DEMO CIRCUIT 1432A
LTC3851AEUD/ LTC3851AEUD-1
QUICK START GUIDE
LTC3851AEUD/
LTC3851AEUD-1
SYNCHRONOUS BUCK CONVERTER
DESCRIPTION
Demonstration circuit 1432A is a synchronous buck
converter featuring the LTC3851AEUD/ LTC3851AEUD-1.
The demo circuit is available in two versions. DC1432A-A
is configured with LTC3851AEUD which provides ISET
function, while the DC1432A-B is configured with
LTC3851AEUD-1 which provides PGOOD function.
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 Forced CCM Mode, Pulse Skip
Mode or Burst Mode operation. Synchronization to an
external clock is also possible on this board.
The demo circuit has an optional DCR sense circuit that
allows using the inductor’s DCR as the current sensing
element to save cost, footprint and improve full load effi-
ciency about 2%.
The LTC3851AEUD/ LTC3851AEUD-1 datasheet gives a
complete description of these parts, operation and appli-
cation information and must be read in conjunction with
this quick start guide for Demonstration circuit 1432A.
Design files for this circuit board are available. Call
the LTC factory.
L
, LTC, LTM, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered
trademarks of Linear Technology Corporation. Adaptive Power, C-Load, DirectSense, Easy
Drive, FilterCAD, Hot Swap, LinearView, µModule, Micropower SwitcherCAD, Multimode
Dimming, No Latency
∆Σ,
No Latency Delta-Sigma, No RSENSE, Operational Filter, PanelPro-
tect, PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT,
UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names
may be trademarks of the companies that manufacture the products.
Table 1. Performance Summary (T
A
=25°C)
PARAMETER
Input Supply Range
Output Voltage Range
Nominal Switching Frequency
Full Load Efficiency
(See Figure 3 for efficiency curves)
CONDITIONS
V
IN
= 12V, I
LOAD
= 15A
V
IN
= 12V, V
OUT
= 1.5V, I
LOAD
= 15A
MIN
4.5
1.47
TYP
1.5
350
88.9
MAX
14
1.53
UNITS
V
V
kHz
%
QUICK START PROCEDURE
Demonstration circuit 1432A is easy to set up to evaluate
the performance of the LTC3851AEUD/ LTC3851AEUD-1.
Refer to Figure 1 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
Vin or Vout and GND terminals. See Figure 2 for proper scope probe
technique.
1.
Place jumpers in the following positions:
FREQ/PLLFLTR
350kHz
RUN
On
MODE/PLLIN
CCM
2.
With power off, connect the input power supply to
Vin and GND.
3.
Turn on the power at the input.
NOTE.
Make sure that the input voltage does not exceed 14V.
1
LTC3851AEUD/ LTC3851AEUD-1
4.
Check for the proper output voltage. Vout = 1.470V
to 1.530V
NOTE.
If there is no output, temporarily disconnect the load to make
sure that the load is not set too high.
the output voltage regulation, ripple voltage, efficien-
cy and other parameters.
6.
Different operating modes can be evaluated by
changing the position of
MODE/PLLIN
jumper and
are discussed in the next section.
5.
Once the proper output voltages are established, ad-
just the loads within the operating range and observe
Iin
A
+
+
V
Vin
Vin supply
-
-
Vout
+
V
Iout
-
A
Vout
load
+
-
Figure 1. Proper Measurement Equipment Setup
Figure 2. Measuring Input or Output Ripple
VIN
GND
2
LTC3851AEUD/ LTC3851AEUD-1
FREQUENCY SYNCHRONIZATION AND MODE SELECTION
Demonstration circuit 1432A’s Mode selector allows
the converter to run in Forced CCM Mode, Pulse Skip
Mode or Burst Mode operation by changing position of
MODE/PLLIN
jumper. For synchronizing to an external
clock source,
FREQ/PLLFLTR
jumper needs to be
Table 2. Mode Selection and Synchronizing Operation Options
CONFIGURATION
Forced CCM Mode Operation
Pulse Skip Mode Operation
Burst Mode Operation
Synchronize to Ext. clock (Ext. clock apply to MODE/PLLIN turret)
placed at “EXTCLK” position and
MODE/PLLIN
jumper
needs to be removed. Apply the external clock from the
MODE/PLLIN
turret to
GND.
Refer to Table 2 and to the
data sheet for more details.
MODE/PLLIN JUMPER
“CCM”
“P.S.”
“BURST”
Remove Jumper
FREQ/PLLFLTR JUMPER
“350K”
“350K”
“350K”
“EXTCLK”
RAIL TRACKING
Demonstration circuit 1432A is configured for an on
board soft start circuit. The soft start ramp rate can be
adjusted by changing the value of C24. Demonstration
Table 3. Output Tracking Options
CONFIGURATION
Soft Start Without Tracking (Default)
Vout Equals External Ramp
Vout Tracking Scaled External Ramp
circuit 1432A can also be modified to track an external
reference. Refer to Table 3 for tracking options and to
the data sheet for more details.
R32
R35
0
OPEN
0
OPEN
Resistor Divider
C24
0.1 F
OPEN
OPEN
TRK/SS PIN
OPEN
External Ramp
External Ramp
OPTIONAL INDUCTOR DCR CURRENT SENSING
Demonstration circuit 1432A provides an optional cir-
cuit for Inductor DCR Current Sensing. Inductor DCR
Current Sensing uses the DCR of the inductor to sense
the inductor current instead of discrete sense resistors.
The advantages of DCR sensing are lower cost, re-
duced board space and higher efficiency, but the dis-
Table 4. Optional Inductor DCR Current Sensing
CONFIGURATION
Current Sense Resistor (Default)
Inductor DCR Current Sensing
RSNSE
2m , 1W
0 Copper
advantage is a less accurate current limit. If DCR sens-
ing is used, be sure to select an inductor current with a
sufficiently high saturation current or use an iron
powder type. Refer to Table 4 for Optional Inductor
DCR Current Sensing setup and to the datasheet for
more details.
R10
100
OPEN
R9
100
OPEN
R1
OPEN
0
C30
1nF
100nF
R27
OPEN
2k
3
LTC3851AEUD/ LTC3851AEUD-1
Demonstration Circuit 1432A (LTC3851AEUD/LTC3851AEUD-1)
Test Efficiency
95
90
85
Efficiency (%)
80
75
Vin=4.5V CCM
Vin=12V CCM
70
Vin=14V CCM
65
0
2
4
6
8
10
Load Current (A)
12
14
16
Figure 3. Efficiency Curve for Demonstration Circuit 1432A
Demonstration Circuit 1432A (LTC3851AEUD/LTC3851AEUD-1)
Test Efficiency
95
90
85
Efficiency (%)
80
75
Vin=12V Current Sensing Resistor
70
Vin=12V Inductor DCR Sensing
65
0
2
4
6
8
10
Load Current (A)
12
14
16
Figure 4. Efficiency Curve for Demonstration Circuit 1432A with Optional Inductor DCR Current Sensing
4
INTVCC
MODE/PLLIN
CCM
1
INTVCC
2
R46
E5
VIN
J1
SW
200K
1%
TG
4
FREQ/PLLFLTR
5
6
3
CIN4
OPT
Q5
OPT
Si4816BDY
JP3
4
B URST P .S .
VIN
VIN
VIN
4.5V - 14V
J2
E4
MODE/PLLIN
3 50K
3
BG
2
1
5
Q1
RJK0305DPB
5
CIN5
OPT
CIN7
OPT
CIN3
22uF
25V
CIN6
22uF
25V
JP1
2
1K
10nF
R30
1
E XT CL K
R31
C22
3
7
8
+
CIN1
150uF
16V
RUN
97.6K
1%
1
ON
OFF
3
R47
C20
1nF
TG
4
Q2
OPT
1
2
3
1
2
3
4
GND
E6
JP2
2
VIN
E1
OPT
GND
RUN
R48
C23
SW
R32 0
16
15
13
14
U1
[1]
C24
INTVCC
5
D2
5
Q3
RJK0329DPB
4
Q4
OPT
1
2
3
1
2
3
TG
1
0.1uF
10V
R35
2
TK/SS
ITH
C19
1uF
FB
BG
ILIM
ILIM
GND
GND
SENSE-
SENSE+
C27
2.2uF
10V
C31
4.7uF
10V
7
INTVCC
10
INTVCC
9
BG
R27
OPT
2.2
VIN
11
R37
VIN
CMDSH-3
3
4
4
OPT
BOOST
0
RUN
12
R36
SW
C25
0.1uF
10V
L1
0.4uH
RSNSE
0.002 OHM
COUT3
47uF
6.3V
+
COUT1
330uF
2.5V
+
COUT2
330uF
2.5V
J4
J3
VOUT
E7
OPT
OPT
E2
TRK/SS
VOUT
VOUT
1.5V / 15A
C26
3300pF
C28
FREQ/PLLFLTR
R40
6.49K
1%
MODE/PLLIN
470pF
1
2
GND
E8
OPTI ONAL DCR
SENSI NG
GND
C29
C32
R38
49.9K
1%
OPT
OPT
R39
43.2K
1%
R29
OPT
VOUT
8
17
5
6
R41
0
C30
1nF
C18
OPT
E9
R1
INTVCC
R42
[1]
OPT
R10
100
R43
OPT
Figure 5. Demonstration Circuit 1432A Schematic
R9
100
R44
OPT
OPT
R45
[1]
[1]
E9
ISET
PGOOD
ASSY
U1
R42
-A
LTC3851AEUD
OPT
-B
LTC3851AEUD-1
100K
LTC3851AEUD/ LTC3851AEUD-1
5
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