LTC3706IGN datasheet(15/20 Pages) LINER

15 / 20 page

LTC3706

3706fb

APPLICATIONS INFORMATION

The REGSD resistor should be selected based upon the

steady-state (DC) thermal impedance of the linear regula-

tor pass device.

REGSD

CC MAX

RISE MAX

= 960

θ • ()

()

where

θJA is the DC thermal impedance of the linear

regulator pass device and TRISE(MAX) is the maximum

junction temperature rise desired for the pass device.

The value for ICC(MAX) depends heavily on the particular

switching MOSFETs used, as well as on the details of

overall system design. Note that it may include the bias

current associated with the primary-side gate driver and

controller, if the LTC3705 is being used. The value for ICC

is best determined experimentally and then guard banded

appropriately to establish ICC(MAX). Using the Typical Ap-

plication circuit on the ﬁrst page of this data sheet as an

example, if a SOT-23 MOSFET is chosen, we might have

θJA = 150°C/W, tRISE(MAX) = 50°C and ICC(MAX) = 35mA so

that RREGSD ≈ 100kΩ. In this case, the linear regulator can

run continuously for any VIN voltage that is less than:

4V = (VIN – VCC)(5μs)(RREGSD)

IN MAX

REGSD

() =

⎛

⎝⎜

⎞

⎠⎟

640

or 13.4V. In addition, a capacitor may be added in parallel

with the REGSD resistor to delay the thermal shutdown

and thereby account for the thermal time constant of the

pass device. When using a delay capacitor, care must be

taken to ensure that the safe operating area (SOA) of the

pass device is not exceeded. The capacitor should be

chosen to provide a time constant that is somewhat faster

than the thermal time constant of the pass device in the

system. This technique will allow for much higher transient

power dissipation, which is particularly useful in larger

(PolyPhase) systems that have a higher VCC bias current.

For the above SOT-23 example, a capacitor CREGSD = 1μF

provides a linear regulator shutdown delay given by:

SHDN

REGSD

REGS

()(

)

()

–

640

⎛

⎝

⎜

⎞

⎠

⎟

or 33ms at VIN = 30V. This delay provides ample time for

linear regulator operation during soft-start, while providing

protection for the pass device during fault conditions such

as input overvoltage or output overcurrent.

Current Sensing

The LTC3706 provides considerable ﬂexibility in current

sensing techniques. It supports two main methods: 1)

resistive current sensing and 2) current transformer cur-

rent sensing. Resistive current sensing is generally simpler,

smaller and less expensive, while current transformer

sensing is more efﬁcient and generally appropriate for

higher (>20A) output currents. For resistive current sens-

ing, the sense resistor may be placed in any one of three

different locations: high side inductor, low side inductor

or low side switch, as shown in Figure 3. Sensing the

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