DEMO MANUAL
DC1185B-A/-B
LTC3850EUF
Dual Phase/Dual Output
Synchronous Buck Converter
DESCRIPTION
Demonstration circuit 1185 is a dual phase/dual output
synchronous buck converter featuring the
LTC3850EUF.
The demo board comes in two versions. The output volt-
ages for version DC1185B-A are 2.0V/10A and 1.8V/10A.
The output voltages for version DC1185B-B are 1.5V/15A
and 1.2V/15A. The input voltage range is 6.5V to 14V for
both versions. For applications that have a 5V ±0.5V input,
the board has an optional resistor to tie the INTVCC pin
to the VIN pin.
The demo board uses a high density, 2-sided drop-in layout.
The power components excluding the bulk output and input
capacitors, fit within a 1.35"
×
0.75" area on the top layer.
The control circuit resides in a 0.60"
×
0.75" area on the
bottom layer. The package style for the LTC3850EUF is a
4mm
×
4mm 28-lead QFN with an exposed ground pad.
The main features of the board include an internal 5V linear
regulator for bias, RUN pins for each output, an EXTVCC
pin and a PGOOD signal. The board can be configured
for either CCM (original setting), Burst Mode
®
, or pulse-
skipping operation with the MODE jumper. The board
also has optional resistors for single output/dual phase
operation, rail tracking, DCR sensing and synchronization
to an external clock.
Design files for this circuit board are available at
http://www.linear.com/demo
L,
LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
Note: Q1-Q4 MOSFETs changed on September 10, 2013.
See Schematic Diagram.
PERFORMANCE SUMMARY
PARAMETER
Minimum Input Voltage
Maximum Input Voltage
Version DC1185B-A
Output Voltage V
OUT1
Output Voltage V
OUT2
Nominal Switching Frequency
Full-Load Efficiency
(See Figure 3 for Efficiency Curves)
Version DC1185B-B
Output Voltage V
OUT1
Output Voltage V
OUT2
Nominal Switching Frequency
Full-Load Efficiency
(See Figure 4 for Efficiency Curves)
Specifications are at T
A
= 25°C
VALUE
6.5V
14V
CONDITION
I
OUT1
= 0A to 10A
I
OUT2
= 0A to 10A
V
OUT1
= 2.0V, I
OUT1
= 10A, V
IN
= 12V
V
OUT2
= 1.8V, I
OUT2
= 10A, V
IN
= 12V
2.0V ±2%
1.8V ±2%
500kHz
90.2%
89.5%
I
OUT1
= 0A to 15A
I
OUT2
= 0A to 15A
V
OUT1
= 1.5V, I
OUT1
= 15A, V
IN
= 12V
V
OUT2
= 1.2V, I
OUT2
= 15A, V
IN
= 12V
1.5V ±2%
1.2V ±2%
400kHz
88.1%
86.5%
dc1185b-a/-bf
1
DEMO MANUAL
DC1185B-A/-B
QUICK START PROCEDURE
Demonstration circuit 1185 is easy to set up to evaluate
the performance of the LTC3850EUF. Refer to Figure 1
for the proper measurement equipment setup and follow
the procedure below:
Note: When measuring the output or input voltage ripple,
care must be taken to avoid a long ground lead on the
oscilloscope probe. See Figure 2 for the proper scope
probe technique. Short, stiff leads need to be soldered
to the (+) and (–) terminals of an output capacitor. The
probe’s ground ring needs to touch the (–) lead and the
probe tip needs to touch the (+) lead.
Place jumpers in the following positions:
JP1
JP2
JP3
RUN1
RUN2
MODE
ON
ON
CCM
Note: Make sure that the input voltage does not exceed 15V.
Check for the proper output voltages.
Version DC1158B-A:
V
OUT1
= 1.960V to 2.040V
V
OUT2
= 1.764V to 1.836V
Version DC1158B-B:
V
OUT1
= 1.470V to 1.530V
V
OUT2
= 1.176V to 1.224V
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.
Note: Do not apply load across the VOSn+ and VOSn–
turrets. These turrets are only intended to Kelvin sense
the output voltage across COUT1 and COUT4. Heavy load
currents may damage the output voltage sense traces.
With power off, connect the input power supply to VIN
and GND.
Turn on the power at the input.
Figure 1. Proper Measurement Equipment Setup
dc1185b-a/-bf
2
DEMO MANUAL
DC1185B-A/-B
QUICK START PROCEDURE
+
COUT
-
VOUT
GND
Figure 2. Measuring Output Voltage Ripple
2.0V/10A and 1.8V/10A Efficiency at V
IN
= 12V and f
SW
= 500kHz
95
2.0V
1.8V
1.5V/15A and 1.2V/15A Efficiency at V
IN
= 12V and f
SW
= 400kHz
95
90
EFFICIENCY (%)
90
EFFICIENCY (%)
1.5V
1.2V
85
85
80
QTOP: RJK030DPB
QBOTTOM: RJK0330DPB
L: TOKO FDU0650-R56M = P3
0.56µH, DCR = 2.45m , 3.2m
R
SENSE
= 3m
0
2
4
6
8
LOAD CURRENT (A)
10
80
QTOP: RJK030DPB
QBOTTOM: RJK0330DPB
L: VITEC 59PR9875
0.4µH, DCR = 0.47m ±10%
R
SENSE
= 2m
0
3
6
9
12
LOAD CURRENT (A)
15
18
75
70
75
MAX
12
70
Figure 3. Efficiency Curves for the DC1185B-A
Figure 4. Efficiency Curves for the DC1185B-B
SINGLE OUTPUT/DUAL PHASE OPERATION
A single output/dual phase converter may be preferred
for high output current applications. The benefits of
single output/dual phase operation is lower ripple current
through the input and output capacitors, improved load
step response and simplified thermal design. To implement
single output/dual phase operation, make the following
modifications:
1. Tie VOUT1 to VOUT2 by tying together the exposed
copper pads near J3 and J5 at the edge of the board.
Use a piece of heavy copper foil.
2. Tie ITH1 to ITH2 by stuffing 0Ω at R49.
3. Tie VFB1 to VFB2 by stuffing 0Ω at R50.
4. Tie TRK/SS1 to TRK/SS2 by stuffing 0Ω at R52.
5. Tie RUN1 to RUN2 by stuffing 0Ω at R55.
6. Remove the redundant ITH compensation network and
VFB divider.
dc1185b-a/-bf
3
DEMO MANUAL
DC1185B-A/-B
RAIL TRACKING
Demonstration circuit 1185 is set up for independent
turn-on of VOUT1 and VOUT2. The ramp-rate for VOUT1 is
determined by the TRK/SS1 capacitor at C2 and the ramp-
rate for VOUT2 is determined by the TRK/SS2 capacitor
at C47. The turn-on of one rail will not affect the other for
the original demo board.
Table 1. VOUT1 Tracking Options for a 1.5V Output
TRACK 1 DIVIDER
CONFIGURATION
Soft-Start without Tracking (Original Board)
External Coincident Tracking
R3
0Ω
17.8kΩ
R2
Not Stuffed
20.0kΩ
TRK/SS1 CAPACITOR
C2
0.1µF
Not Stuffed
This board can be modified on the bench to allow VOUT1
to track an external signal. It can also be modified to
allow VOUT2 to track VOUT1 or to allow VOUT2 to track
an external signal. Tables 2 and 3 cover the rail tracking
options for each rail, with the DC3850B-B version used
as an example.
Table 2. VOUT2 Tracking Options for a 1.2V Output
TRACK 2 DIVIDER
CONFIGURATION
Soft-Start without Tracking (Original Board)
Coincident Tracking to VOUT1 (1.5V)
External Coincident Tracking
R36
0Ω
0Ω
10.0kΩ
R34
Not Stuffed
10.0kΩ
Not Stuffed
R37
Not Stuffed
20.0kΩ
20.0kΩ
TRK/SS2 CAPACITOR
C47
0.1µF
Not Stuffed
Not Stuffed
dc1185b-a/-bf
4
DEMO MANUAL
DC1185B-A/-B
INDUCTOR DCR SENSING
Demonstration circuit 1185 provides an optional circuit
for DCR sensing. DCR sensing uses the DCR of the in-
ductor to sense the inductor current instead of discrete
sense resistors. The advantages of DCR sensing are lower
cost, reduced board space and higher efficiency, but the
disadvantage 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. Tables 3 and 4 show an example of how to modify
the DC1185 for DCR sensing using these parameters:
V
OUT1
= 2.0V/10A
V
OUT2
= 1.8V/10A
V
IN
= 6.5V to 14V
f
SW
= 500kHz, typical
L1, L2 = Toko FDU0650-R56M=P3
(0.56µH, DCR = 2.45mΩ Typ, 3.2mΩ Max)
ILIM = Floating (R42, R44 = Open)
Table 3. V
OUT1
Configured as a 2.0V/10A Converter Using DCR Sensing and Discrete Sense Resistors
R
SENSE
FILTER
RESISTORS
CONFIGURATION
DCR Sensing
RS1
Short with Cu
Strip or Very
Short and Thick
Piece of Wire
3mΩ
2010 Package
L1
Toko
FDU0650-R56M=P3
R29, R30
Open
SENSE FIILTER
CAPACITOR
C14
0.1µF
DCR FILTER/DIVIDER RESISTORS
TOP
R45
2.37kΩ
BOTTOM
R47
6.49kΩ
SENSE1– TO L1–
JUMPER
R61
0Ω
Discrete R
SENSE
(Original Board)
Toko
FDU0650-R56M=P3
100Ω
1nF
Open
Open
Open
Table 4. V
OUT2
Configured as a 1.8V/10A Converter Using DCR Sensing and Discrete Sense Resistors
R
SENSE
FILTER
RESISTORS
CONFIGURATION
DCR Sensing
RS2
Short with Cu
Strip or Very
Short and Thick
Piece of Wire
3mΩ
2010 Package
L2
Toko
FDU0650-R56M=P3
R39, R40
Open
SENSE FIILTER
CAPACITOR
C15
0.1µF
DCR FILTER/DIVIDER RESISTORS
TOP
R51
2.37kΩ
BOTTOM
R53
6.49kΩ
SENSE1– TO L1–
JUMPER
R62
0Ω
Discrete R
SENSE
(Original Board)
Toko
FDU0650-R56M=P3
100Ω
1nF
Open
Open
Open
dc1185b-a/-bf
5
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