DC476A - Demo 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 constant 1.25MHz switching frequency allows for the use
of tiny, surface mount external components. The current-mode control topology yields fast transient response and good loop stability, requiring a minimum
number of external compensation components and
allowing the use of ceramic input and output capacitors. The low RDS(ON) internal power switch (0.09Ω)
maintains high efficiencies (as high as 90%) over a
wide range of input voltages and loads. Its 15µA shutdown current (activated via the SHDN pin) extends
battery life. The wide VIN range of the LT1765 allows
step-down configurations from 3V to 25V input. Synchronization of switching frequencies up to 2MHz is
possible.
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 applications 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 pulseskipping during high-temperature, high-voltage conditions and maintain
control of switch current.
The demonstration circuit is designed to provide either
3.3V at 2A or 5V at 2A output, from a 7V–25V input or
Figure 1. Demonstration Circuit 476
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476
1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER
TYPICAL PERFORMANCE SUMMARY (TA = 25°C)
Table 1. Step-Down Converter (VOUT = 5V)
Table 2. Step-Down Converter (with VOUT = 3.3V)
PARAMETER
VALUE
PARAMETER
VALUE
VIN
7V to 25V
VIN
4.7V to 18V (up to 25V)
VOUT
5.0V
VOUT
3.3V
IOUT
2A(max)
IOUT
2A(max)
Efficiency
Up to 90% at 1A out and up to 89% at
2A output
Efficiency
Up to 88% at 1A out and up to 85%
at 2A output
90
80
VIN = 24V
75
70
65
60
85
75
VIN = 18V
70
65
60
55
55
50
50
45
VIN = 5V
VIN = 8V
VIN = 12V
80
EFFICIENCY (%)
85
EFFICIENCY (%)
90
VIN = 8V
VIN = 12V
VIN = 18V
45
10
1k
100
LOAD CURRENT (mA)
10k
Figure 2. Typical Efficiency of DC476A LT1765EFE, VOUT = 5V
10
1k
100
LOAD CURRENT (mA)
10k
Figure 3. Typical Efficiency of DC476A LT1765EFE, VOUT = 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.
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 (disconnected) if their functions are not being used.
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.
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 regulated output voltage. Maximum duty cycle, switch onresistance, 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 selected output over the full current range of the application. For this demonstration board, with up to 2A output
current, the minimum input voltages required to maintain regulated output voltages is listed below. Customizable output voltages of 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 accordingly from the programmed
output voltage. This mode is called maximum duty cycle
drop-out.
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 voltage to be customized. For output voltages below 3.3V,
the boost diode should be moved from D2 to D3 to provide 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.
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
–
+
+
+
–
–
–
+
+
+
–
– +
–
LOAD
Figure 4. Proper Measurement Equipment Setup
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476
1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER
10-14Vin
2.5V @2.5A
Vin
D3 CMDSH-3
LT1765EFE
Vin
C2
2.2uF 25V X7R 1206
Boost
U1
C8
0.22uF, 16V
CDRH5D28-2R5M
R1
FB
R3
10.7k
Gnd
Vc
C3
4.7nF
L1
Vsw
Sync
/Shdn
Ceramic
Vout
C4
100pF
C7
100pF
D1
R2
10.0k
B320A
C5
22uF 6.3V X5R 1206
Ceramic
3.3k
Other frequency comp. option... R3=2.4k, C3=10nF, C7=open
Figure 5. Modifications to the Demo Circuit that Optimize the Load Response for VIN = 10V–14V, VOUT = 2.5V at 2.5A
12V +/-10%
3.3V @2A
Vin
LT1765EFE
Vin
C2
2.2uF 25V X7R 1206
Boost
U1
C8
0.22uF
D2 CMDSH-3
L1
Vsw
CDRH5D28-2R5M
Sync
/Shdn
Ceramic
R1
FB
R3
17.4k
Gnd
Vc
C3
1000pF
Vout
C4
100pF
C7
62pF
D1
R2
10.0k
B220A
C5
10uF 6.3V X5R 0805
Ceramic
4.7k
Figure 6. Modifications to the Demo Circuit that Optimize the Load Response for VIN = 12V±10%, VOUT = 3.3V at 2A
12V +/-10%
5V @2A
Vin
LT1765EFE
Vin
C2
2.2uF 25V X7R 1206
Boost
U1
C8
0.22uF
D2 CMDSH-3
A916CY-2R7M
R1
FB
R3
2.4k
31.6k
Gnd
Vc
C3
10nF
L1
Vsw
Sync
/Shdn
Ceramic
C4
100pF
Vout
D1
B220A
C7
62pF
R2
10.0k
C5
10uF 6.3V X5R 0805
Ceramic
Figure 7. Modifications to the Demo Circuit that Optimize the Load Response for VIN = 12V±10%, VOUT = 5V at 2A
4
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476
1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER
14-16V
3.3V @2.5A
Vin
LT1765EFE
Vin
C2
2.2uF 25V X7R 1206
Boost
U1
C8
0.22uF
D2 CMDSH-3
L1
Vsw
CDRH5D28-2R5M
Sync
/Shdn
Ceramic
R1
FB
17.4k
Gnd
Vc
C3
1800pF
R3
Vout
C4
100pF
C7
62pF
D1
R2
10.0k
B220A
C5
10uF 6.3V X5R 0805
Ceramic
2.4k
Figure 8. Modifications to the Demo Circuit that Optimize the Load Response for VIN = 14V–16V, VOUT = 3.3V at 2.5A
3-5.5V
1.8V @2A
Vin
D3 CMDSH-3
LT1765EFE
Vin
C2
2.2uF 10V X5R 0805
Boost
U1
Vout
C8
0.22uF
L1
Vsw
CDRH4D28-1R2M
Sync
/Shdn
Ceramic
R1
FB
10nF
R3
4.99k
Gnd
Vc
C3
C4
100pF
D1
R2
10.0k
B220A
C5
10uF 6.3V X5R 0805
Ceramic
1.0k
Figure 9. Modifications to the Demo Circuit that Optimize the Load Response for VIN = 3V–5.5V, VOUT = 1.8V at 2A
5-15V
3.3V @2A
Vin
LT1765EFE
Vin
C2
2.2uF 25V X7R 1206
Boost
U1
C8
0.22uF
D2 CMDSH-3
CDRH6D28-3R0M
R1
FB
R3
3.3k
17.4k
Gnd
Vc
C3
10nF
L1
Vsw
Sync
/Shdn
Ceramic
C4
100pF
Vout
D1
B220A
C7
62pF
R2
10.0k
C5
22uF 6.3V X5R 1206
Ceramic
Figure 10. Modifications to the Demo Circuit that Optimize the Load Response for VIN = 5V–15V, VOUT = 3.3V at 2A
5
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476
1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER
5V +/-10%
2.5V @2A
Vin
D3 CMDSH-3
LT1765EFE
Vin
C2
2.2uF 10V X5R 0805
Boost
U1
C8
0.22uF
CDRH4D28-1R2M
R1
FB
R3
2.2k
10.7k
Gnd
Vc
C3
10nF
L1
Vsw
Sync
/Shdn
Ceramic
Vout
C4
100pF
D1
B220A
R2
10.0k
C5
10uF 6.3V X5R 0805
Ceramic
Figure 11. Modifications to the Demo Circuit that Optimize the Load Response for VIN = 5V±10%, VOUT = 2.5V at 2A
6
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