DEMO MANUAL DC1924A
LT8705
80V V
IN
and V
OUT
Synchronous
4-Switch Buck-Boost DC/DC
Controller
DESCRIPTION
Demonstration circuit 1924A is a high performance buck-
boost converter featuring the
LT
®
8705
that can operate
from input voltages above, below or equal to the output
voltage. The demo board input range is 36V to 80V. The
output is optimized for 48V at 5A, with the output current
limit set at 7A. (The circuit will operate with lower input
voltage than 36V if load current is reduced).
The controller has integrated input current, input voltage,
output current and output voltage regulators. The one
regulator that wants to decrease current gets control over
the compensation pin VC. The inductor current is controlled
by the VC signal that is fed into a current comparator
together with a ramp compensation signal.
While the current mode control limits the inductor current
both in normal and in reverse direction, these current
limits have some variation as input voltage changes. The
input and output current regulators offer more accurate
current limits.
The input voltage regulator is typically used in applica-
tions with solar panels or other high impedance power
sources, and will reduce the current if the input voltage
drops below the set point.
The operating mode of the controller is determined
through the MODE pin (jumper JP1) and can be set to
discontinuous mode, forced continuous mode and Burst
Mode
®
operation.
The LT8705 is capable of bidirectional operation when
operating in forced continuous mode. Additional circuitry
may be needed depending on the application.
The CLKOUT output and the SYNC input can be used
to synchronize two DC1924A circuits with 180 degree
phase shift.
By feeding the LT8705 from a separate low voltage sup-
ply, the power dissipation can be reduced. To supply the
LT8705 chip from an external voltage supply (> 6.4V), cut
the trace as marked on the board to disconnect the EXTVCC
pin from V
OUT
. The LT8705 will start when voltage is ap-
plied at the input (VIN pin), and when it is running it will
draw current from the EXTVCC pin if the voltage is > 6.4V.
Typical efficiency with 5A load is above 97% across a 36V
to 72V input range using the supplied inductor. Lower
core loss can be achieved by using a ferrite core inductor.
The LT8705 data sheet gives a complete description of
the part, operation and application information. The data
sheet should be read in conjunction with this quick start
guide for demo circuit 1924A. The input voltage range of
the LT8705 itself is 2.8V (need EXTVCC > 6.4V) to 80V
and the output range is 1.3V to 80V.
The LT8705EUHF is assembled in a 38-Lead (5mm ×
7mm) plastic QFN package with a thermal pad underneath
the chip. Proper board layout is essential for maximum
thermal and electrical performance. See the data sheet
section Circuit Board Layout Checklist.
Design files for this circuit board are available at
http://www.linear.com/demo/DC1924A
L,
LT, LTC, LTM, Linear Technology Burst Mode and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
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DEMO MANUAL DC1924A
PERFORMANCE SUMMARY
SYMBOL
V
IN
V
OUT
I
IN
I
OUT
f
SW
EFF
PARAMETER
Input Supply Range
Output Voltage
Maximum Load Current
Output Current Limit
Switching Frequency
Efficiency at DC Input
V
IN
= 36V, V
OUT
= 48V, I
OUT
= 5A
V
IN
= 48V, V
OUT
= 48V, I
OUT
= 5A
V
IN
= 72V, V
OUT
= 48V, I
OUT
= 5A
Specifications are at T
A
= 25°C
CONDITIONS
Note: Output Power Is Limited When V
IN
< 30V
MIN
36
TYP
48
48
5
7
200
97.5
97.2
97.5
MAX
80
UNITS
V
V
A
A
kHz
%
%
%
100
99
98
97
EFFICIENCY (%)
96
95
94
93
12
11
10
9
EFFICIENCY (%)
8
7
6
5
DISSIPATION (W)
100
95
90
85
80
75
70
65
60
55
50
45
40
0.01
0.1
1
LOAD CURRENT (A)
EFFICIENCY
EFFICIENCY FERRITE
POWER LOSS
POWER LOSS FERRITE
10
6
5
4
3
2
1
0
POWER LOSS (W)
92
91
90
89
88
30
36
42
48
54 60
V
IN
(V)
66
72
78
4
3
2
1
0
EFFICIENCY
EFFICIENCY FERRITE
DISSIPATION
DISSIPATION FERRITE
DC1924a F01a
DC1924a F01b
Figure 1a. Efficiency and Power Loss as Function of Input
Voltage at 5A Load. Dotted Lines Show Increased Efficiency
When the Inductor Is Replaced by a Ferrite Core Inductor
(Coiltronics SER2918H-223KL)
Figure 1b. Efficiency and Power Loss in CCM Buck Mode.
V
IN
= 56V. Dotted Lines Show Increased Efficiency When
the Inductor Is Replaced by a Ferrite Core Inductor
(Coiltronics SER2918H-223KL)
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DEMO MANUAL DC1924A
QUICK START PROCEDURE
1. Demonstration circuit 1924A is easy to set up to evalu-
ate the performance of the LT8705. Refer to Figure 2
for proper measurement equipment setup and follow
the procedure below.
2. With power off, connect the input power supply to V
IN
(TP1) and GND (TP3).
3. Connect the SHDN terminal to ground with a clip-on
lead to disable the board.
4. Apply 48V to the input. The power source must have
greater than 12A capability if you want to evaluate the
board with full load over the input range.
5. Remove the clip-on lead from SHDN to enable the board.
6. Note that the demo circuit will be enabled at V
IN
> 5.6V,
when V
IN
is rising. If operation at very low input volt-
age is not wanted, increase the value of R11 to set the
undervoltage shutdown at the wanted level.
7. Once the proper output voltage is established, adjust the
load and the input voltage within the operating range
and observe the output voltage regulation, ripple voltage
and efficiency and other parameters as needed. When
measuring input/output voltages, measure at the input/
output terminals of the board to avoid measurement
error caused by voltage drops in cables.
8. To measure input/output voltage ripple, avoid a long
ground lead on the oscilloscope probe, as it may pick
up switching noise. A commonly accepted method is to
remove the oscilloscope probe end cap and ground lead
and set the 20MHz bandwidth limit on the oscilloscope.
Measure the input/output voltage ripple by touching the
probe tip directly to the positive terminal of the input or
output capacitor. Connect the probe ground terminal to
the board’s GND plane near the capacitor with a very
short wire.
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DEMO MANUAL DC1924A
QUICK START PROCEDURE
Figure 2. Test Setup
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DEMO MANUAL DC1924A
PARTS LIST
ITEM
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
QTY
2
2
8
2
4
3
1
1
1
1
2
4
1
2
2
2
1
1
1
1
2
4
1
4
1
1
1
1
1
1
REFERENCE
C13, C47
C14, C15
C41-C44, C48, C49-C51
C46, C65
C45, C52, C55, C64
C56, C57, C58
C59
C61
C62
C63
D1, D2
D3, D4, D5, D6
L1
M1, M2
M3, M4
R8, R9
R11
R13
R14
R16
R27, R46
R38, R41, R51, R58
R40
R42, R43, R44, R45
R47
R49
R50
R57
R62
U1
PART DESCRIPTION
CAP CERM 0.22µF 10% 16V X5R 0603
CAP CER 1000pF 16V 20% X7R 0603
CAP CER 4.7µF 100V X7S 1812
CAP CER 1.0µF 100V X7S 0805
CAP ALUM 220µF 100V 20% RADIAL
CAP CER 4.7µF 16V 10% X5R 0603
CAP CER 1µF 16V 10% X7R 0603
CAP CER 100nF 16V 10% X7R 0603
CAP CER 220pF 25V 5% NP0 0603
CAP CER 3300pF 25V 5% NP0 0603
RECTIFIERS ULTRA FAST RECTIFIER SINGLE
LED SMARTLED GREEN 570NM 0603
INDUCTOR POWER 22µH 11.0A SMD
MOSFET N-CH 80V 55A TDSON-8
MOSFET N-CH 60V 19A TDSON-8
RES 10.0Ω 1/10W 1% 0603 SMD
RES 71.5k 1/10W 1% 0603 SMD
RES 392k 1/10W 0.1% 0603 SMD
RES 20.0k 1/10W 1% 0603 SMD
RES 10.0k 1/10W 0.1% 0603 SMD
RES 100k 1/10W 1% 0603 SMD
RES 2.00Ω 1/10W 1% 0603 SMD
RES 0.01Ω 3W 2512 5% SMD
RES 549Ω 1/10W 1% 0603 SMD
RES 210k 1/10W 1% 0603 SMD
RES 24.3k 1/10W 1% 0603 SMD
RES 56.2k 1/10W 1% 0603 SMD
RES 0.007Ω 1W 1% 2512 SMD
RES 4.02Ω 1/10W 1% 0603 SMD
LT8705 SYNCHRONOUS 4 SWITCH BUCK-BOOST
DC/DC CONTROLLER
MANUFACTURER/PART NUMBER
TAIYO YUDEN EMK107BJ224KA-T
AVX CORPORATION 0603YC102MAT4A
TDK C4532X7S2A475M
TDK C2012X7S2A105K
UCC EKY-101ELL221MK25S
TAIYO YUDEN, EMK107ABJ475KA-T
TDK C1608X7R1C105K
TDK C1608X7R1C104K
NIC NMC0603NPO221J50TRPF
KEMET C0603C332J3GAC
CENTRAL SEMI CMMR1U-02 TR
LG L29K-G2J1-24-Z
WÜRTH ELECTRONICS 74435572200
INFINEON BSC123N08NS3 G
INFINEON BSC039N06NS
VISHAY CRCW060310R0FKEA
PANASONIC ERJ-3EKF7152V
VISHAY MCT06030D3923BP100
VISHAY CRCW060320K0FKEA
PANASONIC ERA-3AEB103V
VISHAY CRCW0603100KFKEA
VISHAY CRCW06032R00FKEA
TT ELECTRONICS LRF3WLF-01-R010-J
VISHAY CRCW0603549RFKEA
VISHAY CRCW0603210KFKEA
VISHAY CRCW060324K3FKEA
VISHAY CRCW060356K2FKEA
VISHAY CRCW25127L000FEA
VISHAY CRCW06034R02FKEA
LINEAR TECH LT8705EUHF#PBF
Required Circuit Components
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