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

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

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476

1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER

TYPICAL PERFORMANCE SUMMARY (T

= 25

Table 1.

Step-Down Converter (V

OUT

= 5V)

PARAMETER VALUE

7V to 25V

OUT

5.0V

OUT

2A(max)

Efficiency Up to 90% at 1A out and up to 89% at

2A output

EFFICIENCY (%)

LOAD CURRENT (mA)

10k10

100 1k

= 24V

= 18V

= 12V

= 8V

Figure 2.

Typical Efficiency of DC476A LT1765EFE, V

OUT

= 5V

Table 2.

Step-Down Converter (with V

OUT

= 3.3V)

PARAMETER VALUE

4.7V to 18V (up to 25V)

OUT

3.3V

OUT

2A(max)

Efficiency Up to 88% at 1A out and up to 85%

at 2A output

EFFICIENCY (%)

LOAD CURRENT (mA)

10k10

100 1k

= 18V

= 12V

= 8V

= 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:

Connect the 7–25V or 4.7–18V input power supply to

the VIN and GND terminals on the board.

Connect an ammeter in series with the input supply to

measure input current.

Connect either power resistors or an electronic load

to the VOUT and GND terminals on the board.

Connect an ammeter in series with output load to

measure output current.

Connect a voltmeter across the VIN and GND termi-

nals to measure input voltage.

Connect a voltmeter across the VOUT and GND termi-

nal to measure output voltage.

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).

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

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

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

100pF

D3 CMDSH-3

10.0k

4.7nF

22uF 6.3V X5R 1206

0.22uF, 16V

10.7k

10-14Vin

Vin Vout

2.5V @2.5A

LT1765EFE

/Shdn

Gnd

Boost

Vsw

Sync

Vin

Ceramic

3.3k

2.2uF 25V X7R 1206

CDRH5D28-2R5M

B320A

Figure 5.

Modifications to the Demo Circuit that Optimize the Load Response for V

= 10V–14V, V

OUT

= 2.5V at 2.5A

Vin

1000pF

2.2uF 25V X7R 1206

Vout

Ceramic

10uF 6.3V X5R 0805

D2 CMDSH-3

0.22uF

3.3V @2A

17.4k

B220A

CDRH5D28-2R5M

12V +/-10%

4.7k

100pF R2

10.0k

Ceramic

62pF

LT1765EFE

/Shdn

Gnd

Boost

Vsw

Sync

Vin

Figure 6.

Modifications to the Demo Circuit that Optimize the Load Response for V

= 12V±10%, V

OUT

= 3.3V at 2A

0.22uF

31.6k

VoutVin

Ceramic

10nF

2.4k

LT1765EFE

/Shdn

Gnd

Boost

Vsw

Sync

Vin

2.2uF 25V X7R 1206

10.0k

12V +/-10%

B220A

D2 CMDSH-3

10uF 6.3V X5R 0805

A916CY-2R7M

5V @2A

Ceramic

62pF

100pF

Figure 7.

Modifications to the Demo Circuit that Optimize the Load Response for V

= 12V±10%, V

OUT

= 5V at 2A

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476

1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER

VoutVin

62pF

LT1765EFE

/Shdn

Gnd

Boost

Vsw

Sync

Vin

2.4k

2.2uF 25V X7R 1206

10uF 6.3V X5R 0805

Ceramic

10.0k

D2 CMDSH-3

B220A

17.4k

14-16V

1800pF

100pF

3.3V @2.5A

CDRH5D28-2R5M

Ceramic

0.22uF

Figure 8.

Modifications to the Demo Circuit that Optimize the Load Response for V

= 14V–16V, V

OUT

= 3.3V at 2.5A

B220A

LT1765EFE

/Shdn

Gnd

Boost

Vsw

Sync

Vin

3-5.5V

Vout

0.22uF

100pF

Vin

10nF

2.2uF 10V X5R 0805

1.0k

4.99k

Ceramic

D3 CMDSH-3

10uF 6.3V X5R 0805

CDRH4D28-1R2M

10.0k

Ceramic

1.8V @2A

Figure 9.

Modifications to the Demo Circuit that Optimize the Load Response for V

= 3V–5.5V, V

OUT

= 1.8V at 2A

Vout

LT1765EFE

/Shdn

Gnd

Boost

Vsw

Sync

Vin

62pF

Vin

100pF

17.4k

5-15V

CDRH6D28-3R0M

Ceramic

10nF

10.0k

3.3k

2.2uF 25V X7R 1206

B220A

0.22uF

Ceramic

D2 CMDSH-3

22uF 6.3V X5R 1206

3.3V @2A

Figure 10.

Modifications to the Demo Circuit that Optimize the Load Response for V

= 5V–15V, V

OUT

= 3.3V at 2A

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 476

1.25MHZ 3A PEAK SWITCH CURRENT MONOLITHIC STEP-DOWN CONVERTER

10nF

D3 CMDSH-3

2.2k

LT1765EFE

/Shdn

Gnd

Boost

Vsw

Sync

Vin

10.7k

Vin Vout

10uF 6.3V X5R 0805

100pF

Ceramic

2.2uF 10V X5R 0805

B220A R2

10.0k

Ceramic

2.5V @2A

0.22uF

5V +/-10%

CDRH4D28-1R2M

Figure 11.

Modifications to the Demo Circuit that Optimize the Load Response for V

= 5V±10%, V

OUT

= 2.5V at 2A

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