QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1349A
HIGH CURRENT POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
1
LTC3853EUJ
DESCRIPTION
Demonstration circuit 1349A is a high current polyphase
3-output step-down supply with tracking featuring the
LTC3853EUJ. The entire circuit, excluding the bulk out-
put capacitors, fits within a 1.5” X 1.5” area on all layers.
The package style for the LTC3850EUJ is a 6mm x 6mm
plastic QFN.
The main features of the board include rail tracking, an
internal 5V linear regulator for bias, RUN pins for each
output, two PGOOD signals and a Mode selector that
allow the converter to run in CCM, pulse skip or Burst
Mode operation. Synchronization to an external clock is
also possible through some minor component changes.
The board is configured for resistor current sensing, but
optional DCR sensing is possible through some compo-
nent changes.
The input voltage range is 6.5V to 14V, but for applica-
tions with narrow, 5V ± 0.5V input range, the board has
an optional resistor to tie the INTVCC pin to the VIN pin.
The LT3853 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 1349A.
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)
PARAMETER CONDITION VALUE
Input Voltage Range 6.5V - 14V
Output Voltage V
OUT1
V
IN
= 6.5V to 14V, I
OUT1
= 0A to 15A 1.8V ±2%
Output Voltage V
OUT2
V
IN
= 6.5V to 14V, I
OUT2
= 0A to 15A 1.2V ±2%
Output Voltage V
OUT3
V
IN
= 6.5V to 14V, I
OUT3
= 0A to 15A 2.5V ±2%
Nominal Switching Frequency 400kHz
V
OUT1
= 1.8V, I
OUT1
= 15A; V
IN
= 12V 89.2% Typical
V
OUT2
= 1.2V, I
OUT2
= 15A; V
IN
= 12V 86.6% Typical
Efficiency
See Figures 4, 5 and 6 for efficiency curves
V
OUT3
= 2.5V, I
OUT3
= 15A; V
IN
= 12V 91.7% Typical
QUICK START PROCEDURE
Demonstration circuit 1349A is easy to set up to evalu-
ate the performance of the LTC3853EUJ. Refer to Figure
1 for proper measurement equipment setup and follow
the procedure below:
NOTE:
When measuring the input or output voltage rip-
ple, 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:
On
On
On
Forced Continuous
2.
With power off, connect the input power supply to Vin
and GND.
3.
Turn on the power at the input.
JP1
JP2
JP3
JP4
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1349A
HIGH CURRENT POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
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NOTE:
Make sure that the input voltage does not ex-
ceed 14V.
4.
Check for the proper output voltages.
Vout1 = 1.765V to 1.836V,
Vout2 = 1.176V to 1.224V,
Vout3 = 2.450V to 2.550V
NOTE:
If there is no output, temporarily disconnect the
load to make sure that the load is not set too high.
5.
Once the proper output voltages are established, ad-
just the loads within the operating range and observe
the output voltage regulation, ripple voltage, efficiency
and other parameters.
6.
Different operating modes can be evaluated by chang-
ing the position of jumper JP4.
Figure 1.
Proper Measurement Equipment Setup
Iout1
A
V
+
-
Vout1
load
+
-
Vout1
Iout2
A
V
+
-
Vout2
load
+
-
Vout2
Iout3
A
V
+
-
Vout3
load
+
-
Vout3
Vin
Vin supply
A
+
-
Iin
V
+
-
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1349A
HIGH CURRENT POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
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Figure 2.
Measuring Input or Output Ripple Directly Across Bulk Capacitor
RAIL TRACKING
Demonstration circuit 1349 is setup for coincident rail
tracking where VOUT1 and VOUT2 track VOUT3 and
the ramp-rate for VOUT3 is determined by the value of
the TK/SS3 capacitor at C44 - See Figure 3. Please
note that turning channel 3 off, will also turn off the
other two channels, as they are tracking channel 3.
This board can be modified on the bench for external
rail tracking or for independent turn-on of the rails. For
the latter case, the ramp-rate for VOUT1 and VOUT2
will be determined by their respective TRK/SS capaci-
tors. Refer to Table 2 for tracking options and to the
data sheet for more details.
Table 2.
Output Tracking Options
TK/SS1 RESISTOR AND
CAPACITORS
TK/SS2 RESISTOR AND
CAPACITORS
TK/SS3 RESISTOR AND
CAPACITORS
CONFIGURATION R31 R32 C46 R33 R34 C45 R35 R36 C44
Soft Start Without Tracking
Vout1
Vout2
Vout3 (Default)
Open
Open
10nF
open
Open
10nF
X
Open
10nF
Vout3 equals External Ramp 0
Ω
Open Open
Ratiometric Tracking:
Vout1 tracking Vout3
Vout2 tracking Vout3
43.2k
Ω
20.0k
Ω
Open
43.2k
Ω
20.0k
Ω
Open
Coincident Tracking (Default):
Vout1 tracking Vout3
Vout2 tracking Vout3
24.9k
Ω
20.0k
Ω
Open
10.0k
Ω
20.0k
Ω
Open
Vout3 tracking ext. ramp
Resistor divider
Open
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1349A
HIGH CURRENT POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
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Figure 3.
Default coincidental startup tracking
FREQUENCY SYNCHRONIZATION
Demonstration circuit 1349’s Mode selector allows the
converter to run in CCM, pulse skip or Burst Mode
operation by changing position of jumper JP4. For
synchronizing to an external clock source, however,
some bench modification is needed. Refer to Table 3
and to the data sheet for more details.
Table 3.
Free Running and Synchronized Operation Options
FREQ PIN COMPONENTS MODE SELECTOR
CONFIGURATION R48 R65 R47 JP4
Free Running 10.0k
Ω
0
Ω
2.55k
Ω
FCC, Pulse Skip or Burst Mode
Synchronized to External Clock open 10k
Ω
0.01µF Burst Mode or Open
INDUCTOR DCR SENSING AND RESISTOR SENSING
The DCR sense circuit uses the resistive voltage drop
across the inductor to estimate the current. In contrast
to the traditional sense resistor current feedback, the
DCR sensing circuit offers lower cost and higher effi-
ciency, but results in less accurate current limit due to
the large variation of the inductor DC resistance. For
modifying the demo board for DCR sensing, please
refer to Table 4. A full load efficiency improvement of
1% - 2% is still possible for optional DCR sensing.
The typical efficiency versus load current for each of
the outputs is given in Figures 4 to 6 respectively for a
range of input voltages.
Table 4.
DCR sensing component selection
REMOVE RSENSE NETWORK DCR NETWORK
Vout1 R21, R22 = Open, RSNS1 = Short R51 = 511
Ω
R52 = OPEN R53 = 0
Ω
C21 = 0.47µF
Vout2 R23, R24 = Open, RSNS2 = Short R52 = 402
Ω
R55 = OPEN R56 = 0
Ω
C21 = 0.47µF
Vout3 R25, R26 = Open, RSNS3 = Short R53 = 511
Ω
R58 = OPEN R59 = 0
Ω
C21 = 0.47µF
Vout3 (2.5V)
Vout1 (1.8V)
Vout2 (1.2V)
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1349A
HIGH CURRENT POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
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SINGLE OUTPUT / DUAL PHASE OPERATION
The demonstration circuit 1349A has been designed to
support a 2+1 configuration (2 phase output plus a
single phase 2
nd
output) as described in the LT3853
datasheet. Although the design of such a two phase
output circuit is beyond the scope of the guide, some
comments on how to implement this on this demo
board should be mentioned.
1.
Make sure the power components for ch1 and ch2
are the same (All components on right side of IC on
schematic for these two channels). Short Vout1 to
Vout2 using the pads provided.
2.
Remove all components connected to VFB2 (pin12)
and ITH2 (pin 13). Remove feedback resistor R37.
3.
Install optional components R63, R64, R66 and
R68 with 0
Ω
jumpers and R67 with a
100k
Ω
ΩΩ
Ω
pull-
up
.
4.
Install the correct feedback (R1, R2 and C1) and
compensation (C4, C5 and R7).
74
76
78
80
82
84
86
88
90
92
94
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Efficiency (%)
Load Current (A)
1.8Vout, 6.5Vin
1.8Vout, 14Vin
Figure 4.
Typical Efficiency vs. Load Current for Vout1 (1.8V) vs. Vin
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1349A
HIGH CURRENT POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
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74
76
78
80
82
84
86
88
90
92
94
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Efficiency (%)
Load Current (A)
1
.
2
Vout,
6
.
5
Vin
1.2Vout, 14Vin
Figure 5.
Typical Efficiency vs. Load Current for Vout2 (1.2V) vs. Vin
74
76
78
80
82
84
86
88
90
92
94
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Efficiency (%)
Load Current (A)
2
.
5
Vout,
6
.
5
Vin
2.5Vout, 14Vin
Figure 6.
Typical Efficiency vs. Load Current for Vout3 (2.5V) vs. Vin
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1349A
HIGH CURRENT POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
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