QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 502
LT1767
DESCRIPTION
Demonstration
circuit
502
is
a
1.25MHz 1.5A monolithic step-down
DC/DC switching converter using the
LT1767 buck regulator. With its fast
switching speed, 1.5A internal power
switch, and wide input voltage range,
the LT1767 is a very versatile and
powerful IC for DC/DC converters that
require compact space and a wide va-
riety of input voltage ranges. The
constant 1.25MHz switching frequency
allows all of the components to be
very small, surface mount devices.
The current-mode control topology
creates fast transient response and
good loop stability with a minimum
number of external compensation com-
ponents and ceramic input and output
capacitors. The low resistance inter-
nal power switch (0.22Ω) maintains
high efficiencies >80% and up to 90%.
The SHDN pin and 6uA shutdown current
extends battery life. The wide V
IN
range of the LT1767 allows step-down
configurations from 3Vin up to 25Vin.
Synchronization of switching frequen-
cies up to 2MHz is possible.
Demonstration Circuit 502 is a step-
down converter using the LT1767EMS8E.
The circuit is designed for 6.6-25V
Table 1. Typical Performance Summary
PARAMETER
Input Voltage Range
Input Voltage Range
V
out
V
out
Maximum Output Current
Typical Switching Frequency
Typical Efficiency
CONDITION
V
out
= 5V (Jumper Selection)
V
out
= 3.3V (Jumper Selection)
V
out
= 5V (Jumper Selection)
V
out
= 3.3V (Jumper Selection)
VALUE
6.6V–25V
4.5V–18V
5V ±3%
3.3V ±3%
1A
1.25MHz
V
in
= 12V, V
out
= 5V, I
out
= 1A 84%
1.25MHZ WIDE INPUT RANGE STEP DOWN DC/DC CONVERTER
input to 5.0V output @1A or for 4.5-
25V
†
input to 3.3V output @1A for ca-
ble modem, handheld, automotive, and
desktop computer applications. The
output voltage is jumper selectable
(5V or 3.3V). This board was designed
for wide input voltage range applica-
tions where simplicity, small circuit
size, and low component count are im-
portant as well as up to 1A of load
current. The use of ceramic capaci-
tors not only demonstrates small size
and low cost, but the advantage of
current-mode control in step-down ap-
plications with a simple compensation
network and a place-holder for a
feedforward capacitor for more rugged
stability and optimal transient re-
sponse if required.
Design files for this circuit board
are available. Call the LTC factory.
Higher input voltages may pulse-skip due to
minimum on-time restrictions. Compensation
component changes may be necessary to opti-
mize pulse-skipping during high-temperature,
high-voltage conditions and maintain control
of switch current.
†
1
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 502
1.25MHZ WIDE INPUT RANGE STEP DOWN DC/DC CONVERTER
QUICK START PROCEDURE
Demonstration circuit 502 is easy to
set up to evaluate the performance of
the LT1767. Refer to Figure 1 for
proper measurement equipment setup
and follow the procedure below:
NOTE:
Make sure that the input voltage
4.
Connect
an ammeter in series with
output load to measure output cur-
rent.
and Gnd terminals to measure input
voltage.
5.
Connect a voltmeter across the Vin
does not exceed 25V.
1.
Connect the 7V–25V or 4.5V–18V in-
6.
Connect a voltmeter across the Vout
put power supply to the Vin and Gnd
terminals on the board.
2.
Connect
and Gnd terminal to measure output
voltage.
7.
After
an ammeter in series with
the input supply to measure input
current.
either power resistors or
an electronic load to the Vout and
Gnd terminals on the board.
3.
Connect
all connections are made,
turn on input power and verify that
the output voltage is either 5.0V
or 3.3V (the output voltage jumper
setting).
synchronization and shutdown
functions are optional and their
pins can be left floating (discon-
nected) if their functions are not
being used.
8.
The
–
+ +
+
–
–
LTC1767EMS8E
Linear Technology
(408) 432-1900
www.linear.com
1.25MHz Wide Input Range
Step Down DC/DC Converter
Demo Circuit 502
Vin
GND
SYNC
–
+
Vout
GND
SHDN
VOUT
3.3V 5V
+
+
–
+
–
–
LOAD
Figure 1. Proper Measurement Equipment Setup
2
2
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 502
1.25MHZ WIDE INPUT RANGE STEP DOWN DC/DC CONVERTER
100
90
3.3V
OUT
EFFICIENCY (%)
80
70
60
50
40
30
20
10
0
0.01
0.1
LOAD CURRENT (mA)
1
5V
IN
8V
IN
12V
IN
18V
IN
100
90
80
5V
OUT
EFFICIENCY (%)
70
60
50
40
30
20
10
0
0.01
0.1
LOAD CURRENT (mA)
1
8V
IN
12V
IN
18V
IN
24V
IN
Figure 2. Efficiency for V
OUT
= 3.3V
Figure 3. Efficiency for V
OUT
= 5V
MINIMUM INPUT VOLTAGE
The minimum LT1767 operating input
voltage is 3.0V. However, a DC/DC
buck converter needs to have an input
voltage greater than the output volt-
age by a certain margin in order to
provide the desired regulated output
voltage. Maximum duty cycle, switch
on-resistance, and inductor and diode
DC losses all play a part in deter-
mining the minimum input voltage for
a selected output over the full cur-
rent range of the application. For
this demonstration board, with up to
1A output current, the minimum input
voltages required to maintain regu-
lated output voltages is listed be-
low. Customizable output voltages
2.5V and 1.8V are very common and are
thus additionally listed in the table
(see
Customizing the Board
below). If
the input voltage falls below the
minimum input voltage listed below,
the output voltage can drop accord-
ingly from the programmed output
voltage. This mode is called maximum
duty cycle drop-out.
Table 2. Minimum Input Voltage for Regulated
Output
V
OUT
5.0V
3.3V
2.5V
1.8V
V
IN
(MINIMUM)
6.6V
4.5V
3.5V
3.0V
CUSTOMIZING THE BOARD
The components built on the board are
optimized for a wide input voltage
range. The bandwidth can be increased
for more specific input voltages such
as 12V ±10% or 5V ±10% if desired.
Some typical applications with typi-
cal input voltage ranges that are
smaller than the wide input voltage
range of the part are included below.
The adjustable feedback resistors al-
low the output voltage to be custom-
ized. For output voltages below 3.0V,
the boost diode should be moved from
D2 to D3 to provide the minimum boost
3
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 502
voltage required for the internal
power switch. Make sure that the
boost capacitor (C8) has a voltage
rating greater than or equal to the
output voltage for applications where
12V +/-10%
Vin
U1
Vin
C2
2.2uF 25V X7R 1206
Boost
1.25MHZ WIDE INPUT RANGE STEP DOWN DC/DC CONVERTER
the boost diode is placed in D2. How-
ever, the boost capacitor must have a
voltage rating greater than the input
voltage whenever the boost diode is
placed in position D3.
5V @1A
LT1767EMS8E
Vsw
CDRH5D18-5R4
R1
FB
Vc
31.6k
C8
0.1uF
D2 CMDSH-3
L1
Vout
Ceramic
Sync
/Shdn
C3
1.5nF
R3
4.7k
C4
100pF
Gnd
D1
UPS120
R2
10.0k
C5
4.7uF 6.3V X5R 0805
Ceramic
12V +/-10%
Vin
U1
Vin
C2
2.2uF 25V X7R 1206
Boost
3.3V @1A
LT1767EMS8E
Vsw
CDRH4D28-4R7
R1
FB
Vc
C3
2.2nF
R3
3.3k
C4
100pF
Gnd
17.4k
C8
0.1uF
D2 CMDSH-3
L1
Vout
Ceramic
Sync
/Shdn
D1
UPS120
R2
10.0k
C5
4.7uF 6.3V X5R 0805
Ceramic
5V +/-10%
Vin
D3 CMDSH-3
U1
Vin
C2
2.2uF 10V X5R 0805
Boost
2.5V @1A
LT1767EMS8E
Vsw
1008PS-272M
R1
FB
Vc
C3
1.5nF
R3
3.3k
C4
100pF
Gnd
10.7k
C8
0.1uF
Vout
L1
Ceramic
Sync
/Shdn
D1
UPS120
R2
10.0k
C5
4.7uF 6.3V X5R 0805
Ceramic
4
4
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 502
3-5.5V
Vin
D3 CMDSH-3
U1
Vin
C2
2.2uF 10V X5R 0805
Boost
1.25MHZ WIDE INPUT RANGE STEP DOWN DC/DC CONVERTER
1.8V @1A
LT1767EMS8E
Vsw
1008PS-272M
R1
FB
Vc
4.99k
C8
0.1uF
Vout
L1
Ceramic
Sync
/Shdn
C3
1.5nF
R3
2.2k
C4
100pF
Gnd
D1
UPS120
R2
10.0k
C5
4.7uF 6.3V X5R 0805
Ceramic
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