DC1441A - Demo Manual

DEMO CIRCUIT 1441A
QUICK START GUIDE
LTC3855EUJ
DUAL OUTPUT
SYNCHRONOUS BUCK CONVERTER
DESCRIPTION
Demonstration circuit DC1441A is a dual output synchronous buck converter featuring the LTC3855EUJ.
The board provides two outputs of 1.8V/17A and
1.2V/17A from an input voltage of 4.5V to 14V at a
switching frequency of 400kHz.
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 1.4” X 0.75” area
on the top layer. The control circuit resides in a 0.8” X
0.9” area on the bottom layer. The package style for the
LTC3855EUJ is a 40-lead 6mm X 6mm QFN.
The main features of the board are listed below:
• Differential amplifier for remote sensing VOUT2
which is configured for 1.2V.
• Diff amp bypass for VOUT2 ≥ 3.3V.
• Optional resistors for single output dual phase
operation.
• Optional resistors for DCR sensing and for NTC
compensated DCR sensing.
• PLLIN pin for synchronization to an external
clock which can be used in conjunction with
PHASMD pin and CLKOUT pin for up to
12-phase operation.
• Selectable light load operating modes of pulse
skip, Burst Mode® or FCM.
• TRACK/SS pins for external rail tracking.
• RUN pins and PGOOD pins for each phase.
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
4.5V
Maximum Input Voltage
14V
Output Voltage VOUT1
IOUT1 = 0A TO 17A, VIN = 4.5V to 14V
1.8V ± 1.75%
Output Voltage VOUT2
IOUT2 = 0A TO 17A, VIN = 4.5V to 14V
1.2V ± 1.50%
Nominal Switching Frequency
400kHz
Efficiency
VOUT1 = 1.8V, IOUT1 = 17A, VIN = 12V
88.3% typical
See Figures 3-5
VOUT2 = 1.2V, IOUT2 = 17A, VIN = 12V
85.0% typical
1
LTC3855EUJ
QUICK START PROCEDURE
Demonstration circuit 1441 is easy to set up to evaluate
the performance of the LTC3855EUJ. Refer to Figure 1
for the proper measurement equipment setup and follow the procedure below:
1.
Place jumpers in the following positions:
JP1 RUN1
ON
JP2
RUN2
ON
JP3 MODE
FCM
2.
With power off, connect the input power supply
to VIN and GND. Turn on the power at the input
and increase the input voltage to 4.5V or
higher.
3.
Check for the proper output voltages.
Vout1 = 1.769V to 1.832V
Vout2 = 1.182V to 1.218V
4.
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.
Do not apply load between the VO1+ and
VO1- pins or between the VO2_SNS+ and VO2_SNSpins. These pins are only intended to Kelvin sense the
output voltage across COUT1 and COUT4. Heavy load
currents applied across the VO1+/- sense pins will
damage these sense traces. Heavy load currents across
the VO2_SNS+/- pins will damage the converter.
Note 1.
When measuring the output or input voltage ripple, do not use the 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.
Note 2.
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:
• Tie VOUT1 to VOUT2 by tying together the
exposed copper pads on the VOUT shapes
with pieces of heavy copper foil.
• Tie ITH1 to ITH2 by stuffing 0Ω at R49.
• Tie VFB1 to VFB2 by stuffing 0Ω at R50.
• Tie TRK/SS1 to TRK/SS2 by stuffing 0Ω at
R52.
• Tie RUN1 to RUN2 by stuffing 0Ω at R55.
• Keep the ILIM pins at the same potential or
tie them together by stuffing 0Ω •at R71.
•
•
•
Keep the ITEMP pins at the same potential
or tie them together by stuffing 0Ω •at R67.
Remove the redundant ITH compensation
network, VFB divider and TRACK/SS cap.
Re-compensate if necessary.
2
LTC3855EUJ
INDUCTOR DCR SENSING
Demonstration circuit 1441 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 since the
controller can not detect saturation when DCR sensing
is used. This means using a ferrite with a high saturation current rating or using an iron powder type whose
inductance will drop off much more gradually. Refer to
tables 2 and 3 to see an example of how to setup the
two rails for DCR sensing. The original RSENSE setup is
shown for comparison.
These parameters are used:
•
•
•
•
•
•
•
•
VOUT1 = 1.8V / 17A
VOUT2 = 1.2V / 17A
VIN = 12V
Fsw = 400kHz, typical
L1,2 = Vishay IHLP4040DZ-01 0.56uH
(0.56uH, DCR = 1.7mΩ typ, 1.8mΩ max)
ILIM = FLOATING
No temperature compensation
The DC1441A also has footprints for an NTC compensation network on the ITEMP pins for DCR sensing
which increases the current sense threshold as the inductor temperature increases. As a result, the current
limit falls less with temperature. RN1 detects the temperature for L1, RN2 detects the temperature for L2.
RN3 is used for single output dual phase applications
and is placed next to both inductors. See the data sheet
for more details on NTC compensated DCR sensing.
VOUT2 >= 3.3V
The 1.2V output on phase 2 uses the internal differential
amplifier to sense the output voltage. However, its
common mode range only goes up to 3.5V. Therefore,
for margin, the nominal output voltage should not exceed 3.3V. For outputs of 3.3V and higher, the diff amp
should be bypassed. To bypass the diff amp, follow
these steps:
•
Remove R11 to disconnect the output of the
diff amp from the feedback divider.
•
Stuff R12 with 0 Ω to tie the feedback divider to
VOUT2 (through R65).
•
Remove R70.
•
Stuff R69 with 0 Ω.
3
LTC3855EUJ
Table 2.
CONFIGURATION
DCR Sensing
Discrete RSENSE
(original board)
Table 3.
CONFIGURATION
DCR Sensing
Discrete RSENSE
(original board)
VOUT1 Setup for 1.8V/17A with DCR Sensing and with Discrete Sense Resistors
RSENSE
SENSE
DCR FILTER/DIVIDER
FILTER
FIILTER
RESISTORS
RESISTORS
CAP
TOP
BOTTOM
RS1
Short with
Cu strip or
very short &
thick piece of
wire
2mΩ
2010 pkg
L1
Vishay
SENSE1- TO L1JUMPER
R29, R30
Open
C14
0.1uF
R56
3.09kΩ
R57
100kΩ
R58
0Ω
100Ω
1nF
Open
Open
Open
IHLP4040DZ-01
0.56uH
Iron powder
Vitec 59PR9875
0.4uH ferrite,
Isat = 23A
VOUT2 Setup for 1.2V/17A with DCR Sensing and with Discrete Sense Resistors
RS2
Short with
Cu strip or
very short &
thick piece of
wire
2mΩ
2010 pkg
L2
Vishay
RSENSE
FILTER
RESISTORS
SENSE
FIILTER
CAP
DCR FILTER/DIVIDER
RESISTORS
TOP
BOTTOM
SENSE2- TO L2JUMPER
R40,R39
Open
C15
0.1uF
R59
3.09kΩ
R60
Not
stuffed
R62
0Ω
100Ω
1nF
Open
Open
Open
IHLP4040DZ-01
0.56uH
Iron powder
Vitec 59PR9875
0.4uH ferrite,
Isat = 23A
4
LTC3855EUJ
Vout1 V
+
-
Iin
A
Iout1
+
Vin
supply
Vout1
load
+
A
A
-
Iout2
+
Vout2
load
-
-
V
+
+
Vout2 V
-
Vin
Figure 1. Proper Measurement Equipment Setup
+
COUT
VOUT
-
GND
Figure 2. Measuring Output Voltage Ripple
5
LTC3855EUJ
1.8V / 17A and 1.2V / 17A Efficiency
at VIN = 12V, FSW = 400kHz, mode = FCM
95
Efficiency (%)
90
1.8V
1.2V
85
For each phase
● QT = RJK0305DPB
● QB = RJK0330DPB
● L = Vitec 59PR9875 (0.4uH)
● Rsense = 2mOhms
80
75
0
2
4
6
8
10
12
14
16
18
Load current (Amps)
Figure 3. Typical Efficiency Curves in FCM
VOUT = 1.8V, VIN = 12V, FSW = 400kHz
100
90
80
Efficiency (%)
70
60
FCM
Burst Mode
Pulse Skip
50
40
Parameters
● QT = RJK0305DPB
● QB = RJK0330DPB
● L = Vitec 59PR9875 (0.4uH)
● Rsense = 2mOhms
30
20
10
0
0.01
0.10
1.00
10.00
100.00
Load current (Amps)
Figure 4. Typical Efficiency Curves for the 1.8V rail in FCM, Burst Mode and Pulse Skip Mode.
6
LTC3855EUJ
VOUT = 1.2V, VIN = 12V, FSW = 400kHz
100
90
80
Efficiency (%)
70
60
FCM
Burst Mode
Pulse Skip
50
40
Parameters
● QT = RJK0305DPB
● QB = RJK0330DPB
● L = Vitec 59PR9875 (0.4uH)
● Rsense = 2mOhms
30
20
10
0
0.01
0.10
1.00
10.00
100.00
Load current (Amps)
Figure 5. Typical Efficiency Curves for the 1.2V rail in FCM, Burst Mode and Pulse Skip Mode.
7
R37
PGOOD1
X2
1
PGOOD2
X1
on PCB
Tooling Holes
1
JP3
0.1uF
C47
OPT
1uF
C50
4
1
2
3
R46
100K
1%
3
R40
100
20.0K
R43
S2-
OPT
TRK/SS1 R52
TRK/SS2
ITEMP1
SGND
DIFFP
SENSE2-
SENSE2+
ILIM2
RUN2
OPT
R42
OPT
R69
R67 OPT
ITEMP2
OPT
R71
OPT
OPT
VFB1 R50
ILIM1
RUN1 R55
OPT
VFB2
ITH2
0
R70
1nF
TK/SS2
ITH2
VFB2
SGND
VFB1
ITH1
39
ILIM1
OPT
RN1
ITEMP1
R44
OPT
R30
100
C14
TK/SS1
R11 0
41
10
9
8
7
6
5
RUN2
INTVCC
R39
100
1nF
C15
TRK/SS2
ITH2
VFB2
3
VFB1
4
2
1
ITH1
TRK/SS1
S1+
S1-
R29
100
ITEMP1
U1
LTC3855EUJ
O PT IONAL JUM PERS FOR
SINGLE OUT PUT /DUAL PHASE
OPERAT ION
OPT
R12
OPT
C49
20.0K
R33
20.0K
S2+
40.2K
OPT
R32
R27
OPT
OPT
R10 0
R28
C38
C37
JP1
ITH1 R49
R66
100K
1%
INTVCC
OPT
OPT
OPT
R38
R41
VIN
1.5nF
C48
INTVCC
JP2
R35
5.49K 1%
150pF
C42
C44
C41 1nF
C43 150pF
R31
18.2K
1%
0.1uF
C2
R34
EXTVCC
OPT
1
E16
E4
OPT
R2
EXTVCC
Repres ents
PGOOD2
PGOOD1
E2
E3
0
R3
20.0K
R36 0
VOUT1
E9
E10
GND
EXTVCC
TRK/SS2
TRK/SS1
PLLIN
E1
R68
2
INTVCC
3
RUN1
R7
100K
1%
INTVCC
1
R63
OPT
21
22
23
24
25
26
27
28
CMDSH-3
D2
1uF
C17
2.2
R18
OPT
R45
OPT
R47
OPT
C51
OPT
RN3
INTVCC
4.7uF
10V
C11
4
4
Q3
RJK0305DPB
Q2
RJK0330DPB
4
4
Q4
RJK0330DPB
INTVCC
VIN
INTVCC
4
Q1
RJK0305DPB
CMDSH-3
D1
0.1uF
C20
E15
4
VIN
4
4
Q6
OPT
VIN
D4
OPT
D3
Q8 OPT
OPT
Q7
OPT
OPT
RN2
ITEMP2
OPT
R51
OPT
R53
OPT
C52
OPT
R62
OPT
L2-
OPT
R60
L1-
OPT
R57
COUT1
100uF
6.3V
+ COUT2
330uF
2.5V
+ COUT3
330uF
2.5V
VOUT1
CIN1
270uF
16V
C32
10uF
10V
+
J1
E5
VIN
VIN+
VIN-
GND
J3
VOUT1
VO1+
E11
E6
J2
J4
GND
VO1-
E12
2010
0.002
RS2
R64
10
COUT4
100uF
6.3V
R65
10
COUT5 COUT6
+ 330uF + 330uF
2.5V
2.5V
VOUT2
E13
E14
C36
10uF
10V
J5
VOUT2
NOTE 4,
ONLY APPLY LOAD FROM J3 TO J4 FOR VOUT1
ONLY APPLY LOAD FROM J5 TO J6 FOR VOUT2
DO NOT CONNECT SCOPE GROUND TO E14
2010
0.002
RS1
VIN
1
NOTE 3,
FOR VOUT2 >= 3.3V, REMOVE R11 AND R70. STUFF R12
AND R69 WITH 0 OHM RESISTORS.
0.4uH
L2
R59 OPT
CIN5
10uF
R58
OPT
CIN3
0.4uH
L1
R56 OPT
OPT
10uF
Q5
OPT
CIN4
CIN2
NOTE 2,
TO IMPLEMENT DCR SENSING ON PHASE 2, SHORT RS2.
REMOVE R39 AND R40. STUFF R62 WITH A 0 OHM RESISTOR.
THE DCR SENSE FILTER CONSISTS OF C15, R59 AND R60.
CALCULATE THESE VALUES PER THE DATA SHEET.
OPT
INTVCC
0.1uF
C21
R25
0
0
BG2
EXTVCC
BG1
TG1
R9
29
SW1
CLKOUT
30
R61
SW2
TG2
BOOST2
PGND2
BG2
EXTVCC
INTVCC
VIN
BG1
PGND1
BOOST1
TG1
JP4
4
CLKOUT
TEMPERATURE COMPENSATION NETWORK FOR DCR SENSING
ILIM2
2
OPT
3
1
2
35
NOTE 1,
TO IMPLEMENT DCR SENSING ON PHASE 1, SHORT RS1.
REMOVE R29 AND R30. STUFF R58 WITH A 0 OHM RESISTOR.
THE DCR SENSE FILTER CONSISTS OF C14, R56 AND R57.
CALCULATE THESE VALUES PER THE DATA SHEET.
1
2
3
ITEMP2
40
S E N S E 1D IF F N
11
S E N S E 1+
38
13
D IF F O U T
12
R UN 1
37
2
1
S 1+
R26
2
1
N FF
O O
2
GND
VO2_SNSVO2_SNS+
J6
A
71
/
V
2.
1
1
R UN2
34
FR E Q
36
IT E M P 1
ILIM 1
14
IT E M P 2
ILIM 2
15
M O D E /P LLIN
P G O O D1
16
P G O O D2
17
33
P HS A S MD
NC
18
31
SW1
32
C LK O U T
SW2
19
TG 2
20
5
6
7
8
5
6
7
8
1
2
3
5
6
7
8
1
2
3
5
6
7
8
1
2
3
5
6
7
8
1
2
3
5
6
7
8
1
2
3
1
2
1
1
2
3
5
6
7
8
5
6
7
8
1
2
3
2
N
U
R
2
M
E C
D F
O M
M B
S
P
S 1-
F
F
O
A
71
/
V
8.
1
S 2+
N
O
G
E
D
G
E
D
G
E
D
V
4
1V
5.
4
S 2-
1
N
U
R
06
D
M
S 09
A
H
P 02
1
2
VIN
LTC3855EUJ
8
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