LTC3722, LTC4440, LTC3901, AND LT1431

2

OPERATING PRINCIPLES

The LTC3722 Synchronous Dual Mode Phase Modu-

lated Full Bridge Controller is used on the primary and

works together with the LTC3901 Secondary Side

Synchronous Driver to provide a synchronous recti-

fied output. When an input voltage is applied, the

LTC3722 begins a controlled soft-start of the output

voltage. As this voltage begins to rise, the LT1431

Programmable Reference is quickly powered up via

the output voltage. The LT1431 provides feedback via

optocoupler ISO1 to set the output voltage at 12V. The

LTC4440 High Voltage Gate Driver is used to level

shift the high side primary MOSFETs gate signals. The

LTC3722 provides precise control of gate signals to

primary MOSFETs and secondary MOSFETs via T3

and U1 (LTC3901). The LTC3901 includes a timer and

current sense to limit reverse inductor current.

For large values of input inductance, a 100V, 47uF

electrolytic capacitor can be added across the input

terminals to damp the input filter and provide ade-

quate stability.

See Linear Technology Application

Note AN19 for a discussion on input filter stability

analysis.

A recommended part is the Sanyo

100MV47AX.

QUICK START PROCEDURE

Demonstration circuit 607 is easy to set up to evaluate

the performance of the LTC3722, LTC4440, LTC3901,

and LT1431. Refer to Figure 1 for proper measurement

equipment setup and follow the procedure below:

NOTE. When measuring the input or output voltage ripple, care must be

taken to avoid a long ground lead on the oscilloscope probe. Measure the

output (or input) voltage ripple by touching the probe tip and probe ground

directly across the +Vout and –Vout (or +Vin and –Vin) terminals or in-

put/output capacitors. See Figure 2 for proper scope probe technique.

1.

Set an input power supply that is capable of 36V to

72V to a voltage of 36V. Then turn off the supply.

2.

With power off, connect the supply to the input ter-

minals +Vin and –Vin.

a.

Input voltages lower than 36V can keep the con-

verter from turning on due to the undervoltage

lockout feature of the LTC3722.

b.

If efficiency measurements are desired, an am-

meter capable of measuring 15Adc or a resistor

shunt can be put in series with the input supply

in order to measure the DC607A’s input current.

c.

A voltmeter with a capability of measuring at

least 72V can be placed across the input termi-

nals in order to get an accurate input voltage

measurement.

3.

Turn on the power at the input.

NOTE. Make sure that the input voltage never exceeds 72V.

4.

Check for the proper output voltage of 12V ± 1%.

Turn off the power at the input.

5.

Once the proper output voltage is established, con-

nect a variable load capable of sinking 35A at 12V to

the output terminals +Vout and –Vout. Set the cur-

rent for 0A.

a.

If efficiency measurements are desired, an am-

meter or a resistor shunt that is capable of han-

dling 35Adc can be put in series with the output

load in order to measure the DC607A’s output

current.

b.

A voltmeter with a capability of measuring at

least 12V can be placed across the output termi-

nals in order to get an accurate output voltage

measurement.

6.

Turn on the power at the input.

NOTE. If there is no output, temporarily disconnect the load to make

sure that the load is not set too high.

7.

Once the proper output voltage is again established,

adjust the load within the operating range and ob-

serve the output voltage regulation, ripple voltage,

efficiency and other desired parameters.