DC1254A - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1254A
POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
LTC3853EUJ
DESCRIPTION
Demonstration circuit 1254A is a polyphase 3-output
step-down supply with tracking featuring the
LTC3853EUJ. The entire circuit, excluding the bulk output capacitors, fits within a 1.5” X 1” area on all layers.
The package style for the LTC3853EUJ 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 component changes.
The input voltage range is 6.5V to 24V. For applications
with narrow, 5V ± 0.5V input range, the board has an
optional resistor to tie the INTVCC pin to the VIN pin.
The LTC3853 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 1254A.
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 (TA = 25°C)
PARAMETER
CONDITION
VALUE
Input Voltage Range
6.5V - 24V
Output Voltage VOUT1
VIN = 6.5V to 24V, IOUT1 = 0A to 5A
2.5V ±2%
Output Voltage VOUT2
VIN = 6.5V to 24V, IOUT2 = 0A to 5A
1.8V ±2%
Output Voltage VOUT3
VIN = 6.5V to 24V, IOUT3 = 0A to 5A
3.3V ±2%
Nominal Switching Frequency
Efficiency
See Figures 4, 5 and 6 for efficiency curves
500kHz
VOUT1 = 2.5V, IOUT1 = 5A; VIN = 12V
89.2% Typical
VOUT2 = 1.8V, IOUT2 = 5A; VIN = 12V
86.6% Typical
VOUT3 = 3.3V, IOUT3 = 5A; VIN = 12V
91.7% Typical
QUICK START PROCEDURE
Demonstration circuit 1254A is easy to set up to evaluate the performance of the LTC3853EUJ. Refer to Figure
1 for proper measurement equipment setup and follow
the procedure below:
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.
NOTE: When
Place jumpers in the following positions:
JP1
On
JP2
On
JP3
On
JP4
Pulse Skip
2.
With power off, connect the input power supply to Vin
and GND.
3.
Turn on the power at the input.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1254A
POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
NOTE: Make
sure that the input voltage does not ex-
5.
Once the proper output voltages are established, adjust 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 changing the position of jumper JP4.
ceed 24V.
4.
Check for the proper output voltages.
Vout1 = 2.450V to 2.550V,
Vout2 = 1.765V to 1.836V,
Vout3 = 3.234V to 3.366V
NOTE: If there is no output, temporarily disconnect the
load to make sure that the load is not set too high.
Iout1
A
+
+
V
Vout1
-
Vout1
load
Iout2
A
+
+
V
Vout2
-
Iin
Vout2
load
-
A
Iout3
+
Vin supply
-
Vin +
A
V
-
+
+
V
-
Vout3
Vout3
load
-
Figure 1. Proper Measurement Equipment Setup
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1254A
POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
GND
VIN
Figure 2. Measuring Input or Output Ripple Across Terminals or Directly Across Bulk Capacitor
RAIL TRACKING
Demonstration circuit 1231 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 C46 - See Figure 3. Please
note that turning channel 3 off, will also turn off the
other two channels, as they track channels 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 capacitors. 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
CONFIGURATION
R31
R32
TK/SS2 RESISTOR AND
CAPACITORS
C44
R33
R34
TK/SS3 RESISTOR AND
CAPACITORS
C45
R35
R36
C46
Soft Start Without Tracking
Vout1 (Default)
Open
Open
10nF
Vout2
open
Open
10nF
Vout3
Vout3 equals External Ramp
X
Open
10nF
0Ω
Open
Open
Ratiometric Tracking:
Vout1 tracking Vout3
63.4kΩ
20.0kΩ
Open
Vout2 tracking Vout3
63.4kΩ
20.0kΩ
Open
24.9kΩ
20.0kΩ
Open
Coincident Tracking (Default):
Vout1 tracking Vout3
Vout2 tracking Vout3
Vout3 tracking ext. ramp
43.2kΩ
20.0kΩ
Open
Resistor divider
Open
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1254A
POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
Vout3 (3.3V)
Vout1 (2.5V)
Vout2 (1.8V)
Figure 3. Default coincidental startup tracking
FREQUENCY SYNCHRONIZATION
Demonstration circuit 1231’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
R63
R47
JP4
Free Running
10kΩ
0Ω
3.16kΩ
FCC, Pulse Skip or Burst Mode
Synchronized to External Clock
open
10kΩ
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 efficiency, 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. An efficiency improvement of 1% or
more 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.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1254A
POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
Table 4. DCR sensing component selection
REMOVE RSENSE NETWORK
ADD DCR SENSE NETWORK
Vout1
R21, R22 = Open, RSNS1 = Short
R51 = 1.69kΩ
R52 = 22.6kΩ
R53 = 0Ω
C21 = 0.1µF
Vout2
R23, R24 = Open, RSNS2 = Short
R52 = 1.69kΩ
R55 = 22.6k Ω
R56 = 0Ω
C22 = 0.1µF
Vout3
R25, R26 = Open, RSNS3 = Short
R53 = 2.74kΩ
R58 = 7.5kΩ
R59 = 0Ω
C23 = 0.1µF
95
Efficiency (%)
90
85
80
6.5Vin, 2.5Vout
75
12Vin, 2.5Vout
16Vin, 2.5Vout
24Vin, 2.5Vout
70
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Load Current (A)
Figure 4. Typical Efficiency vs. Load Current for Vout1 (2.5V) vs. Vin
95
Efficiency (%)
90
85
80
6.5Vin, 1.8Vout
75
12Vin, 1.8Vout
16Vin, 1.8Vout
24Vin, 1.8Vout
70
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Load Current (A)
Figure 5. Typical Efficiency vs. Load Current for Vout2 (1.8V) vs. Vin
5
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1254A
POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
95
Efficiency (%)
90
85
80
6.5Vin, 3.3Vout
75
12Vin, 3.3Vout
16Vin, 3.3Vout
24Vin, 3.3Vout
70
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Load Current (A)
Figure 6. Typical Efficiency vs. Load Current for Vout3 (3.3V) vs. Vin
6
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1254A
POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1254A
POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
8
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