LTC3707IGN datasheet(19/32 Pages) LINER

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LTC3707

3707fb

APPLICATIONS INFORMATION

Topside MOSFET Driver Supply (CB, DB)

External bootstrap capacitors CB connected to the BOOST

pins supply the gate drive voltages for the topside MOSFETs.

Capacitor CB in the functional diagram is charged though

external diode DB from INTVCC when the SW pin is low.

When one of the topside MOSFETs is to be turned on,

the driver places the CB voltage across the gate-source

of the desired MOSFET. This enhances the MOSFET and

turns on the topside switch. The switch node voltage, SW,

rises to VIN and the BOOST pin follows. With the topside

MOSFET on, the boost voltage is above the input supply:

VBOOST = VIN + VINTVCC. The value of the boost capacitor

CB needs to be 100 times that of the total input capacitance

of the topside MOSFET(s). The reverse breakdown of the

external Schottky diode must be greater than VIN(MAX).

When adjusting the gate drive level, the ﬁnal arbiter is the

total input current for the regulator. If a change is made

and the input current decreases, then the efﬁciency has

improved. If there is no change in input current, then there

is no change in efﬁciency.

Output Voltage

The LTC3707 output voltages are each set by an external

feedback resistive divider carefully placed across the output

capacitor. The resultant feedback signal is compared with

the internal precision 0.800V voltage reference by the error

ampliﬁer. The output voltage is given by the equation:

OUT = 0.8V 1+

⎛

⎝⎜

⎞

⎠⎟

SENSE+/SENSE– Pins

The common mode input range of the current comparator

sense pins is from 0V to (1.1)INTVCC. Continuous linear

operation is guaranteed throughout this range allowing

output voltage setting from 0.8V to 7.7V, depending upon

the voltage applied to EXTVCC. A differential NPN input

stage is biased with internal resistors from an internal 2.4V

source as shown in the Functional Diagram. This requires

that current either be sourced or sunk from the SENSE

pins depending on the output voltage. If the output voltage

is below 2.4V current will ﬂow out of both SENSE pins to

the main output. The output can be easily preloaded by

the VOUT resistive divider to compensate for the current

comparator’s negative input bias current. The maximum

current ﬂowing out of each pair of SENSE pins is:

ISENSE+ + ISENSE– = (2.4V – VOUT)/24k

Since VOSENSE is servoed to the 0.8V reference voltage,

we can choose R1 in Figure 2 to have a maximum value

to absorb this current.

(MAX) = 24k

0.8V

2.4V – V

OUT

⎛

⎝⎜

⎞

⎠⎟

for VOUT < 2.4V

Regulating an output voltage of 1.8V, the maximum value

of R1 should be 32K. Note that for an output voltage above

2.4V, R1 has no maximum value necessary to absorb the

sense currents; however, R1 is still bounded by the VOSENSE

feedback current.

Soft-Start/Run Function

The RUN/SS1 and RUN/SS2 pins are multipurpose pins

that provide a soft-start function and a means to shut down

the LTC3707. Soft-start reduces the input power source’s

surge currents by gradually increasing the controller’s

current limit (proportional to VITH). This pin can also be

used for power supply sequencing.

An internal 1.2μA current source charges up the CSS ca-

pacitor.When the voltage on RUN/SS1 (RUN/SS2) reaches

1.5V, the particular controller is permitted to start operating.

As the voltage on RUN/SS increases from 1.5V to 3.0V,

the internal current limit is increased from 25mV/RSENSE

to 75mV/RSENSE. The output current limit ramps up slowly,

taking an additional 1.25s/μF to reach full current. The

output current thus ramps up slowly, reducing the start-

ing surge current required from the input power supply.

If RUN/SS has been pulled all the way to ground there is

a delay before starting of approximately:

DELAY =

1.5V

1.2µA

SS = 1.25s / µF

()C

IRAMP =

−1.5V

1.2µA

SS = 1.25s / µF

()C

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