1
DEMO MANUAL DC187/DC188
DEMO MANUAL DC187/DC188
LT1374 Monolithic 4A Switcher
5.5V to 25V Input
3.3V or 5V Output
NO DESIGN SWITCHER
DESCRIPTIO
U
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-
, LTC and LT are registered trademarks of Linear Technology Corporation.
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.
DC187 DC188
BOARD PHOTOS
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.
PERFORmANCE SU ARY
UW
WW
T
A
= 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) 4.91 5.02 5.20 V
Jumper J1 Inserted 3.23 3.30 3.42 V
Maximum I
LOAD
(Note 2) 4 A
Input Voltage Range (Note 3) 5.5 25 V
Switching Frequency 460 500 540 kHz
Output Ripple Voltage 60 mV
P-P
Line Regulation 5.5V to 25V 8 mV
Load Regulation I
LOAD
= 10mA to 4A 5 mV
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 1000 kHz
Supply Current SHDN = 0V 20 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.
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DEMO MANUAL DC187/DC188
TYPICAL PERFOR A CE CHARACTERISTICS
UW
LOAD CURRENT (A)
0
84
86
90
3
187/88 TA01
82
80
12 4
78
76
88
EFFICIENCY (%)
10VIN, 5VOUT Efficiency
TIME (SECONDS)
0
DIE TEMPERATURE RISE (°C)
40
60
187/88 TA03
20
010 20 30
100 V
IN
= 10V
V
OUT
= 5V I
OUT
= 4A
I
OUT
= 3A
I
OUT
= 2A
I
OUT
= 1A
80
LOAD CURRENT (A)
0
DIE TEMPERATURE RISE (°C)
50
60
70 DC188
DC187
4
187/88 TA02
40
30
0123
20
10
90
80
V
IN
= 10V
V
OUT
= 5V
PACKAGE A D SCHE ATIC DIAGRA SM
W UW
DC187
Temperature Rise vs Time,
DC188
Temperature Rise
vs Load Current
+
BOOST
D3
OPTIONAL
E1
V
IN
5.5V TO 25V
E2
S/D
E3
GND
D1
MBRD835L
D2
MMBD914LT1
C4
0.33µF
16V L1
6.8µH
J1
3.3V/5V
LINK
V
IN
*SYNC FUNCTION REPLACES SHDN
FOR LT1374CR-SYNC PARTS
6
3
7
5
2*
14
U1
LT1374CR
SHDN
C3
10µF
50V
+
C5
100µF
10V
C1
1500pF
50V
+
C6
0.47µF
25V
R2
5.36k
1%
R1
OPTIONAL
C2
OPTIONAL
R4
2.74k
1%
R3
4.99k
1%
+
C7
OPTIONAL
E5
V
OUT
3.3V/4A OR
5V/4A
E4
GND
DC187 SD
SW
FB
GND V
C
FB
BOOST
V
IN
GND
V
C
R PACKAGE
7-LEAD PLASTIC DD PAK
FRONT VIEW
TAB
IS
GND
7
6
5
4
3
2
1
V
SW
SHDN (SYNC)
LT1374CR (LT1374CR-SYNC)
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DEMO MANUAL DC187/DC188
PACKAGE A D SCHE ATIC DIAGRA SM
W UW
DC188
REFERENCE
DESIGNATOR QUANTITY PART NUMBER DESCRIPTION VENDOR TELEPHONE
C1 1 08055C152MAT2S 1500pF 50V X7R Chip Capacitor AVX (803) 946-0362
C2, C7 2 Optional Capacitor
C3 1 THCR60E1H106ZT 10µF 50V Y5U Chip Capacitor Marcon (847) 696-2000
C4 1 0805YC334MAT2S 0.33µF 16V X7R Chip Capacitor AVX (803) 946-0362
C5 1 TPSD107M010R0100 100µF 10V TPS Tantalum Capacitor AVX (207) 282-5111
C6 1 12063G474MAT3S 0.47µF 25V Y5V Chip Capacitor AVX (803) 946-0362
D1 1 MBRD835L SMT Diode Motorola (602) 244-3576
D2 1 MMBD914LT1 Diode Motorola (602) 244-3576
D3 1 Optional Diode
E1 to E5 5 2501-2 Turret 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 Optional Resistor
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
PARTS LIST
+
BOOST
D3
OPTIONAL
E1
V
IN
5.5V TO 25V
E2
S/D
E3
GND
D1
MBRD835L
D2
MMBD914LT1
C4
0.33µF
16V L1
6.8µH
J1
3.3V/5V
LINK
V
IN
2
8
5
3
1
7*
64
U1
LT1374CS8
SHDN
C3
10µF
50V
+
C5
100µF
10V
C1
1500pF
50V
+
C6
0.47µF
25V
R2
5.36k
1%
R1
OPTIONAL
C2
OPTIONAL
R4
2.74k
1%
R3
4.99k
1%
+
C7
OPTIONAL
E5
V
OUT
3.3V/4A OR
5V/4A
E4
GND
DC188 SD
SW
BIAS
FB
GND V
C
*SYNC FUNCTION REPLACES SHDN
FOR LT1374CS8-SYNC PARTS
1
2
3
4
8
7
6
5
TOP VIEW
S8 PACKAGE
8-LEAD PLASTIC SO
VIN
BOOST
FB
FGND
VSW
SHDN (SYNC)
VC
BIAS
LT1374CS8 (LT1374CS8-SYNC)
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DEMO MANUAL DC187/DC188
OPERATIO
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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) occu-
pies 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.
LT1374 Operation
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.
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 V
IN
and GND terminals and the load across the
V
OUT
and GND terminals. When measuring load/line regu-
lation, remember to Kelvin connect to the turrets. Also,
when measuring output ripple voltage with an oscillo-
scope 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.
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, switch-
ing is disabled. This is typically used for input undervolt-
age 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 logic-
level 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 subhar-
monic oscillation—refer to the data sheet for more details.
COMPONENTS
Inductor L1
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 I
SAT
rating. The equivalent Coiltronics UP2-
6R8 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 I
SAT
rating. With this coil, ripple
at 10V
IN
, 5V
OUT
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.
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 I
OUT
/2. Tantalum
capacitors become resistive at higher frequencies, requir-
ing careful ripple-rating selection to prevent excessive
5
DEMO MANUAL DC187/DC188
OPERATIO
U
heating. Ceramic capacitors’ ESL (effective series induc-
tance) tends to dominate their ESR, making them less
susceptible to ripple-induced heating. The output capaci-
tor, 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 fre-
quencies 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 para-
sitic 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 cur-
rent. 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 V
C
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 satu-
rated. 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 V
IN
.
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 precau-
tions that must be taken when laying out high frequency
converter circuits. The high frequency switching path runs
from ground, through C3 to the V
IN
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 associ-
ated 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 V
C
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 V
OUT
as required, but keep
the divider network close to the LT1374 to prevent noise
pick-up on the FB node. Noise pickup on the V
C
pin
appears as various problems, including poor load regula-
tion, subharmonic oscillation and instability. Thermal man-
agement 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 de-
partment.
6
DEMO MANUAL DC187/DC188
PCB LAYOUT A D FIL
UW
Component Side Solder Mask
Component Side Silkscreen Component Side
Solder Side
Solder Side Solder Mask Pastemask
DC187
7
DEMO MANUAL DC187/DC188
PCB LAYOUT A D FIL
UW
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 represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
Component Side Solder Mask
Component Side Silkscreen Component Side
Solder Side
Solder Side Solder Mask Pastemask
DC188
8
DEMO MANUAL DC187/DC188
dc1878 LT/TP 0498 500 • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 1998
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
●
(408) 432-1900
FAX: (408) 434-0507
●
TELEX: 499-3977
●
www.linear-tech.com
PC FAB DRAWI GS
U
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
A
B
C
D
DIAMETER
0.020
0.037
0.072
0.095
TOTAL HOLES
NUMBER
OF HOLES
29
2
2
5
38
2.000
2.000
187 FD
A
B
C
C
D
D
A
A
A
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
A
B
C
D
E
DIAMETER
0.020
0.037
0.025
0.072
0.095
TOTAL HOLES
NUMBER
OF HOLES
38
2
5
2
5
38
2.000
2.000
188 FD
A
A
B
C
C
C
D
D
EE
DC187
DC188
