DEMO MANUAL DC355/DC356
NO-DESIGN SWITCHER
LT1959 Monolithic 4A Switcher
5V to 15V Input
1.8V Output
handheld devices and, in larger systems, as local onboard
regulators. High frequency switching allows the use of
small inductors, making this all surface mount solution
ideal for space conscious systems.
, LTC and LT are registered trademarks of Linear Technology Corporation.
DESCRIPTIO
Demonstration circuits DC355 and DC356 are complete
DC/DC step-down regulators using the LT
®
1959 constant
frequency, high efficiency converter in 7-pin DD (DC356)
and SO-8 (DC355) packages. These circuits are primarily
used in personal computers, disk drives, portable
PERFOR A CE SU
PARAMETER
Output Voltage
Maximum I
LOAD
Input Voltage Range
Switching Frequency
Output Ripple Voltage
Line Regulation
Load Regulation
SHDN Lockout Threshold
SHDN Shutdown Threshold
Synchronization Range
Supply Current
T
A
= 25°C, V
IN
= 5V, I
LOAD
= 2A, V
OUT
= 1.8V, SHDN and SYNC pins open, unless otherwise specified.
CONDITIONS
(Note 1)
(Note 2)
MIN
1.75
4.3
4.5
460
5V to 15V
I
LOAD
= 10mA to 4A
2.3
0.15
DC355 Only
SHDN = 0V
580
20
500
25
4
10
2.38
0.37
2.46
0.6
1000
15
540
TYP
1.8
MAX
1.85
UNITS
V
A
V
kHz
mV
P-P
mV
mV
V
V
kHz
µA
Note 1:
Output voltage variations include
±1%
tolerance of feedback
divider network.
Note 2:
For DC355, additional thermal restrictions apply.
BOARD PHOTOS
DC355 Component Side
DC356 Component Side
WW
U
ARY
U W
1
DEMO MANUAL DC355/DC356
NO-DESIGN SWITCHER
TYPICAL PERFOR A CE CHARACTERISTICS
1.8V Output Efficiency
90
85
EFFICIENCY (%)
DIE TEMPERATURE RISE (°C)
80
75
70
65
60
5V
IN
12V
IN
0
1
LOAD CURRENT (A)
DC355/356 TA01
PACKAGE A D SCHE ATIC DIAGRA SM
D2*
OPTIONAL
C4
1µF
10V
DC355
E1
V
IN
5V TO 15V
D3*
MMBD914LT1
L1
6.8µH
D1
MBRD835L
+
C3
10µF
25V
2
1
6
BOOST
V
IN
U1 SW
LT1959CS8
SYNC
FB
V
C
5
R1
3.3k
C2
3.3nF
8
7
3
+
C5
100µF
10V
+
E2
SHDN
E3
GND
SHDN
GND
4
E6
SYNC
*MOVE D3 TO POSITION D2
FOR OUTPUT VOLTAGES > 3.3V
C1
OPTIONAL
D2*
OPTIONAL
C4
1µF
10V
DC356
E1
V
IN
5V TO 15V
D3*
MMBD914LT1
L1
6.8µH
D1
MBRD835L
+
C3
10µF
25V
6
5
BOOST
V
IN
SW
U1
LT1959CR
FB
V
C
1
R1
3.3k
C2
3.3nF
3
+
C5
100µF
10V
+
E2
SHDN
E3
GND
2
SHDN
GND
4
7
*MOVE D3 TO POSITION D2
FOR OUTPUT VOLTAGES > 3.3V
C1
OPTIONAL
2
W
W
U W
2
Temperature Rise vs Load Current
120
100
DC355
80
60
40
20
0
3
4
0
1
2
LOAD CURRENT (A)
DC355/356 TA02
V
IN
= 5V
V
OUT
= 1.8V
DC356
3
4
U
C7
OPTIONAL
R2
1.21k
1%
R3
2.49k
1%
+
C6
0.47µF
10V
E5
V
OUT
3.3V/4A
V
IN
1
E4
GND
BOOST 2
FB 3
GND 4
TOP VIEW
8 V
SW
7 SYNC
6 SHDN
5 V
C
S8 PACKAGE
8-LEAD PLASTIC SO
DC355 SCHEM
LT1959CS8
C7
OPTIONAL
R2
1.21k
1%
R3
2.49k
1%
+
C6
0.47µF
10V
E5
V
OUT
3.3V/4A
E4
GND
FRONT VIEW
7
6
5
4
3
2
1
FB
BOOST
V
IN
GND
V
SW
SYNC OR SHDN
V
C
TAB
IS
GND
DC356 SCHEM
R PACKAGE
7-LEAD PLASTIC DD PAK
LT1959CR
DEMO MANUAL DC355/DC356
NO-DESIGN SWITCHER
PARTS LISTS
DC355
REFERENCE
DESIGNATOR
C1
C2
C3
C4
C5
C6
C7
D1
D2
D3
E1 to E6
R1
R2
R3
L1
U1
QUANTITY
0
1
1
1
1
1
1
1
1
1
6
1
1
1
1
1
2501-2
CR10-332JM
CR10-1821F-T
CR10-4991F-T
DO3316P-682
LT1959CS8
MBRD835L
MMBD914LT1
08055C332MAT2S
GRM235Y5V106Z
0805ZC105MAT2S
TPSD107M010R0080
0603ZG474MAT3S
PART NUMBER
DESCRIPTION
Optional Capacitor
3300pF 50V X7R Chip Capacitor
10µF 25V Y5V Chip Capacitor
1µF 10V X7R Chip Capacitor
100µF 10V TPS Tantalum Capacitor
0.47µF 10V Y5V Chip Capacitor
Optional Capacitor
SMT Diode
1N914 Diode
Optional Diode
Pad Turret
3.3k 1/8W 5% Chip Resistor
1.82k 1/8W 1% Chip Resistor
4.99k 1/8W 1% Chip Resistor
6.8µH 20% Inductor
SO-8 Linear IC
Mill-Max
AAC
AAC
AAC
Coilcraft
LTC
(516) 922-6000
(714) 255-9186
(714) 255-9186
(800) 508-1521
(847) 639-6400
(408) 432-1900
ON Semiconductor
ON Semiconductor
(602) 244-6600
(602) 244-6600
AVX
Murata
AVX
AVX
AVX
(843) 946-0362
(770) 436-1300
(843) 946-0362
(207) 282-5111
(843) 946-0362
VENDOR
TELEPHONE
DC356
REFERENCE
DESIGNATOR
C1
C2
C3
C4
C5
C6
C7
D1
D2
D3
E1 to E6
R1
R2
R3
L1
U1
QUANTITY
0
1
1
1
1
1
1
1
1
1
6
1
1
1
1
1
2501-2
CR10-332JM
CR10-1821F-T
CR10-4991F-T
DO3316P-682
LT1959CR
MBRD835L
MMBD914LT1
08055C332MAT2S
GRM235Y5V106Z
0805ZC105MAT2S
TPSD107M010R0080
0603ZG474MAT3S
PART NUMBER
DESCRIPTION
Optional Capacitor
3300pF 50V X7R Chip Capacitor
10µF 25V Y5V Chip Capacitor
1µF 10V X7R Chip Capacitor
100µF 10V TPS Tantalum Capacitor
0.47µF 10V Y5V Chip Capacitor
Optional Capacitor
SMT Diode
1N914 Diode
Optional Diode
Pad Turret
3.3k 1/8W 5% Chip Resistor
1.82k 1/8W 1% Chip Resistor
4.99k 1/8W 1% Chip Resistor
6.8µH 20% Inductor
7-Pin DD Pak Linear IC
Mill-Max
AAC
AAC
AAC
Coilcraft
LTC
(516) 922-6000
(714) 255-9186
(714) 255-9186
(800) 508-1521
(847) 639-6400
(408) 432-1900
ON Semiconductor
ON Semiconductor
(602) 244-6600
(602) 244-6600
AVX
Murata
AVX
AVX
AVX
(843) 946-0362
(770) 436-1300
(843) 946-0362
(207) 282-5111
(843) 946-0362
VENDOR
TELEPHONE
3
DEMO MANUAL DC355/DC356
NO-DESIGN SWITCHER
OPERATIO
DC355 vs DC356 (Temperature vs Package Size)
The DC355 and DC356 demonstration boards are
intended for evaluation of the LT1959 switching regulator
in the SO-8 and 7-pin DD packages, respectively. The 7-pin
DD package used on DC356 has no SYNC pin. The primary
reason for choosing the SO-8 over the DD package is board
space. The DC356 (DD package) occupies an active board
area of approximately 0.75 square inches. By optimizing
the DC355 board, using a Sumida coil and removing the
layout options, a total active area of 0.4 square inches can
be achieved. The DD package is more suitable for higher
power or higher ambient temperature applications.
Although both boards will supply 4A of output current,
DC355 must be thermally derated to 3A continuous cur-
rent at 22°C ambient to prevent excessive die tempera-
tures. DC356 can run at 60°C ambient at 4A output cur-
rent. However, the SO-8 package can be used for dynamic
loads up to the full rated switch current.
LT1959 Operation
The LT1959 data sheet gives a complete description of the
part, operation and applications information. The data
sheet should be read in conjunction with this demo manual.
Hook-Up
Solid turret terminals are provided for easy connection to
supplies and test equipment. Connect a 0V to 15V, 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 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 excessive
noise. For improved results, the test hook should be
removed from the tip of 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
For normal operation, the SHDN pin can be left floating.
SHDN has two output-disable modes: lockout and
shutdown. When the pin is taken below the lockout
4
U
threshold, switching is disabled. This is typically used for
input undervoltage lockout. Grounding the SHDN pin
places the LT1959 in shutdown mode. This reduces total
board supply current to 20µA.
Synchronization
Synchronization is available on DC355 only. For normal
demo board operation, the SYNC pin can be left floating.
If it is not used in the application, it is advisable to tie this
pin to ground. To synchronize switching to an external
clock, apply a logic-level signal to the SYNC pin. The
amplitude must be from a logic low to greater than 2.2V,
with a duty cycle between 10% and 90%. The synchroni-
zation frequency must be greater than the free-running
oscillator frequency and less than 1MHz. Additional cir-
cuitry may be required to prevent subharmonic oscilla-
tion. Refer to the LT1959 data sheet for more details.
COMPONENTS
Inductor L1
The inductor is a Coilcraft DO3316P-682, a 6.8µH
unshielded ferrite unit. It was 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, solder
pads are available for the Sumida CD43-1R8 inductor.
This 1.8µH unit has a 2.9A I
SAT
rating. Ripple at 5V
IN
is
±0.6A.
This gives a maximum output current of
(4.5A – 0.6A) = 3.9A.
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 capaci-
tors become resistive at higher frequencies, requiring
careful ripple-rating selection to prevent excessive heat-
ing. Ceramic capacitors’ ESL (effective series inductance)
tends to dominate their ESR, making them less suscep-
tible to ripple-induced heating.
DEMO MANUAL DC355/DC356
NO-DESIGN SWITCHER
OPERATIO
The output capacitor C5 is an AVX tantalum capacitor. A
ceramic is not recommended as the main output capaci-
tor, since loop stability relies on a resistive characteristic
at higher frequencies to form a zero. The AVX TPS series
was specifically designed to have the low ESR required in
switch-mode power supplies. At switching frequencies,
ripple voltage is more a function of ESR than of absolute
capacitance value. If lower output ripple voltage is re-
quired, use the optional capacitor C7 to reduce ESR rather
than increasing the capacitance of C5. For very low ripple,
an additional LC filter in the output may be a less expensive
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, trace inductance and local bypass capaci-
tors will perform this function, negating the need for C6.
Catch Diode D1
Use diodes designed for switching applications, with
adequate current rating and fast turn-on times, such as
Schottky or ultrafast diodes. 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 LT1959 data sheet. R1 + C2 from V
C
to ground
give a stable loop response over a wide range of input
and output conditions. Optional capacitor C1 is included
for optimization of the dynamic response for a specific
application.
U
Boost Voltage: 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 above 3.3V, diode D2 can
replace D3 and provide sufficient boost voltage to C4.
PCB LAYOUT
In many cases, the layout of the demonstration board
may be dropped directly into the application with mini-
mal changes. If not, there are several precautions that
must be taken when laying out high frequency con-
verter circuits. The high frequency switching path runs
from ground, through C3, to the V
IN
pin of the LT1959,
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 LT1959 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 line
to V
OUT
as required but keep the divider network close
to the LT1959 to prevent noise pickup on the FB node.
Noise pickup on the V
C
pin appears as various prob-
lems, 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 manner.
For more information or advice, contact the LTC Appli-
cations department.
5
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