DC187A - Demo Manual

DEMO MANUAL DC187/DC188
NO DESIGN
SWITCHER
DEMO MANUAL
DC187/DC188
DESCRIPTIO
LT1374 Monolithic 4A Switcher
5.5V to 25V Input
3.3V or 5V Output
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Demonstration circuits DC187/DC188 are complete
DC/DC step-down regulators using the LT ®1374, constant
frequency, high efficiency converter in 7-pin DD (DC187)
and SO-8 (DC188) packages. These circuits are primarily
used in personal computers, disk drives, portable hand-
held devices and in larger systems, as local onboard
regulators. High frequency switching allows the use of
small inductors, making these all surface mount solutions
ideal for space-conscious systems.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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PERFORmANCE SU
ARY
TA = 25°C, VIN = 10V, ILOAD = 2A, VOUT = 3.3V (Jumper J1 inserted), SHDN pin open unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Output Voltage
Jumper J1 Removed (Note 1)
Jumper J1 Inserted
4.91
3.23
5.02
3.30
5.20
3.42
V
V
Maximum ILOAD
(Note 2)
4
Input Voltage Range
(Note 3)
5.5
Switching Frequency
460
Output Ripple Voltage
A
25
500
V
540
kHz
60
mVP-P
8
mV
Line Regulation
5.5V to 25V
Load Regulation
ILOAD = 10mA to 4A
SHDN Lockout Threshold
Shutdown Option Boards
2.3
2.38
2.46
V
SHDN Shutdown Threshold
Shutdown Option Boards
0.15
0.37
0.6
V
Synchronization Range
SYNC Option Boards
580
Supply Current
SHDN = 0V
Note 1: Output voltage variations include the ±1% tolerance of the
feedback-divider network. For tighter voltage range, use higher tolerance
resistors or a fixed 5V output device, the LT1374-5.
5
mV
1000
20
kHz
µA
Note 2: For DC188 additional thermal restrictions apply.
Note 3: For operating voltages down to 4V consult LTC Marketing for
details on the LT1506.
BOARD PHOTOS
DC187
DC188
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DEMO MANUAL DC187/DC188
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TYPICAL PERFOR A CE CHARACTERISTICS
Temperature Rise
vs Load Current
90
90
DIE TEMPERATURE RISE (°C)
EFFICIENCY (%)
86
84
82
80
78
100
VIN = 10V
VOUT = 5V
80
88
Temperature Rise vs Time,
DC188
DC188
DIE TEMPERATURE RISE (°C)
10VIN, 5VOUT Efficiency
70
60
DC187
50
40
30
20
VIN = 10V
VOUT = 5V
IOUT = 4A
80
60
IOUT = 3A
40
IOUT = 2A
20
IOUT = 1A
10
76
0
0
1
2
4
3
0
2
1
3
0
4
20
10
TIME (SECONDS)
0
LOAD CURRENT (A)
LOAD CURRENT (A)
187/88 TA02
187/88 TA01
187/88 TA03
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PACKAGE A D SCHE ATIC DIAGRA SM
DC187
D2
MMBD914LT1
E1
VIN
5.5V TO 25V
+
C3
10µF
50V
2*
BOOST
SW
VIN
U1
LT1374CR
SHDN
GND
E3
GND
4
L1
6.8µH
+
6
5
E2
S/D
C4
0.33µF
16V
D3
OPTIONAL
FB
3
D1
MBRD835L
+
C7
OPTIONAL
R2
5.36k
1%
R4
2.74k
1%
+
C6
0.47µF
25V
DC187 SD
R3
4.99k
1%
VC
1
C1
1500pF
50V
FRONT VIEW
TAB
IS
GND
7
6
5
4
3
2
1
R PACKAGE
7-LEAD PLASTIC DD PAK
LT1374CR (LT1374CR-SYNC)
2
E5
VOUT
3.3V/4A OR
5V/4A
E4
GND
J1
3.3V/5V
LINK
7
R1
OPTIONAL
C2
OPTIONAL
*SYNC FUNCTION REPLACES SHDN
FOR LT1374CR-SYNC PARTS
C5
100µF
10V
30
FB
BOOST
VIN
GND
VSW
SHDN (SYNC)
VC
DEMO MANUAL DC187/DC188
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PACKAGE A D SCHE ATIC DIAGRA SM
DC188
D2
MMBD914LT1
C4
0.33µF
16V
D3
OPTIONAL
E1
VIN
5.5V TO 25V
+
C3
10µF
50V
7*
E2
S/D
+
2
1
BOOST
U1 SW
LT1374CS8
BIAS
VIN
SHDN
GND
FB
8
D1
MBRD835L
C5
100µF
10V
+
C7
OPTIONAL
R2
5.36k
1%
R4
2.74k
1%
+
5
3
R3
4.99k
1%
6
R1
OPTIONAL
C2
OPTIONAL
E5
VOUT
3.3V/4A OR
5V/4A
C6
0.47µF
25V
E4
GND
J1
3.3V/5V
LINK
VC
4
E3
GND
L1
6.8µH
DC188 SD
TOP VIEW
VIN 1
8 VSW
BOOST 2
C1
1500pF
50V
*SYNC FUNCTION REPLACES SHDN
FOR LT1374CS8-SYNC PARTS
7 SHDN (SYNC)
FB 3
6 VC
FGND 4
5 BIAS
S8 PACKAGE
8-LEAD PLASTIC SO
LT1374CS8 (LT1374CS8-SYNC)
PARTS LIST
REFERENCE
DESIGNATOR
QUANTITY
PART NUMBER
DESCRIPTION
VENDOR
08055C152MAT2S
1500pF 50V X7R Chip Capacitor
AVX
(803) 946-0362
10µF 50V Y5U Chip Capacitor
Marcon
(847) 696-2000
0.33µF 16V X7R Chip Capacitor
AVX
(803) 946-0362
100µF 10V TPS Tantalum Capacitor
AVX
(207) 282-5111
0.47µF 25V Y5V Chip Capacitor
AVX
(803) 946-0362
MBRD835L
SMT Diode
Motorola
(602) 244-3576
MMBD914LT1
Diode
Motorola
(602) 244-3576
C1
1
C2, C7
2
C3
1
THCR60E1H106ZT
C4
1
0805YC334MAT2S
C5
1
TPSD107M010R0100
C6
1
12063G474MAT3S
D1
1
D2
1
TELEPHONE
Optional Capacitor
D3
1
E1 to E5
5
2501-2
Turret
Optional Diode
Mill-Max
(516) 922-6000
J1
1
2802S-02-G2
0.079" Center 2-Pin Header
Comm Con
(818) 301-4200
P1
1
CCIJ2MM-138-G
0.079" Center 2-Pin Shunt
Comm Con
(818) 301-4200
L1
1
DO3316P-682
6.8µH 20% Inductor
Coilcraft
(847) 639-1469
R1
1
R2
1
CR10-5361F-T
5.36k 1/8W 1% Chip Resistor
TAD
(714) 255-9123
R3
1
CR10-4991F-T
4.99k 1/8W 1% Chip Resistor
TAD
(714) 255-9123
R4
1
CR10-2741F-T
2.74k 1/8W 1% Chip Resistor
TAD
(714) 255-9123
U1 (DC187)
1
LT1374CR (-SYNC)
IC
LTC
(408) 432-1900
U1 (DC188)
1
LT1374CS8 (-SYNC)
IC
LTC
(408) 432-1900
Optional Resistor
3
DEMO MANUAL DC187/DC188
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OPERATIO
DC187 vs DC188 (Temperature vs Package Size)
The DC187 and DC188 demonstration boards are
intended for evaluation of the LT1374 switching regulator
in the 7-pin DD and SO-8 packages, respectively. The
boards are electrically identical; they differ only in package
layout (the BIAS pin is not available in the DD package).
The primary reason for choosing the SO-8 over the DD
package is board space. The DC187 (DD package) occupies an active board area of approximately 0.75 square
inches. Optimizing the DC188 board, by using a Sumida
coil and removing the layout options and voltage-selector
jumper, a total active area of 0.4 square inches is possible.
The DD package is more suitable for higher power or
higher ambient-temperature applications. Although both
boards will supply 4A of output current, the DC188 must
be thermally derated to 3A continuous at 40°C ambient to
prevent excessive die temperatures. The DC187 can run at
70°C ambient at 4A output current. The SO-8 package can,
however, be used for dynamic loads up to the full rated
switch current. The Temperature Rise vs Time, DC188
graph shows the dynamic thermal response of the DC188
board for loads up to 4A.
Shutdown Option
(Device Markings LT1374CR or 1374)
For normal operation, the S/D pin can be left floating. S/D
has two output-disable modes, lockout and shutdown.
When the pin is taken below the lockout threshold, switching is disabled. This is typically used for input undervoltage lockout. Grounding the S/D pin places the LT1374 in
shutdown mode. This reduces total board supply current
to 20µA.
Synchronization Option
(Device Markings LT1374CR-SYNC or 1374SN)
For normal operation the S/D pin can be left floating. To
synchronize switching to an external clock, apply a logiclevel signal to the S/D pin. Amplitude must be from a
logical low to greater than 2.2V with a duty cycle from 10%
to 90%. Synchronization frequency must be greater than
that of the free-running oscillator and less than 1MHz.
Additional circuitry may be required to prevent subharmonic oscillation—refer to the data sheet for more details.
COMPONENTS
LT1374 Operation
Inductor L1
The LT1374 data sheet gives a complete description of the
part, operation and applications information. The data
sheet must be read in conjunction with this demo manual.
The inductor is a Coilcraft DO3316P-682, a 6.8µH
unshielded ferrite unit. It is selected for its low cost, small
size and 4.6A ISAT rating. The equivalent Coiltronics UP26R8 unit can be substituted. If board space is at a premium
and higher ripple current is acceptable, DC188 has the
solder pads available for the Sumida CD43-1R8 inductor.
This 1.8µH unit has a 2.9A ISAT rating. With this coil, ripple
at 10VIN, 5VOUT is ±1.5A. This gives a maximum output
current of (4.5A – 1.5A) = 3A. At input voltages above 15V,
C7 should be inserted in parallel with C5 to increase output
capacitor ripple rating.
Hook-Up
Solid turret terminals are provided for easy connection to
supplies and test equipment. The jumper should be
inserted if a 3.3V output is required. It should be removed
for 5V operation. Connect a 0V to 25V, 4.5A power supply
across the VIN and GND terminals and the load across the
VOUT and GND terminals. When measuring load/line regulation, remember to Kelvin connect to the turrets. Also,
when measuring output ripple voltage with an oscilloscope probe, the wire from the probe to the ground clip will
act as an antenna, picking up noise. For improved results,
the ground clip should be removed from the probe. The tip
should be touched against the output turret, with the bare
ground shield pressed against the ground turret. This
reduces the noise seen on the waveform.
4
Input/Output Capacitors C3, C5, C6 and C7
The input capacitor, C3, is a Tokin ceramic capacitor. It
was selected for its small size, high voltage rating and low
ESR (effective series resistance). The input ripple current
for a buck converter is high, typically IOUT/2. Tantalum
capacitors become resistive at higher frequencies, requiring careful ripple-rating selection to prevent excessive
DEMO MANUAL DC187/DC188
U
OPERATIO
heating. Ceramic capacitors’ ESL (effective series inductance) tends to dominate their ESR, making them less
susceptible to ripple-induced heating. The output capacitor, C5, is an AVX tantalum capacitor. A ceramic is not
recommended as the main output capacitor since loop
stability relies on a resistive characteristic at higher frequencies to form a zero. The AVX TPS series, specifically
designed for use in switch mode power supplies, has very
low ESR. At switching frequencies, ripple voltage is more
a function of ESR than of absolute capacitance value. If
lower output ripple voltage is required, use the optional
capacitor, C7, to reduce ESR, rather than increasing the
capacitance of C5. For very low ripple, an additional LC
filter on the output may be a cheaper solution. The output
contains very narrow voltage spikes because of the parasitic inductance of C5. A small ceramic capacitor, C6,
removes these spikes on the demo board. In application
circuits, trace inductance and local bypass capacitors may
perform this function, negating the need for C6.
Catch Diode D1
Use diodes designed for switching applications, such as
Schottky or ultrafast diodes, with adequate current rating
and fast turn-on times. In selecting a diode, the basic
parameters of interest are forward voltage, maximum
reverse voltage, average operating current and peak current. Lower forward voltage yields higher circuit efficiency
and lowers power dissipation in the diode. The MBRD835L
has a maximum forward drop of 0.4V at 3A. The reverse
voltage rating must be greater than the input voltage.
Average diode current is always less than output current,
but under a shorted output condition, diode current can
equal the switch current limit. If the application must
withstand this condition, the diode must be rated for
maximum switch current.
Compensation: C1, C2 and R1
A detailed discussion of frequency compensation can be
found in the LT1374 data sheet. C1, a 1500pF capacitor
from VC to ground, gives a stable loop response over a
wide range of input and output conditions. Options R1 and
C2 are included to optimize the dynamic response for
specific applications.
Boost: D2, D3 and C4
A boost voltage of at least 2.8V is required throughout the
on time of the switch to guarantee that it remains saturated. For output voltages of 3.3V or more, diode D2
provides sufficient boost voltage to C4. Below 3.3V, D2
can be moved to position D3, powering boost from VIN.
PCB LAYOUT
In many cases, the layout of the demonstration board may
be dropped directly into the application with minimal
changes. If this is not practical, there are several precautions that must be taken when laying out high frequency
converter circuits. The high frequency switching path runs
from ground, through C3 to the VIN pin of the LT1374, out
of the SW pin, through D1 and back to ground. This loop
acts as an antenna and will radiate noise if not kept as short
as possible. Also, at higher switching currents the associated trace inductance can cause excessive voltage spikes
across the switch. The use of a ground plane will reduce
many noise problems. The ground pin of the LT1374
contains some high frequency signal currents, but more
importantly, it is the 0V reference for the output voltage.
Connect the ground pin directly to the ground plane. The
FB and VC components should be kept away from the
power components as much as possible. The ground for
these components should be separated from power
grounds. Run a Kelvin sense to VOUT as required, but keep
the divider network close to the LT1374 to prevent noise
pick-up on the FB node. Noise pickup on the VC pin
appears as various problems, including poor load regulation, subharmonic oscillation and instability. Thermal management must also be considered. The SO-8 package has
a fused ground pin. Soldering this pin to a large copper
area will significantly reduce its thermal resistance. Solder
filled feedthroughs close to the ground pin provide a good
thermal path to the ground plane. For the DD package, the
grounded tab should be treated in the same way. For more
information or advice, contact the LTC Applications department.
5
DEMO MANUAL DC187/DC188
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PCB LAYOUT A D FIL
DC187
6
Component Side Silkscreen
Component Side
Component Side Solder Mask
Solder Side
Solder Side Solder Mask
Pastemask
DEMO MANUAL DC187/DC188
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PCB LAYOUT A D FIL
DC188
Component Side Silkscreen
Component Side
Component Side Solder Mask
Solder Side
Solder Side Solder Mask
Pastemask
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
7
DEMO MANUAL DC187/DC188
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PC FAB DRAWI GS
DC187
2.000
C
D
A
D
B
A
A
2.000
NOTES: UNLESS OTHERWISE SPECIFIED
1. MATERIAL: FR4 OR EQUIVALENT EPOXY,
2 OZ COPPER CLAD, THICKNESS 0.062 ±0.006
TOTAL OF 2 LAYERS
2. FINISH: ALL PLATED HOLES 0.001 MIN/0.0015 MAX
COPPER PLATE, ELECTRODEPOSITED TIN-LEAD COMPOSITION
BEFORE REFLOW, SOLDER MASK OVER BARE COPPER (SMOBC)
3. SOLDER MASK: BOTH SIDES USING GREEN SR1020 OR EQUIVALENT
4. SILKSCREEN: USING WHITE NONCONDUCTIVE EPOXY INK
5. ALL DIMENSIONS IN INCHES
A
SYMBOL
DIAMETER
NUMBER
OF HOLES
A
0.020
29
B
0.037
2
C
0.072
2
D
0.095
5
TOTAL HOLES
38
C
187 FD
DC188
2.000
D
E
E
A
A
C
2.000
NOTES: UNLESS OTHERWISE SPECIFIED
1. MATERIAL: FR4 OR EQUIVALENT EPOXY,
2 OZ COPPER CLAD, THICKNESS 0.062 ±0.006
TOTAL OF 2 LAYERS
2. FINISH: ALL PLATED HOLES 0.001 MIN/0.0015 MAX
COPPER PLATE, ELECTRODEPOSITED TIN-LEAD COMPOSITION
BEFORE REFLOW, SOLDER MASK OVER BARE COPPER (SMOBC)
3. SOLDER MASK: BOTH SIDES USING GREEN SR1020 OR EQUIVALENT
4. SILKSCREEN: USING WHITE NONCONDUCTIVE EPOXY INK
5. ALL DIMENSIONS IN INCHES
SYMBOL
DIAMETER
NUMBER
OF HOLES
A
0.020
38
B
0.037
2
C
0.025
5
D
0.072
2
E
0.095
5
TOTAL HOLES
38
B
C
C
D
8
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417● (408) 432-1900
FAX: (408) 434-0507● TELEX: 499-3977 ● www.linear-tech.com
188 FD
dc1878 LT/TP 0498 500 • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 1998
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