Linear Technology LT8608 User manual

1
DEMO MANUAL DC2521A
UG-1403 Rev. 0
DESCRIPTION
LT8608/LT8608B
42V, 1.5A Micropower
Synchronous Step-Down Regulator
Demonstration circuit 2521A is a 42V, 1.5A micropower
synchronous step-down regulator featuring the LT8608/
LT8608B. There are two assembly versions. DC2521A-A
is for the LT8608 and DC2521A-B is for the LT8608B.
The LT8608/LT8608B is a compact, high efficiency, high
speed synchronous monolithic step-down switching
regulator. Top and bottom power switches, compensa-
tion components and other necessary circuits are inside
of the 2mm × 2mm DFN package to minimize external
components and simplify design. The difference LT8608
and LT8608B is the LT8608 works under low ripple
Burst Mode
®
operation at light load condition, while the
LT8608B has pulse-skipping mode for light load. Due
to the Burst Mode operation, LT8608 consumes only
1.7μA of quiescent current when output is regulated at
5V. LT8608B has higher quiescent current because of its
pulse-skipping mode operation.
All registered trademarks and trademarks are the property of their respective owners.
PERFORMANCE SUMMARY
The demo board is designed for 5V output from a 5.5V
to 42V input. The wide input range allows a variety of
input sources, such as automotive batteries and industrial
supplies.
The demo board has an EMI filter installed. The radiated
EMI performances of the board (with EMI filter) are shown
in Figure 2 and Figure 3. The red lines in Figure 2 and
Figure 3 are CISPR 25 Class 5 peak limit. To use the EMI
filter, the input should be tied to VEMI, not VIN. An induc-
tor L2, which is a 0Ω jumper on the board by default
now, can be added in the EMI filter to further reduce the
conducted emission.
The LT8608 data sheet gives a complete description of
the part, operation and application information. The data
sheet must be read in conjunction with this demo manual
for DC2521A.
Design files for this circuit board are available.
Specifications are at TA= 25°C
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VIN Input Voltage Range 5.5 42 V
VOUT Output Voltage 4.8 5 5.2 V
IOUT Maximum Output Current 1.5 A
fSW Switching Frequency 1.85 2 2.15 MHz
EFE Efficiency at DC VIN = 12V, IOUT = 0.75A 92 %

2
DEMO MANUAL DC2521A
UG-1403 Rev. 0
QUICK START PROCEDURE
DC2521A is easy to set up to evaluate the performance
of the LT8608/LT8608B. Refer to Figure 4 for proper
measurement equipment setup and follow the proce-
durebelow:
NOTE:When measuring the input or output voltage
ripple, care must be taken to avoid a long ground lead
on the oscilloscope probe. See Figure 5 for the proper
scope technique.
1. Set an input power supply that is capable of 42V/1.5A.
Then turn off the supply.
2. With power off, connect the supply to the input termi-
nals VIN and GND.
3. Turn on the power at the input.
NOTE:Make sure that the input voltage never
exceeds42V.
4. Check for the proper output voltage of 5V. Turn off the
power at the input.
5. Once the proper output voltage is established, connect
a variable load capable of sinking 1.5A at 5V to the
output terminals V
OUT
and GND. Set the current for0A.
a. If efficiency measurements are desired, an amme-
ter can be put in series with the output load in
order to measure the DC2521A’s output current.
b. A voltmeter can be placed across the output ter-
minals in order to get an accurate output voltage
measurement.
6. Turn on the power at the input.
NOTE:If there is no output, temporarily disconnect the
load to make sure that the load is not set too high.
7. Once the proper output voltage is established again,
adjust the load and/or input within the operating range
and observe the output voltage regulation, ripple volt-
age, efficiency and other desired parameters.
Figure 2. LT8608/LT8608B Demo Circuit EMI Performance in
CISPR 25 Radiated Emission Test, Antenna Polarization: Vertical
(VIN = 14V, VOUT = 5V, IOUT = 1.5A, 2MHz Switching Frequency)
LOAD CURRENT (A)
0
0.5
1
1.5
50
55
60
65
70
75
80
85
90
95
100
EFFICIENCY (%)
DC2521A F01
LT8608 (Burst Mode OPERATION)
LT8608B (PULSE-SKIPPING MODE)
Figure 1. Efficiency vs Load Current at 12V Input and 2MHz Switching Frequency
Figure 3. LT8608/LT8608B Demo Circuit EMI Performance in
CISPR25 Radiated Emission Test, Antenna Polarization:Horizontal
(VIN = 14V, VOUT = 5V, IOUT = 1.5A, 2MHz Switching Frequency)

3
DEMO MANUAL DC2521A
UG-1403 Rev. 0
QUICK START PROCEDURE
Figure 4. Proper Measurement Equipment Setup
Figure 5. Measuring Output Ripple
V
OUT
GND

4
DEMO MANUAL DC2521A
UG-1403 Rev. 0
PARTS LIST
ITEM QTY REFERENCE PART DESCRIPTION MANUFACTURER/PART NUMBER
Required Circuit Components
1 1 C2 CAP., X7R, 4.7μF, 50V, 10% 1206 MURATA, GRM31CR71H475K
2 1 C4 CAP., X7R, 0.22μF, 16V, 10%, 0603 AVX, 0603YC224KAT2A
3 1 C5 CAP., C0G, 10pF, 25V, 5%, 0603 AVX, 06033A100JAT2A
4 1 C6 CAP., X7R, 47μF, 10V, 10%, 1210 MURATA, GRM32ER71A476KE20L
5 1 C7 CAP., X7R, 1.0μF, 25V, 10%, 0603 MURATA, GRM188R71E105KA12D
6 1 L1 IND., 2.2µH WURTH ELEKTRONIK, 74438336022
7 1 R2 RES., CHIP, 18.2k, 1/10W, 1%, 0603 VISHAY, CRCW060318K2FKEA
8 1 R3 RES., CHIP, 49.9k, 1/10W, 1%, 0603 VISHAY, CRCW060349K9FKEA
9 2 R1, R4 RES., CHIP, 1MEG, 1/10W, 1%, 0603 VISHAY, CRCW06031M00FKEA
10 1 R5 RES., CHIP, 187k, 1/10W, 1%, 0603 VISHAY, CRCW0603187KFKEA
DC2521A-A Version Specific Components
1 1 U1 IC, REGULATOR, DFN-8 ANALOG DEVICES, LT8608EDC#PBF
DC2521A-B Version Specific Components
1 1 U1 IC, REGULATOR, DFN-8 ANALOG DEVICES, LT8608BEDC#PBF
Additional Demo Board Circuit Components
1 1 C1 CAP., ALUM 22μF 63V SUN ELECT, 63CE22BS
2 2 C3, C10 CAP., X7R, 4.7μF, 50V, 10% 1206 MURATA, GRM31CR71H475K
3 1 C8 CAP., X7R, 1μF, 50V, 10%, 0805 TDK, C2012X7R1H105K
4 1 C9 CAP., X7R, 4.7μF, 16V, 10%, 0805 MURATA, GRM21BR71C475K73L
5 2 C11, C12 CAP., X7R, 0.1μF, 50V, 10%, 0603 MURATA, GRM188R71H104KA93D
6 1 FB1 FERRITE BEAD 0805 TDK, MPZ2012S221AT000
7 1 L2 RES., CHIP, 0Ω, 3/4W, 2010 VISHAY, CRCW20100000Z0EF
8 0 R6 (OPT.) RES., 0603
Hardware For Demo Board Only
1 7 E1 − E7 TESTPOINT, TURRET, 0.094" MILL-MAX, 2501-2-00-80-00-00-07-0
2 1 E8 (OPT.) TESTPOINT, TURRET, 0.094"
3 4 MH1 − MH4 STAND-OFF, NYLON 0.50 TALL KEYSTONE, 8833 (SNAP ON)

5
DEMO MANUAL DC2521A
UG-1403 Rev. 0
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog
Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications
subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
SCHEMATIC DIAGRAM
5
5
4
4
3
3
2
2
1
1
DD C C
B B
A A
ALL CAPACITORS ARE 0603.
NOTES: UNLESS OTHERWISE SPECIFIED
1. ALL RESISTORS ARE 0603.
SUNCON 63CE22BS
(2MHz)
*
ASSEMBLY TYPE
VERSION
DC2521A-A
DC2521A-B
LT8608EDC
U1
LT8608BEDC
VCC
C12 0.1µF 50V
R6 (Opt)
C5
10pF
R5 187k
C10 4.7µF 50V 1206
C6
47µF 10V 1210
R1 1.00M
R4
1.00M
E6
EN/UV
U1 *
VIN
7
EN
8
VCC
3
RT
4
FB 5
PG 6
SW 2
BST 1
GND
9
+
C1 22µF 63V
C11 0.1µF 50V
C7
1µF
C3 4.7µF 50V 1206
E4
GND
C8 1µF
50V
0805
L1
2.2µH
74438336022
E8
VIN
5.5V - 42V
R2
18.2k
FB1
MPZ2012S221AT000
E2
GND
C4
0.22µF
L2
0 2010
E7 PG
E3
VOUT
5V / 1.5A
R3 49.9k
E5
GND
C2 4.7µF 50V 1206
E1
VEMI
5.5V - 42V
C9 4.7µF 16V 0805
VOUT

6
DEMO MANUAL DC2521A
UG-1403 Rev. 0
 ANALOG DEVICES, INC. 2018
UG17098-0-9/18(0)
www.analog.com
ESD Caution
ESD (electrostatic discharge) sensitive device. Charged devices and circuit boards can discharge without detection. Although this product features patented or proprietary protection
circuitry, damage may occur on devices subjected to high energy ESD. Therefore, proper ESD precautions should be taken to avoid performance degradation or loss of functionality.
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