DC1231A - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1231A
DUAL OUTPUT, DUAL PHASE 5A SYNCHRONOUS BUCK CONVERTER
LTC3850EGN
DESCRIPTION
Demonstration circuit 1231A is a Dual Output, Dual
Phase 5A Synchronous Buck Converter featuring the
LTC3850EGN. The entire circuit, including the bulk output capacitors, fit within a 1.04” X 0.94” area on all layers. The package style for the LTC3850EGN is a 28-lead
narrow plastic SSOP.
The main features of the board include rail tracking, an
internal 5V linear regulator for bias, RUN pins for each
output and a PGOOD signal 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.
Two versions of the board are available. DC-1231A-A has
an on-board sense resistor for current feedback, while
the DC1231A-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 resis-
tors to save cost and board space and improves efficiency.
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 LT3850 datasheet gives a complete description of
the part, operation and application information. The
datasheet must be read in conjunction with this quick
start guide for demo circuit 1231A.
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
Minimum Input Voltage
6.5V
Maximum Input Voltage
24V
Output Voltage VOUT1
VIN = 6.5V to 24V, IOUT1 = 0A to 5A
3.3V ±2%
Output Voltage VOUT2
VIN = 6.5V to 24V, IOUT2 = 0A to 5A
2.5V ±2%
VIN = 24V, IOUT1 = 5A (20MHz BW)
<30mVP–P#
VIN = 24V, IOUT2 = 5A (20MHz BW)
<30mVP–P#
Typical Output Ripple VOUT
Output Regulation
Line
±0.05%
Load
±0.1%
Nominal Switching Frequency
Efficiency
See Figures 3 and 4 for efficiency curves
400kHz
DC1231A-A
DC1231A-B
VOUT1 = 3.3V, IOUT1 = 5A; VIN = 24V
90.3%* Typical
91.2%* Typical
VOUT2 = 2.5V, IOUT1 = 5A; VIN = 24V
88.6%* Typical
89.5%* Typical
*Optional Mosfet (Si4816BDY) will result in up to 0.4% improvement in efficiency at full load
#
Measured at bulk output capacitor
QUICK START PROCEDURE
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1231A
DUAL OUTPUT, DUAL PHASE 5A SYNCHRONOUS BUCK CONVERTER
Demonstration circuit 1231A is easy to set up to evaluate the performance of the LTC3850EGN. 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.
2.
3.
NOTE: Make sure that the input voltage does not exceed 26V.
4.
On
JP2
On
JP3
CCM
Check for the proper output voltages.
Vout1 = 3.234V to 3.366V,
Vout2 = 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, 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 position of jumper JP3.
Place jumpers in the following positions:
JP1
Turn on the power at the input.
With power off, connect the input power supply to Vin
and GND.
Iout1
A
Iin
A
+
Vin supply
-
+
+
V Vout1
-
Vin +
V
-
-
V
Vout1
load
Vout2
+
A
Vout2
load
+
Iout2
Figure 1. Proper Measurement Equipment Setup
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1231A
DUAL OUTPUT, DUAL PHASE 5A SYNCHRONOUS BUCK CONVERTER
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 VOUT2 tracks VOUT1 and the ramprate for VOUT1 is determined by the value of the
TRK/SS1 capacitor at C6.
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.
This board can be modified on the bench for external
rail tracking or for independent turn-on of the rails. For
Table 2. Output Tracking Options
TRACK1 DIVIDER
CONFIGURATION
TRK/SS1 CAP
TRACK2 DIVIDER
R1
R4
C
open
open
10nF
43.2kΩ
20.0kΩ
Open
63.4kΩ
20.0kΩ
open
R10
R12
C
0Ω
open
10nF
TRK/SS2 CAP
Soft Start Without Tracking
Vout1
Vout2
Coincident Tracking:
Vout1 tracking External Ramp
Vout2 tracking Vout1
63.4kΩ
20.0kΩ
open
Ratiometric Tracking:
Vout2 tracking Vout1
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1231A
DUAL OUTPUT, DUAL PHASE 5A SYNCHRONOUS BUCK CONVERTER
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 JP3. 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
R2
R3
R7
JP3
Free Running
10kΩ
0Ω
2.49kΩ
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 two different versions of the board offer either an
on-board sense resistor or an inductor DCR sense circuit for current feedback. 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 in
the inductor resistance.
Furthermore, this indirect current sensing method
cannot detect inductor saturation and requires the use
of ‘soft’ saturating inductors (such as powder iron)
resulting in increased core losses or ‘hard’ saturating
inductors (such as ferrite) with sufficiency high current ratings resulting in increased inductor size.
The demonstration circuit 1231A-B is intended to
demonstrate the feasibility of a high performance, high
efficiency synchronous buck converter using a ‘hard’
saturating ferrite inductor with DCR sensing.
The typical efficiency versus load current for each of
the outputs is given in Figure 3 and Figure 4 respectively. The efficiency is measured at both the minimum and maximum input voltage. The typical efficiency curves for both the sense resistor (A-A) and
DCR sensing (A-B) boards are shown. An efficiency
improvement of up to 1% is possible for the DCR
sensing version.
If further improvement in efficiency is required, the
switching devices can be replaced by the optional
mosfets Q3 and Q4.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1231A
DUAL OUTPUT, DUAL PHASE 5A SYNCHRONOUS BUCK CONVERTER
Efficiency vs. Load Current, 3.3Vout, 400kHz, CCM
98
96
94
92
90
Efficiency (%)
88
86
84
82
80
78
76
A-A, 6.5Vin
A-B, 6.5Vin
A-A, 24Vin
A-B, 24Vin
74
72
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 3. Typical Efficiency vs. Load Current for A-A and A-B boards – 3.3Vout
Efficiency vs. Load Current, 2.5Vout, 400kHz, CCM
98
96
94
92
90
E fficiency (% )
88
86
84
82
80
78
76
A-A, 6.5Vin
A-B, 6.5Vin
A-A, 24Vin
A-B, 24Vin
74
72
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 A-A and A-B boards – 2.5Vout
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1231A
DUAL OUTPUT, DUAL PHASE 5A SYNCHRONOUS BUCK CONVERTER
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