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LTC3706

3706fb

APPLICATIONS INFORMATION

Start-Up Considerations

In self-starting applications, the LTC3705 will initially begin

the soft-start of the converter in an open-loop fashion.

After bias is obtained on the secondary side, the LTC3706

assumes control and completes the soft-start interval. In

order to ensure that control is properly transferred from the

LTC3705 (primary-side) to the LTC3706 (secondary-side),

it is necessary to limit the rate of rise on the primary-side

soft-start ramp so that the LTC3706 has adequate time to

wake up and assume control before the output voltage gets

too high. This condition is satisﬁed for many applications

if the following relationship is maintained:

CSS,SEC ≤ CSS PRI

However, care should be taken to ensure that soft-start

transfer from primary-side to secondary-side is completed

well before the output voltage reaches its target value. A

good design goal is to have the transfer completed when

the output voltage is less than one-half of its target value.

Note that the fastest output voltage rise time during pri-

mary-side soft-start mode occurs with maximum input

voltage and minimum load current.

The open-loop start-up frequency on the LTC3705 is set

by placing a resistor from the FB/IN+ pin to GND. Although

the exact start-up frequency on the primary side is not

critical, it is generally good practice to set this approxi-

mately equal to the operating frequency on the secondary

side. The FS/IN– start-up resistor for the LTC3705 may be

selected using the following:

fHz

PRI

FS IN

()

.•

–

32 10

In the event that the secondary-side circuitry fails to

properly start up and assume control of switching, there

are several fail-safe mechanisms to help avoid overvoltage

conditions. First, the LTC3705 contains a volt-second

clamp that will keep the primary-side duty cycle at a level

that cannot produce an overvoltage condition. Second,

the LTC3705 contains a time-out feature that will detect

a FAULT if the LTC3706 fails to start up and deliver PWM

signals to the primary side. Finally, the LTC3706 has an

independent overvoltage detection circuit that will crowbar

the output of the DC/DC converter using the synchronous

MOSFET switch.

In the event that a short circuit is applied to the output of

the DC/DC converter prior to start-up, the LTC3706 will

generally not receive enough bias voltage to operate. In

this case, the LTC3705 will detect a FAULT for one of two

reasons: 1) the start-up time-out feature will be activated

since the LTC3706 never sends signals to the primary side

or 2) the primary-side overcurrent circuit will be tripped

because of current buildup in the output inductor. In either

case, the LTC3705 will initiate a shutdown followed by a

soft-start retry. See the LTC3705 data sheet for further

details.

OPERATION

Slave Mode Operation

When two or more LTC3706 devices are used in PolyPhase

systems, one device becomes the “master” controller, while

the others are used as “slaves.” Slave mode is activated

by connecting the FB pin to VCC. In this mode, the ITH pin

becomes a high impedance input, allowing it to be driven by

the master controller. In this way, equal inductor currents

are established in each of the individual phases. Also, in

slave mode the soft-start charge/discharge currents are

disabled, allowing the master device to control the charging

and discharging of the soft-start capacitor.

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