DC1185A - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1185
DUAL PHASE/DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER
LTC3850EUF
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
voltages for version -A are 2.0V/10A and 1.8V/10A and
the output voltages for version -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, two sided drop-in
layout. The power components, excluding the bulk output and input capacitors, fit within a 1.35” X 0.75” area
on the top layer. The control circuit resides in a 0.60” X
0.75” area on the bottom layer. The package style for the
LTC3850EUF is a 4mm X 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 skip 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. Call
the LTC factory.
Table 1. Performance Summary (TA = 25°C)
PARAMETER
CONDITION
VALUE
Minimum Input Voltage
6.5V
Maximum Input Voltage
14V
Version -A
Output Voltage VOUT1
IOUT1 = 0A to 10A
2.0V ±2%
Output Voltage VOUT2
IOUT2 = 0A to 10A
1.8V ±2%
Nominal Switching Frequency
500kHz
Full Load Efficiency
VOUT1 = 2.0V, IOUT1 = 10A , VIN = 12V
88.0%
(see Figure 3 for efficiency curves)
VOUT2 = 1.8V, IOUT2 = 10A , VIN = 12V
87.0%
Output Voltage VOUT1
IOUT1 = 0A to 15A
1.5V ±2%
Output Voltage VOUT2
IOUT2 = 0A to 15A
1.2V ±2%
Version -B
Nominal Switching Frequency
400kHz
Full Load Efficiency
VOUT1 = 1.5V, IOUT1 = 15A , VIN = 12V
87.4%
(see Figure 4 for efficiency curves)
VOUT2 = 1.2V, IOUT2 = 15A , VIN = 12V
85.3%
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1185
DUAL PHASE/DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER
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
RUN1
ON
JP2
RUN2
ON
JP3
MODE
CCM
Check for the proper output voltages.
Version –A:
Vout1 = 1.960V to 2.040V
Vout2 = 1.764V to 1.836V
Version –B:
Vout1 = 1.470V to 1.530V
Vout2 = 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
With power off, connect the input power supply to VIN
and GND.
not apply load across the VOSn+ and VOSnturrets. 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.
Turn on the power at the input.
NOTE: Make sure that the input
voltage does not exceed
15V.
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:
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.
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.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1185
DUAL PHASE/DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER
RAIL TRACKING
Demonstration circuit 1185 is setup for independent
turn-on of VOUT1 and VOUT2. The ramp-rate for VOUT1
is determined by the TRK/SS1 cap at C2 and the ramprate for VOUT2 is determined by the TRK/SS2 cap at
C47. The turn-on of one rail will not affect the other for
the original demo board.
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 –B version used
as an example.
Table 1. VOUT1 Tracking Options for a 1.5V Output.
TRACK1 DIVIDER
TRK/SS1 CAP
CONFIGURATION
R3
R2
C2
Soft Start Without Tracking (original board)
0Ω
Not stuffed
0.1uF
External Coincident Tracking
17.8kΩ
20.0kΩ
Not Stuffed
Table 2. VOUT2 Tracking Options for a 1.2V Output.
TRACK2 DIVIDER
TRK/SS2 CAP
CONFIGURATION
R36
R34
R37
C47
Soft Start Without Tracking (original board)
0Ω
Not stuffed
Not stuffed
0.1uF
Coincident Tracking to VOUT1 (1.5V)
0Ω
10.0kΩ
20.0kΩ
Not Stuffed
External Coincident Tracking
10.0kΩ
Not stuffed
20.0kΩ
Not Stuffed
INDUCTOR DCR SENSING
Demonstration circuit 1185 provides an optional circuit
for DCR sensing. DCR sensing uses the DCR of the inductor 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 sensing 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:
VOUT1 = 2.0V / 10A
VOUT2 = 1.8V / 10A
VIN = 6.5V to 14V
Fsw = 500kHz, typical
L1,2 = Toko FDU0650-R56M=P3
(0.56uH, DCR = 2.45mΩ typ, 3.2mΩ max)
ILIM = FLOATING (R42,R44 = OPEN)
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1185
DUAL PHASE/DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER
Table 3. VOUT1 Configured as a 2.0V/10A Converter Using DCR Sensing and Discrete Sense Resistors
CONFIGURATION
RS1
DCR Sensing
Short with Cu
strip or very
short & thick
piece of wire
L1
Toko
SENSE
FIILTER
CAP
DCR FILTER/DIVIDER
RESISTORS
SENSE1- TO L1JUMPER
TOP
BOTTOM
R29,R30
C14
R45
R47
R61
Open
0.1uF
2.37kΩ
6.49kΩ
0Ω
100Ω
1nF
Open
Open
Open
FDU0650-R56M=P3
Discrete RSENSE 3mΩ
Toko
(original board)
FDU0650-R56M=P3
2010 pkg
RSENSE
FILTER
RESISTORS
Table 4. VOUT2 Configured as a 1.8V/10A Converter Using DCR Sensing and Discrete Sense Resistors
CONFIGURATION
RS2
DCR Sensing
Short with Cu
strip or very
short & thick
piece of wire
L2
Toko
SENSE
FIILTER
CAP
DCR FILTER/DIVIDER
RESISTORS
SENSE1- TO L1JUMPER
TOP
BOTTOM
R39,R40
C15
R51
R53
R62
Open
0.1uF
2.37kΩ
6.49kΩ
0Ω
100Ω
1nF
Open
Open
Open
FDU0650-R56M=P3
Discrete RSENSE 3mΩ
Toko
(original board)
FDU0650-R56M=P3
2010 pkg
RSENSE
FILTER
RESISTORS
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1185
DUAL PHASE/DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER
SYNCHRONIZATION TO AN EXTERNAL CLOCK
The LTC3850 uses a phase lock loop which forces its
internal clock to be synchronized to an external clock.
Once synchronized, the rising edge of the top FET gate is
aligned to the rising edge of the external clock. The external clock signal needs to be applied to the LTC3850’s
MODE pin which is tied to the turret labeled SYNC. The
internal phase lock loop is stabilized by a network on the
FREQ pin of the LTC3850. To setup the DC1185 for synchronization to an external clock, follow the steps below.
2. Stuff 10kΩ at R8.
3. Stuff 10nF at R10.
4. Leave 1nF at C12.
5. Float the MODE pin by placing the MODE
jumper in the BM position.
6. Apply the external clock from the turret labeled SYNC to GND.
1. Remove R7.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1185
DUAL PHASE/DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER
Vout1 V
+
-
Iin
A
Iout1
+
Vin
supply
A
A
-
Iout2
Vout1
load
+
+
Vout2
load
-
+
-
V
Vin
Vout2 V
-
+
Figure 1. Proper Measurement Equipment Setup
+
COUT
VOUT
-
GND
Figure 2. Measuring Output Voltage Ripple
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1185
DUAL PHASE/DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER
2.0V / 10A and 1.8V / 10A efficiency at
Vin = 12V and Fsw = 500kHz
Efficiency
95%
90%
2.0V
1.8V
85%
Qtop = Si4420BDY
Qbottom = Si4630DY
No schottky
L = Toko FDU0650 0.56uH
Rsense = 3m
80%
75%
70%
0
2
4
6
8
10
12
Load current (Amps)
Figure 3. Efficiency Curves for the DC1185A-A.
1.5V / 15A and 1.2V / 15A efficiency at
Vin = 12V and Fsw = 400kHz
95%
Efficiency
90%
1.5V
1.2V
85%
Qtop = RJK0305DPB
Qbottom = RJK0301DPB
No schottky
L = Vitec 59PR9875 (0.4uH)
Rsense = 2m
80%
75%
70%
0
3
6
9
12
15
18
Load current (Amps)
Figure 4. Efficiency Curves for the DC1185A-B.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1185
DUAL PHASE/DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER
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