DEMO MANUAL DC204
NO-DESIGN SWITCHER
DESCRIPTIO
LT1676/LT1776 High Voltage
High Efficiency Step-Down
DC/DC Converters
turn-on rise time. This eliminates pulse skipping, which
helps reduce output ripple voltage and switching noise in
the audio frequency spectrum. Additionally, the supply
current can be shut down to less than 20µA in standby
mode. Demonstration circuit DC204 Version B is also a
high efficiency step-down regulator using the LT1776
switching regulator. The LT1776 is identical to the LT1676
with the exception that its nominal operating frequency is
200kHz.
Gerber files for this circuit board are available.
Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Demonstration circuit DC204 Version A is a high efficiency
step-down (buck) regulator using the LT1676 switching
regulator. Typical applications include automotive DC/DC
conversion, telecom 48V step-down and IEEE1394 step-
down converters. This controller includes an onboard
700mA peak-current switch and is optimized for use with
a high supply voltage. The input voltage can range from 7V
to 60V. The output voltage is jumper selectable to either
3.3V or 5V. The LT1676 uses a 100kHz switching fre-
quency and current mode control, resulting in very high
efficiency, low ripple and fast transient response. At low
output currents, the LT1676 automatically slows down the
PERFOR A CE SU
SYMBOL
V
IN
V
OUT
I
O
V
RIP
f
O
I
Q
PARAMTER
Input Voltage Range
Output Voltage (Jumper Selectable)
Maximum Output Load Current
Typical Output Ripple
Nominal Operating Frequency
Supply Current in Shutdown
TYPICAL PERFOR A CE CHARACTERISTICS A D BOARD PHOTO
LT1676 Output Efficiency
V
IN
= 40V, V
OUT
= 5V
90
EFFICIENCY (%)
80
70
60
0
100
200
300
400
LOAD CURRENT (mA)
500
DC204 BP
DC204 F00
U
WW
U W
U
U W
ARY
T
A
= 25°C
CONDITIONS
Version A (LT1676)
Version B (LT1776)
See Figure 1
V
IN
> 12V
V
IN
≤
12V , V
OUT
= 5V
I
O
= 500mA
Version A (LT1676)
Version B (LT1776)
J1
Closed
J1
J1
Closed
Open
JUMPER POSITION
VALUE
7V to 60V
7V to 60V
5V
3.3V
500mA
450mA
16mV
P-P
100kHz
200kHz
20µA
Component Side
1
DEMO MANUAL DC204
NO-DESIGN SWITCHER
PACKAGE A D SCHE ATIC DIAGRA SM
E1
SHDN
V
IN
(SEE TABLE)
E2
1
5
8
C1
47µF
SANYO
100V
E3
GND
C2
0.15µF
100V
LT1676/LT1776
SHDN
V
IN
V
C
GND
U1
V
SW
V
CC
FB
SYNC
3
2
7
6
R3
7.15k
1%
J2
J1
DC204A F01
+
C3
100pF
50V R4
(SEE
TABLE)
C4
(SEE
TABLE)
C5
(SEE
TABLE)
4
ASSEMBLY
VERSION A
VERSION B
V
IN
7V TO 60V
7V TO 60V
U1
LT1676CS8
LT1776CS8
C4
1000pF
180pF
C5
100pF
39pF
L1
CTX200-4
CTX100-2
R4
47k
100k
Figure 1. LT1676/LT1776 High Efficiency Step-Down DC/DC Converter Schematic
PARTS LIST
REFERENCE
DESIGNATOR
C1
C2
C3
C4
C5
C6
C7
D1
E1 to E6
J1, J2
P1, P2
L1
R1
R2
R3
R4
U1
QUANTITY PART NUMBER
1
1
1
1
1
1
1
1
1
1
6
2
2
1
1
1
1
1
1
1
1
1
100MV47GX
VJ1210Y154KXBAT
08055A101KAT
08055C102KAT
08055C181KAT
08055A101KAT
08055A390KAT
TPSD107M010R0080
0805ZC105KAT2A
MBRS1100T3
2501-2
2802S-02-G2
CCIJ2MM-138-G
CTX200-4
CTX100-2
CR10-1002FM
CR10-5901FM
CR10-7151FM
CR10-473JM
CR10-104JM
LT1676CS8
LT1776CS8
DESCRIPTION
47µF 100V TPS Tantalum Capacitor
0.15µF 100V X7R Chip Capacitor
100pF 50V NPO Chip Capacitor
1000pF 50V X7R Chip Capacitor (Version A)
180pF 50V Chip Capacitor (Version B)
100pF 50V NPO Chip Capacitor (Version A)
39pF 50V Chip Capacitor (Version B)
100µF 10V TPS Tantalum Capacitor
1µF 10V X7R Chip Capacitor
1A 100V Schottky Diode
Testpoint Turret
0.079” 2X1 Header
0.079” Center Shunt
200µH Inductor (Version A)
100µH Inductor (Version B)
10k 1/8W 1% Chip Resistor
5.9k 1/8W 1% Chip Resistor
7.15k 1/8W 1% Chip Resistor
47k 1/8W 5% Chip Resistor (Version A)
100k 1/8W 5% Chip Resistor (Version B)
IC, SO8 (Version A)
IC, SO8 (Version B)
VENDOR
SANYO
Vitramon
AVX
AVX
AVX
AVX
AVX
AVX
AVX
Motorola
Mill-Max
Comm Con
Comm Con
Coiltronics
Coiltronics
TAD
TAD
TAD
TAD
TAD
LTC
LTC
TELEPHONE
(619) 661-6835
(203) 268-6261
(803) 946-0362
(803) 946-0362
(803) 946-0362
(803) 946-0362
(803) 946-0362
(207) 282-5111
(803) 946-0362
(800) 441-2447
(516) 922-6000
(626) 301-4200
(626) 301-4200
(561) 241-7876
(561) 241-7876
(714) 255-9123
(714) 255-9123
(714) 255-9123
(714) 255-9123
(714) 255-9123
(408) 432-1900
(408) 432-1900
2
W
W
U
L1
(SEE TABLE)
D1
MBRS1100T3
R1
10k
1%
R2
5.9k
1%
E5
+
C6
100µF
10V
TPS
C7
1µf
10V
V
OUT
3.3V/5V
(SEE TABLE) E4
SYNC
E6
GND
TOP VIEW
OUTPUT VOLTAGE SELECT
J1
CLOSED
5V
J1
OPEN
3.3V
SHDN 1
V
CC
2
V
SW
3
GND 4
8
7
6
5
V
C
FB
SYNC
V
IN
S8 PACKAGE
8-LEAD PLASTIC SO
LT1676CS8/LT1776CS8
DEMO MANUAL DC204
NO-DESIGN SWITCHER
QUICK START GUIDE
Refer to Figure 2 for proper measurement equipment
setup and follow the procedure outlined below:
1. Connect the input power supply to the V
IN
and GND
terminals. The input voltage must be between 7V and
60V.
2. Connect an ammeter in series with the input supply to
measure input current.
3. Connect either power resistors or an electronic load
to the V
OUT
and GND terminals.
4. Connect an ammeter in series with output load to
measure output current.
5. The SHDN pin should be left floating for normal
operation and tied to GND for shutdown.
6. Leave jumper J2 connected to operate the LT1676
(Version A) at its nominal switching frequency of
100kHz or the LT1776 (Version B) at 200kHz.
7. Set the output voltage with jumper J1, as shown in
the table below.
8. After all connections are made, turn on input power
and verify that the output voltage is correct.
POSITION
Jumper J1 Closed
Jumper J1 Open
OUTPUT VOLTAGE
5V
3.3V
OPERATIO
Introduction
The circuit in Figure 1 highlights the capabilities of the
LT1676/LT1776. The application circuit is set up for an
output voltage of either 3.3V or 5V by means of jumper J1.
The demo board comes equipped with input, output, GND,
SYNC and SHDN terminals to make bench testing
convenient.
Since the LT1676/LT1776 demo board allows such a wide
input range, it uses two techniques to optimize efficiency.
First, the internal control circuitry draws power from the
V
CC
pin, which is normally connected to the output supply.
During start-up, the LT1676/LT1776 draw power from
V
IN
. After the switching supply output voltage reaches
2.9V, the LT1676/LT1776 draw power from the output.
This reduces quiescent power consumption by hundreds
of milliwatts when operating at a high input voltage.
Second, the LT1676/LT1776 switch circuitry maintains a
fast rise time at high loads. Both of these factors help in
maximizing efficiency with high loads and high line volt-
ages. At light loads, the LT1676/LT1776 slow down the
rise time to avoid pulse skipping, maintaining a constant
frequency at all loads. This helps significantly in reducing
output ripple voltage and prevents switching noise from
folding back into the audio frequency spectrum.
U
Theory of Operation
During normal operation, the internal power transistor is
turned on during each cycle when the oscillator sets a latch
and turned off when the main current comparator resets
the latch. While the internal switch is off, the Schottky
diode (D1) carries the inductor current until it tries to
reverse or until the beginning of the next cycle. The voltage
on the V
C
pin, which is the output of the error amplifier,
controls the peak inductor current. The FB pin provides the
error amplifier with an output feedback voltage, V
FB
, from
an external resistor divider. When the load current in-
creases, it causes a slight decrease in V
FB
relative to the
1.24V reference, which, in turn, causes the voltage on the
V
C
pin to increase until the average inductor current
matches the new load current.
SYNC Pin
This pin is used to synchronize the internal oscillator to an
external clock with a frequency between 130kHz and
250kHz for the LT1676 and between 250kHz and 400kHz
for the LT1776. If a switching frequency higher than
nominal is desired, remove jumper J2 and tie an external
oscillator of the desired frequency between the SYNC
terminal and the input GND terminal. The amplitude of the
3
DEMO MANUAL DC204
NO-DESIGN SWITCHER
OPERATIO
external oscillator can be a TTL compatible level or 3.3V
logic levels, with a duty cycle between 10% and 90%.
Refer to the “Selecting Power Inductor” section in the
LT1676/LT1776 data sheet for how to optimize the induc-
tor value if running at a higher frequency.
How to Measure Voltage Regulation and Efficiency
When measuring load regulation or efficiency, voltage
measurements should be made directly across the V
OUT
and GND terminals, not at the end of test leads at the load.
Similarly, input voltage should be measured directly at the
V
IN
and GND terminals of the LT1676/LT1776 demo
board. Input and output current should be measured by
placing an ammeter in series with the input supply and
load. Refer to Figure 2 for the proper monitoring equip-
ment setup.
A
L
O
A
D
+
+
Figure 2. Proper Measurement Setup
4
U
How to Measure Output Voltage Ripple
When measuring output voltage ripple, care must be taken
to avoid a long ground lead on the oscilloscope probe. A
sturdy wire should be soldered to the output side of the
GND terminal. The other end of the wire is looped around
the ground side of the probe and should be kept as short
as possible. The tip of the probe is touched directly to V
OUT
(see Figure 3). Bandwidth is generally limited to 20MHz for
ripple measurements. Also, if multiple pieces of line-
powered test equipment are used, be sure to use isolation
transformers on their power lines to prevent ground
loops, which can cause erroneous results. Figure 4 shows
the output voltage ripple with a steady-state load of
500mA for the LT1676.
SHDN
SYNC
PROBE
DEMO CIRCUIT 204A
LT1676/LT1776
V
OUT
V
IN
I
IN
A
+
DC204A
LT1676/LT1776
+
V
OUT
GND
GND
GND
DC204A F02
DC204A F03
Figure 3. Measuring Output Voltage Ripple
10mV/DIV
10µs/DIV
DC204 F04
Figure 4. Output Voltage Ripple for
the LT1676, I
L
= 500mA
DEMO MANUAL DC204
NO-DESIGN SWITCHER
OPERATIO
Heat Dissipation Issues
Since the LT1676/LT1776 include a 500mA onboard power
switch, care must be taken not to exceed the maximum
junction temperature for the part. A simple technique is to
use the PC board as a heat sink. On the LT1676/LT1776
demo board, the power IC is surrounded by ground plane
on both sides of the PC board. The two sides are connected
through vias to better handle the power dissipation. If the
LT1676/LT1776 are laid out on a multilayer board, there
should be metal on the inner layers directly underneath
the LT1676/LT1776. This helps in spreading heat and
improves the power dissipation capability of the PCB.
Layout Guidelines
Since the LT1676/LT1776 are switching regulators, a
good layout is essential for good load regulation and
minimizing radiated/conducted noise. Be sure to follow
these layout guidelines:
U
1. The LT1676/LT1776’s V
SW
pin and the Schottky diode,
D1, should be placed as close as possible to each
other.
2. The anode of Schottky diode D1 should be tied to input
capacitor C1’s ground by means of a wide trace, not via
the ground plane.
3. Keep the trace from the FB pin to the junction of R1 and
R2 short and use a long trace from the top of resistor
R1 to the output terminal, rather than vice versa.
4. The grounds of output capacitors C6 and C7 should be
tied directly to the ground plane.
5. The ground of the feedback resistors and the loop
compensation resistor/capacitor (connected to the V
C
pin) should be referenced to the chip ground pin,
which, in turn, is directly tied to the input bulk capaci-
tors’ ground.
6. C2 should be as close as possible to Pin 5.
5
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