Analog Devices LTC3728EG User manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 542

DUAL PHASE, DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER

LTC3728EG

DESCRIPTION

Demonstration circuit 542 is a dual output, dual

phase step down converter, which is controlled

by the LTC3728EG. Output #1 can be configured

for either 5.0V or 3.3V and output #2 can be

configured for either 3.3V or 2.5V. The maximum

load for both rails is 12A. The input voltage

range of the DC542 is 8V to 24V.

The DC542 also contains a footprint for a 5.5V

boost converter bias supply whose output is

connected to EXTVCC. See the “Optional Bias

Supply” section for more details.

Design files for this circuit board are avail-

able. Call the LTC factory.

Table 1.Performance Summary

PARAMETER CONDITION VALUE

Minimum Input Voltage 8V

Maximum Input Voltage 24V

OUT1

= 8V to 24V, I

OUT

= 0A to 12A 5.0V ± 3% / 3.3V ± 3%

OUT2

= 8V to 24V, I

OUT

= 0A to 12A 3.3V ± 3% / 2.5V ± 3%

QUICK START PROCEDURE

Demonstration circuit 542 is easy to set up. Re-

fer to Figure 1 for the proper setup of the meas-

urement equipment and follow the procedure

below:

1. With power off, connect the input voltage

source and the load. The load cables should

be sized for 12 Amps and the input cables

should be sized for 15 Amps.

2. The jumpers should be placed in their default

positions as shown in Figure 1.

3. If a constant current electronic load is used,

preset the load current to about 1A or less

before applying power to the input. Other-

wise, the foldback current limit function of the

LTC3728 may be triggered during startup.

4. Apply power to the input and set the input

voltage to 12V. The voltage for output #1

should be 5.0V ± 3% and the voltage for out-

put #2 should be 3.3V ± 3%. Increase the

load on both rails to 12.0A. The two outputs

should still be within regulation.

5. Keep the input voltage at 12.0V and the load

at 12.0A for both rails. Measure the DC input

current. It should be less than 9.4A.

6. Remove input power from the converter. Set

the voltage for output #1 to 3.3V using

jumper JP5 and the voltage for output #2 to

2.5V using JP4. Repeat step #4. The voltage

for output #1 should be 3.3V ± 3% and the

voltage for output #2 should be 2.5V ± 3%.

7. Adjust the input voltage and load current to

the desired levels within their limits and ob-

serve the regulation, output ripple, load step

response, efficiency and other parameters.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 542

DUAL PHASE, DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER

Note: When measuring the output voltage ripple,

care must be taken to avoid a long ground lead

on the oscilloscope probe. Connect the probe tip

to VOUT and the probe’s ground ring to GND.

Make the connections as short as possible. For

output #1, measure the ripple directly across

C35 and for output #2, measure across C36.

See Figure 2.

Figure 1. Setup for measurement equipment.

Figure 2. Output ripple measurement.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 542

DUAL PHASE, DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER

OPTIONAL BIAS SUPPLY

The DC542 also contains a footprint for a 5.5V

boost converter bias supply whose output is

connected to EXTVCC. The input of this bias

supply can be connected to one of the following:

1) VIN, if VIN<5V

2) VOUT1 for VOUT1 <5V and VIN>5V.

The latter connection will provide higher effi-

ciency. If this bias supply is stuffed, please keep

the following points in mind:

1) If the input voltage of the optional bias supply

is connected to VIN, then the board will need

to be slightly modified to ensure that the

voltage on the EXTVCC pin of the

LTC3728EG does not exceed the voltage on

the VIN pin. These are the modifications:

a) Remove resistor R8.

b) Tie the (+) end of C3 to the (+) end of

C21.

2) If the input voltage of the optional bias supply

is connected to output #1, precautions need

to be taken to make sure that the voltage on

the EXTVCC pin of the LTC3728EG does not

exceed the voltage on the VIN pin during

turn-on of the main converter. Follow the

steps below:

a) Put jumper JP6 in the “0” position to tie

the RUN1 signal to ground.

b) Turn-on the main input voltage and set to

desired level.

c) Put jumper JP6 in the “1” position to turn-

on output #1.

3) If a different power MOSFET configuration is

used or the switching frequency is increased,

then be sure to measure the current flowing

into the EXTVCC pin and make sure the cur-

rent will not exceed the maximum output cur-

rent of the bias supply. At 3.3V input, the

maximum output current of the bias supply is

200mA.

For further assistance, contact the factory.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 542

DUAL PHASE, DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 542

DUAL PHASE, DUAL OUTPUT SYNCHRONOUS BUCK CONVERTER

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Authorized Distributor

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DC542A

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