Linear Technology LT1765EFE User manual
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476
1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER
LT1765EFE
DESCRIPTION
Demonstration circuit 476 is a 1.25MHz 3A monolithic
step-down DC/DC switching converter using the
LT1765EFE. The LT1765 features fast switching speed,
a 3A internal power switch, and a wide input voltage
range, making it a versatile and powerful IC that fits
easily into space-constrained applications. The con-
stant 1.25MHz switching frequency allows for the use
of tiny, surface mount external components. The cur-
rent-mode control topology yields fast transient re-
sponse and good loop stability, requiring a minimum
number of external compensation components and
allowing the use of ceramic input and output capaci-
tors. The low R
DS(ON)
internal power switch (0.09
Ω
)
maintains high efficiencies (as high as 90%) over a
wide range of input voltages and loads. Its 15µA shut-
down current (activated via the SHDN pin) extends
battery life. The wide V
IN
range of the LT1765 allows
step-down configurations from 3V to 25V
input. Syn-
chronization of switching frequencies up to 2MHz is
possible.
The demonstration circuit is designed to provide either
3.3V at 2A or 5V at 2A output, from a 7V–25V input or
4.7V–25V
†
input, respectively, covering the common
values used in cable modems, handhelds, automotive,
and desktop computer applications. The 5V or 3.3V
output voltage is jumper selectable.
This board is designed for applications that require 2A
of load current from a wide input voltage range plus
simplicity, small circuit size, and low component
count. The use of ceramic capacitors in this circuit 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
feedforward capacitor for more rugged stability and
excellent transient response.
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 optimize pulse-
skipping during high-temperature, high-voltage conditions and maintain
control of switch current.
Figure 1.
Demonstration Circuit 476
1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476
1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER
TYPICAL PERFORMANCE SUMMARY (T
A
= 25
°
C)
Table 1.
Step-Down Converter (V
OUT
= 5V)
PARAMETER VALUE
V
IN
7V to 25V
V
OUT
5.0V
I
OUT
2A(max)
Efficiency Up to 90% at 1A out and up to 89% at
2A output
EFFICIENCY (%)
LOAD CURRENT (mA)
10k10
90
45
100 1k
50
55
60
65
70
75
80
85
V
IN
= 24V
V
IN
= 18V
V
IN
= 12V
V
IN
= 8V
Figure 2.
Typical Efficiency of DC476A LT1765EFE, V
OUT
= 5V
Table 2.
Step-Down Converter (with V
OUT
= 3.3V)
PARAMETER VALUE
V
IN
4.7V to 18V (up to 25V)
V
OUT
3.3V
I
OUT
2A(max)
Efficiency Up to 88% at 1A out and up to 85%
at 2A output
EFFICIENCY (%)
LOAD CURRENT (mA)
10k10
90
45
100 1k
50
55
60
65
70
75
80
85
V
IN
= 18V
V
IN
= 12V
V
IN
= 8V
V
IN
= 5V
Figure 3.
Typical Efficiency of DC476A LT1765EFE, V
OUT
= 3.3V
QUICK START PROCEDURE
Demonstration circuit 476 is easy to set up to evaluate
the performance of the LT1765EFE. Refer to Figure 4 for
proper measurement equipment setup and follow the
procedure below:
1.
Connect the 7–25V or 4.7–18V input power supply to
the VIN and GND terminals on the board.
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 VOUT and GND terminals on the board.
4.
Connect an ammeter in series with output load to
measure output current.
5.
Connect a voltmeter across the VIN and GND termi-
nals to measure input voltage.
6.
Connect a voltmeter across the VOUT and GND termi-
nal to measure output voltage.
7.
After 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).
8.
The synchronization and shutdown functions are op-
tional and their terminals can be left floating (discon-
nected) if their functions are not being used.
MINIMUM INPUT VOLTAGE
The minimum LT1765EFE operating input voltage is
3.0V. Nevertheless, a DC/DC buck converter must have
an input voltage that is greater than the output voltage
by a certain margin in order to provide the desired regu-
lated output voltage. Maximum duty cycle, switch on-
resistance, and inductor and diode DC losses all play a
2
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476
1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER
part in determining the minimum input voltage for a se-
lected output over the full current range of the applica-
tion. For this demonstration board, with up to 2A output
current, the minimum input voltages required to main-
tain regulated output voltages is listed below. Custom-
izable output voltages of 2.5V and 1.8V are very com-
mon 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 out-
put voltage can drop accordingly from the programmed
output voltage. This mode is called maximum duty cycle
drop-out.
Table 3.
Minimum Input Voltage vs Output Voltage
MINIMUM INPUT VOLTAGE VIN (V) OUTPUT VOLTAGE (V)
7 5
4.7 3.3
3.7 2.5
3 1.8
CUSTOMIZING THE BOARD
The components used in this demonstration circuit are
optimized for a wide input voltage range. Nevertheless,
the bandwidth can be increased for more specific input
voltages such as 12V±10% or 5V±10% if desired. Some
typical applications are shown in Figure 5 to Figure 11.
The adjustable feedback resistors allow the output volt-
age to be customized. For output voltages below 3.3V,
the boost diode should be moved from D2 to D3 to pro-
vide the minimum boost 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 the boost diode is placed
in D2. However, the boost capacitor must have a voltage
rating greater than the input voltage whenever the boost
diode is placed in position D3.
+
–
LOAD
+–
+–
+–
+–
+–
+–
Figure 4.
Proper Measurement Equipment Setup
3
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476
1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER
Ceramic
Other frequency comp. option... R3=2.4k, C3=10nF, C7=open
C4
100pF
C7
100pF
D3 CMDSH-3
R2
10.0k
C3
4.7nF
C5
22uF 6.3V X5R 1206
C8
0.22uF, 16V
R1
10.7k
10-14Vin
Vin Vout
2.5V @2.5A
U1
LT1765EFE
/Shdn
FB
Vc
Gnd
Boost
Vsw
Sync
Vin
Ceramic
R3
3.3k
C2
2.2uF 25V X7R 1206
L1
CDRH5D28-2R5M
D1
B320A
Figure 5.
Modifications to the Demo Circuit that Optimize the Load Response for V
IN
= 10V–14V, V
OUT
= 2.5V at 2.5A
Vin
C3
1000pF
C2
2.2uF 25V X7R 1206
Vout
Ceramic
C5
10uF 6.3V X5R 0805
D2 CMDSH-3
C8
0.22uF
3.3V @2A
R1
17.4k
D1
B220A
L1
CDRH5D28-2R5M
12V +/-10%
R3
4.7k
C4
100pF R2
10.0k
Ceramic
C7
62pF
U1
LT1765EFE
/Shdn
FB
Vc
Gnd
Boost
Vsw
Sync
Vin
Figure 6.
Modifications to the Demo Circuit that Optimize the Load Response for V
IN
= 12V±10%, V
OUT
= 3.3V at 2A
C8
0.22uF
R1
31.6k
VoutVin
Ceramic
C3
10nF
R3
2.4k
U1
LT1765EFE
/Shdn
FB
Vc
Gnd
Boost
Vsw
Sync
Vin
C2
2.2uF 25V X7R 1206
R2
10.0k
12V +/-10%
D1
B220A
D2 CMDSH-3
C5
10uF 6.3V X5R 0805
L1
A916CY-2R7M
5V @2A
Ceramic
C7
62pF
C4
100pF
Figure 7.
Modifications to the Demo Circuit that Optimize the Load Response for V
IN
= 12V±10%, V
OUT
= 5V at 2A
4
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476
1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER
VoutVin
C7
62pF
U1
LT1765EFE
/Shdn
FB
Vc
Gnd
Boost
Vsw
Sync
Vin
R3
2.4k
C2
2.2uF 25V X7R 1206
C5
10uF 6.3V X5R 0805
Ceramic
R2
10.0k
D2 CMDSH-3
D1
B220A
R1
17.4k
14-16V
C3
1800pF
C4
100pF
3.3V @2.5A
L1
CDRH5D28-2R5M
Ceramic
C8
0.22uF
Figure 8.
Modifications to the Demo Circuit that Optimize the Load Response for V
IN
= 14V–16V, V
OUT
= 3.3V at 2.5A
D1
B220A
U1
LT1765EFE
/Shdn
FB
Vc
Gnd
Boost
Vsw
Sync
Vin
3-5.5V
Vout
C8
0.22uF
C4
100pF
Vin
C3
10nF
C2
2.2uF 10V X5R 0805
R3
1.0k
R1
4.99k
Ceramic
D3 CMDSH-3
C5
10uF 6.3V X5R 0805
L1
CDRH4D28-1R2M
R2
10.0k
Ceramic
1.8V @2A
Figure 9.
Modifications to the Demo Circuit that Optimize the Load Response for V
IN
= 3V–5.5V, V
OUT
= 1.8V at 2A
Vout
U1
LT1765EFE
/Shdn
FB
Vc
Gnd
Boost
Vsw
Sync
Vin
C7
62pF
Vin
C4
100pF
R1
17.4k
5-15V
L1
CDRH6D28-3R0M
Ceramic
C3
10nF
R2
10.0k
R3
3.3k
C2
2.2uF 25V X7R 1206
D1
B220A
C8
0.22uF
Ceramic
D2 CMDSH-3
C5
22uF 6.3V X5R 1206
3.3V @2A
Figure 10.
Modifications to the Demo Circuit that Optimize the Load Response for V
IN
= 5V–15V, V
OUT
= 3.3V at 2A
5
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476
1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER
C3
10nF
D3 CMDSH-3
R3
2.2k
U1
LT1765EFE
/Shdn
FB
Vc
Gnd
Boost
Vsw
Sync
Vin
R1
10.7k
Vin Vout
C5
10uF 6.3V X5R 0805
C4
100pF
Ceramic
C2
2.2uF 10V X5R 0805
D1
B220A R2
10.0k
Ceramic
2.5V @2A
C8
0.22uF
5V +/-10%
L1
CDRH4D28-1R2M
Figure 11.
Modifications to the Demo Circuit that Optimize the Load Response for V
IN
= 5V±10%, V
OUT
= 2.5V at 2A
6
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