
GND
ILIM
1
2
3
1
2
3
10
1
2
2
VIN
GND
E8
12
INTVCC
11
INTVCC
R27
[1]
GND
C26
2200pF
R40
6.8K
1%
B
6
C28
330pF
R38
49.9K
1%
C29
20pF
C32
OPT
R39
43.2K
1%
C30
7
8
1000pF
10
9
C27
2.2uF
10V
C31
4.7uF
10V
R29
C18
[1]
[1]
B
INTVCC
R42
VOUT
R41
0
0
ILIM
R28
OPT
R43
[1]
R10
R9
[1]
[1]
[1]
A
CONTRACT NO.
APPROVALS
DATE
10/10/01
TECHNOLOGY
TITLE
1630 McCarthy Blvd.
Milpitas, CA 95035
Phone: (408)432-1900
Fax: (408)434-0507
A
ASSY
-A
-B
DESC
RSENSE
DCR SENSE
RSNSE
3m OHM
10 AWG WIRE
R9, R10
10 OHM
DO NOT STUFF
R27
DO NOT STUFF
3.01K 1%
R29
DO NOT STUFF
30.1K 1%
C18
DO NOT STUFF
0.1uF 50V X7R
R43
DO NOT STUFF
0 OHM
DRAWN MI
CHECKED
APPROVED
ENGINEER
DESIGNER
SCH, LTC3851AEGN, SYNCHRONOUS BUCK
RSENSE / DCR SENSE CONVERTER
SIZE CAGE CODE
DWG NO
DC1171A
FILENAME: 1171A-3.1.DSN
SHEET
1
B
Friday, March 04, 2011
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