DEMO MANUAL DC1798A
LTC3869IGN-2
Dual, 2-Phase Synchronous
Step-Down DC/DC Controllers
DESCRIPTION
Demonstration circuit 1798A is a Dual Output, Dual
Phase 5A Synchronous Buck Converter featuring the
LTC3869IGN-2. The entire circuit, including the bulk output
capacitors, fit within a 1.04"
×
0.94" area on all layers. The
package style for the LTC3869IGN-2 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, a PGOOD signal and a mode selector that allows
the converter to run in CCM, pulse-skipping or Burst
Mode operation. Synchronization to an external clock is
also possible through some minor component changes.
Two versions of the board are available.
DC1798A-A has an on-board sense resistor for current
feedback, while the DC1798A-B is configured with a DCR
sense circuit that allows the converter to use the inductors
DCR as the sense element instead of the on-board sense
resistors to save cost and board space and improves
efficiency.
The input voltage range is 4.5V to 26V. The LTC3869 data-
sheet gives a complete description of the part, operation
and application information. The datasheet must be read
in conjunction with this demo manual for DC1798A.
Design files for this circuit board are available at
http://www.linear.com/demo
L,
LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
PERFORMANCE SUMMARY
PARAMETER
Minimum Input Voltage
Maximum Input Voltage
Output Voltage V
OUT1
Output Voltage V
OUT2
Typical Output Ripple V
OUT
Nominal Switching Frequency
Efficiency
See Figures 3 and 4 for efficiency
curves
*Measured at bulk output capacitor
CONDITIONS
(T
A
= 25°C)
VALUE
4.5V
26V
V
IN
= 6.5V to 26V, I
OUT1
= 0A to 5A
V
IN
= 6.5V to 26V, I
OUT2
= 0A to 5A
V
IN
= 26V, I
OUT1
= 5A (20MHz BW)
V
IN
= 26V, I
OUT2
= 5A (20MHz BW)
3.3V±2%
2.5V±2%
<30mV
P-P
*
<30mV
P-P
*
400kHz
DC1798A-A
DC1798A-B
91.0%** Typical
88.8%* Typical
V
OUT1
= 3.3V, I
OUT1
= 5A; V
IN
= 16V
V
OUT2
= 2.5V, I
OUT2
= 5A; V
IN
= 16V
90%** Typical
87.5%* Typical
**Optional Mosfet (Si4816BDY) will result in up to 0.4% improvement in efficiency at full load
dc1798af
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DEMO MANUAL DC1798A
QUICK START PROCEDURE
Demonstration circuit 1798A is easy to set up to evaluate
the performance of the LTC3869IGN-2. 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 V
IN
or
V
OUT
and GND terminals. See Figure 2 for proper scope
probe technique.
1. Make sure jumpers are in the following positions:
JUMPER
JP1
JP2
JP3
POSITION
ON
ON
CCM
3. Turn on the power at the input.
NOTE.
Make sure that the input voltage does not exceed
28V.
4. Check for the proper output voltages.
V
OUT1
= 3.234V to 3.366V,
V
OUT2
= 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.
2. With power off, connect the input power supply to V
IN
and GND. Connect active loads to outputs.
A
4
A
V
POWER
SUPPLY
V
V
OUT1
Load
A
V
V
OUT2
Load
Figure 1. Proper Measurement Equipment Setup
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DEMO MANUAL DC1798A
QUICK START PROCEDURE
Figure 2. Measuring Output Voltage Ripple
Rail Tracking
Demonstration circuit 1798A is set up for coincident rail
tracking where V
OUT2
tracks V
OUT1
and the ramp rate for
V
OUT1
is determined by the value of the TRK/SS1 capaci-
tor at C6.
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 V
OUT1
and V
OUT2
will be
determined by their respective TRK/SS capacitors. Refer
to Table 1 for tracking options and to the data sheet for
more details.
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 cir-
cuit 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 cur-
rent feedback, the DCR sensing circuit offers lower cost
Table 1. Output Tracking Options
TRACK 1 DIVIDER
CONFIGURATION
Soft Start Without Tracking
V
OUT1
V
OUT2
Coincident Tracking:
V
OUT1
tracking External Ramp
V
OUT2
tracking V
OUT1
Ratiometric Tracking:
V
OUT2
tracking V
OUT1
R10
0Ω
R12
OPEN
TRK/SS1 CAPACITOR
C
10nF
TRACK 1 DIVIDER
R1
OPEN
R4
OPEN
TRK/SS1 CAPACITOR
C
10nF
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
1798A 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. 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 MOSFET Q3 and Q4.
63.4kΩ
20.0kΩ
OPEN
43.2kΩ
20.0kΩ
OPEN
OPEN
OPEN
OPEN
63.4kΩ
20.0kΩ
OPEN
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DEMO MANUAL DC1798A
QUICK START PROCEDURE
100
95
90
EFFICIENCY (%)
85
80
75
70
65
0
1
1
2
2
3
3
4
4
5
5
LOAD CURRENT (A)
Figure 3. Typical Efficiency vs Load Current for A-A(R
SENSE
) and A-B(DCR SENSE) Boards, 3.3V
OUT
, 400kHz
6.5V
IN
, 3.3V
OUT
, DCR SENSE, 400kHz
16V
IN
, 3.3V
OUT
, DCR SENSE, 400kHz
6.5V
IN
, 3.3V
OUT
, R
SENSE
, 400kHz
16V
IN
, 3.3VV
OUT
, R
SENSE
, 400kHz
dc1798af
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DEMO MANUAL DC1798A
100
95
90
EFFICIENCY (%)
85
80
75
70
65
0
1
1
2
2
3
3
4
4
5
5
LOAD CURRENT (A)
Figure 4. Typical Efficiency vs Load Current for A-A(R
SENSE
) and A-B(DCR SENSE) Boards, 3.3V
OUT
, 400kHz
6.5V
IN
, 2.5V
OUT
, DCR SENSE, 400kHz
16V
IN
, 2.5V
OUT
, DCR SENSE, 400kHz
6.5V
IN
, 2.5V
OUT
, R
SENSE
, 400kHz
16V
IN
, 2.5V
OUT
, R
SENSE
, 400kHz
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