DEMO MANUAL DC242
DC/DC CONVERTER
LT1777 High Voltage,
Low Noise Step-Down
DC/DC Converter
DESCRIPTIO
Demonstration Circuit DC242 is a high input voltage, low
noise step-down (buck) regulator using the LT
®
1777
switching regulator IC. Typical applications are automo-
tive cellular and GPS receivers, low noise telecom and
industrial-instrument power supplies. This controller in-
cludes an onboard 700mA peak-current switch and is
optimized for use with a high supply voltage. The input
voltage can range from 7.4V to 48V. In order to achieve low
noise, the LT1777 is equipped with dI/dt limiting circuitry,
which is programmed via a small external inductor in the
power path. It also contains internal circuitry to limit the
PERFOR A CE SU
PARAMETER
Input Voltage Range
Output Voltage (Jumper Selectable)
Maximum Output Load Current
Typical Output Ripple Voltage
Nominal Operating Frequency
I
Q
, Supply Current in Shutdown
*I
O
= 450mA for V
IN
≤
12V and V
OUT
= 5V
TYPICAL PERFOR A CE CHARACTERISTICS A D BOARD PHOTO
V
SW
Switching Waveforms
Component Side
V
SW
VOLTAGE
10V/DIV
V
SW
CURRENT
200mA/DIV
500ns/DIV
DC242 TA01
U
WW
U W
U
dV/dt turn-on and turn-off ramp rates. By slowing down
the sharp edges during turn-on and turn-off of the power
switch, conducted and radiated EMI are significantly re-
duced. The output voltage is jumper selectable to either
3.3V or 5V. The LT1777 uses a 100kHz switching fre-
quency and current mode control, which offers excellent
dynamic input supply rejection and short-circuit protec-
tion. Additionally, the supply current can be shut down to
less than 30µA in standby mode.
Gerber files for this
circuit are available. Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
U W
ARY
T
A
= 25°C
VALUE
7.4V to 48V
3.3V, 5V
I
O
= 500mA*
17mV
P-P
100kHz
30µA
CONDITIONS
I
OUT
= 0mA to 500mA
V
IN
> 12V, V
OUT
= 5V
I
OUT
= 500mA, V
IN
= 24V
V
IN
= 7.4V to 48V
1
DEMO MANUAL DC242
DC/DC CONVERTER
SCHE ATIC A D PACKAGE DIAGRA S
TP2
V
IN
7.4V TO 48V
TP1
GND
TP6
SYNC
TP5
SHDN
C3
100pF
C6
100pF
R4
68k
C7
680pF
+
C4
100µF
63V
C5
0.1µF
100V
12
3
14
2
11
15
7
10
V
IN
SYNC
SHDN
V
C
NC
NC
NC
SGND
U1
LT1777
4
V
CC
V
SW
V
D
FB
GND
GND
GND
GND
6
5
13
1
8
9
16
L2
0.47µH
SML32-470k
JP2
OPEN TO
EXT SYNC
Figure 1. 100kHz Low Noise Step-Down Switching Regulator
TOP VIEW
GND 1
NC 2
SHDN 3
V
CC
4
V
D
5
V
SW
6
SGND 7
GND 8
16 GND
15 NC
14 V
C
13 FB
12 SYNC
11 NC
10 V
IN
9
GND
S PACKAGE
16-LEAD PLASTIC SO
LT1777CS
PARTS LIST
REFERENCE
DESIGNATOR
C1
C2
C3, C6
C4
C5
C7
D1
JP1, JP2
JP1, JP2
L1
L2
QUANTITY
1
1
2
1
1
1
1
2
2
1
1
PART NUMBER
0805ZC105MAT1A
TPSD107M010R0065
06035A101JAT1A
63CV100BS
12061C104MAT1A
06033A681JAT1A
10BQ100TR
2802S-02G2
CCIJ2mm-138-G
CTX200-4
SML32-470K
DESCRIPTION
1µF 10V 20% X7R Capacitor
100µF 10V 20% Tantalum Capacitor
100pF 50V 5% NPO Capacitor
100µF 63V 20% Electrolytic Capacitor
0.1µF 100V 20% X7R Capacitor
680pF 25V 5% NPO Capacitor
100V 1A Schottky Diode
2mm 2-Pin Jumper
2mm 2-Pin Shunt
200µH 0.6A SMT Inductor
470nH 450mA SMT Inductor
VENDOR
AVX
AVX
AVX
Sanyo
AVX
AVX
IR
Comm Con
Comm Con
Coiltronics
Gowanda
TELEPHONE
(843) 946-0362
(207) 282-5111
(843) 946-0362
(619) 661-6835
(843) 946-0362
(843) 946-0362
(310) 322-3331
(626) 301-4200
(626) 301-4200
(561) 241-7876
(716) 532-2234
2
W
U
W
L1
200µH
CTX200-4
4
1
D1
10BQ100
3
2
TP3
V
OUT
3.3V OR 5V
500mA
TP4
GND
+
C2
100µF
10V
C1
1µF
10V
R3
35.7k
1%
JP1
V
OSEL
OPEN = 3.3V
CLOSE = 5V
R2
26.1k
1%
R1
21.5k
1%
DC242 F01
DEMO MANUAL DC242
DC/DC CONVERTER
PARTS LIST
REFERENCE
DESIGNATOR
L3
R1
R2
R3
R4
TP1 to TP6
U1
QUANTITY
1
1
1
1
1
6
1
PART NUMBER
DT1608C-152
CR16-2152FM
CR16-2612FM
CR16-3572FM
CR16-683JM
2501-2
LT1777CS
DESCRIPTION
1.5µH Inductor (Alternate for L2)
21.5k 1/16W 1% Chip Resistor
26.1k 1/16W 1% Chip Resistor
35.7k 1/16W 1% Chip Resistor
68k 1/16W 5%
0.090 Turret Testpoint
16-Lead SO DC/DC Controller IC
VENDOR
Coilcraft
Tad
Tad
Tad
Tad
Mill-Max
LTC
TELEPHONE
(847) 639-6400
(714) 255-9123
(714) 255-9123
(714) 255-9123
(714) 255-9123
(516) 922-6000
(408) 432-1900
QUICK START GUIDE
Refer to Figure 2 for proper measurement equipment
set-up and follow the procedure outlined below:
1. Connect the input power supply to the V
IN
and GND
terminals. Input voltage is limited to between 7.4V and
48V.
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 the output load to
measure output current.
5. The SHDN pin should be left floating for normal
operation or tied to GND for shutdown.
A
VOUT
LOAD
GND
GND
6. Leave jumper JP2 connected for switching the LT1777
at its nominal frequency of 100kHz.
7. Set the output voltage with jumper JP1, as shown in
Table 1.
8. After all connections are made, turn on the input
power and verify that the output voltage is correct.
Table 1
POSITION
Jumper JP1 Closed
Jumper JP1 Open
OUTPUT VOLTAGE
5V
3.3V
VIN
A
LT1777CS
DC242
DC242 F02
Figure 2. Proper Measurement Set-Up
3
DEMO MANUAL DC242
DC/DC CONVERTER
OPERATIO
Introduction
The schematic in Figure 1 highlights the capabilities of the
LT1777. The application circuit is set up for an output
voltage of either 3.3V or 5V, set by jumper JP1.The demo
board comes equipped with input, output, GND, SYNC and
SHDN terminals to make bench testing convenient.
Since the LT1777 allows such a wide input range, from
7.4V to 48V, the internal control circuitry draws power
from the V
CC
pin, which is normally connected to the
output supply. During start-up, the LT1777 draws power
from V
IN
. However, after the switching supply output
voltage reaches 2.9V, the LT1777 uses the output to
supply its internal control circuitry, thereby reducing
quiescent power by hundreds of milliwatts when operat-
ing at high input voltage. This helps maximize efficiency
at high line voltages.
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, 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 an output feedback voltage, V
FB,
from an external
resistor divider. When the load current increases, 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.
Sense Inductor
The LT1777 uses an external sense inductor to program
the dI/dt limiting circuitry. The minimum usefull induc-
tance value for the sense inductor is 470nH, which pro-
duces a dI/dt of approximately 2.2A/µs. Larger sense
inductors yield lower current slew rates, resulting in
reduced high frequency RFI emissions at the expense of
slightly reduced efficiency. Refer to the “Selecting Sense
EFFICIENCY (%)
4
U
Inductor” section in the LT1777 data sheet for information
on programming a different dI/dt rate through the main
power inductor.
SYNC Pin
This pin is used to synchronize the internal oscillator to an
external clock with a frequency between 130kHz and
250kHz. If a switching frequency higher than nominal is
desired, remove jumper JP2 and tie an external oscillator
of the desired frequency between the SYNC terminal and
the input GND terminal. The external oscillator can be a
TTL compatible level or 3.3V, with a duty cycle between
10% and 90%. Refer to the “Selecting Main Inductor”
section in the LT1777 data sheet for information on
optimizing the inductor value for running at a higher
frequency.
How to Measure Voltage Regulation and Efficiency
When trying to measure load regulation or efficiency,
voltage measurements should be made directly across the
V
OUT
and GND terminals rather than 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 DC242. Input
and output current should be measured by placing an
ammeter in series with the input supply and load. Figure 3
shows the typical efficiency curve for L2 (sense inductor)
= 470nH, V
IN
= 12V, V
OUT
= 5V.
85
V
IN
= 12V
V
OUT
= 5V
L2 = 470nH
80
75
70
65
60
0
100
300
400
200
LOAD CURRENT (mA)
500
DC242 F03
Figure 3. LT1777 Output Efficiency
DEMO MANUAL DC242
DC/DC CONVERTER
OPERATIO
How to Measure Output Voltage Ripple
In order to measure output voltage ripple, care must be
taken to avoid a long ground lead on the oscilloscope
probe. A sturdy wire should be soldered on the output GND
terminal. The other end of the wire is looped around the
ground connection of the probe and should be kept as
short as possible. The tip of the probe is touched directly
to V
OUT
(see Figure 4). 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
5 shows the output voltage ripple with a steady state load
of 500mA.
Figure 4. Measuring Output Voltage Ripple
Figure 5. Output Voltage Ripple for the LT1777, I
L
= 500mA
U
Heat Dissipation Issues
Since the LT1777 includes 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. The four corner pins on
the LT1777 IC are connected to the ground plane on both
sides of the PC board. The two sides are connected
through vias to better handle the power dissipation. If the
LT1777 is placed on a multilayer board, it is desirable to
have metal on the inner layers directly underneath the
LT1777. This helps in spreading heat and improves the
power dissipation capability of the PCB.
Layout Guidelines
Since the LT1777 is a switching regulator, attention to
proper layout is essential for good load regulation and to
minimize radiated/conducted noise. Be sure to follow
these layout guidelines:
DC242
LT1777
V
OUT
GND
DC242 F04
PROBE
1. The LT1777’s V
SW
pin, sense inductor L2 and Schottky
diode D1 should be placed as close as possible to each
other.
2. The anode of Schottky diode D1 should be tied directly
to the ground plane.
3. Keep the trace from the FB pin to the junction of R3 and
R2 short and use a long trace from the top of resistor R3
to the output terminal, rather than vice versa.
4. Output capacitor C2’s grounds 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 SGND pin, which,
in turn, is directly tied to the ground plane.
DC242 F05
6. C5 should be as close as possible to Pin 10.
5
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