LTC3707IGN datasheet(11/32 Pages) LINER

11 / 32 page

LTC3707

3707fb

OPERATION

be forced back into the main power supply potentially

boosting the input supply to dangerous voltage levels—

BEWARE!

Frequency Setting

The FREQSET pin provides frequency adjustment of the

internal oscillator from approximately 140kHz to 310kHz.

This input is nominally biased through an internal resistor

to the 1.19V reference, setting the oscillator frequency to

approximately 220kHz. This pin can be driven from an ex-

ternal AC or DC signal source to control the instantaneous

frequency of the oscillator. Voltage should not be applied

to the FREQSET pin prior to the application of voltage to

the VIN pin.

INTVCC/EXTVCC Power

Power for the top and bottom MOSFET drivers and most

other internal circuitry is derived from the INTVCCpin.When

the EXTVCC pin is left open, an internal 5V low dropout

linear regulator supplies INTVCC power. If EXTVCC is taken

above 4.7V, the 5V regulator is turned off and an internal

switch is turned on connecting EXTVCC to INTVCC. This

allows the INTVCCpowertobederivedfromahighefﬁciency

external source such as the output of the regulator itself

or a secondary winding, as described in the Applications

Information.

Standby Mode Pin

The STBYMD pin is a three-state input that controls com-

mon circuitry within the IC as follows: When the STBYMD

pin is held at ground, both controller RUN/SS pins are pulled

to ground providing a single control pin to shut down both

controllers. When the pin is left open, the internal RUN/SS

currents are enabled to charge the RUN/SS capacitor(s),

allowing the turn-on of either controller and activating

necessary common internal biasing. When the STBYMD

pin is taken above 2V, both internal linear regulators are

turned on independent of the state on the RUN/SS pins

of the two switching regulator controllers, providing an

output power source for “wake-up” circuitry. Decouple the

pin with a small capacitor (0.01μF) to ground if the pin is

not connected to a DC potential.

Output Overvoltage Protection

An overvoltage comparator, 0V, guards against transient

overshoots (>7.5%) as well as other more serious condi-

tions that may overvoltage the output. In this case, the top

MOSFET is turned off and the bottom MOSFET is turned

on until the overvoltage condition is cleared.

Power Good (PGOOD) Pin

The PGOOD pin is connected to an open drain of an in-

ternal MOSFET. The MOSFET turns on and pulls the pin

low when both the outputs are not within

±7.5% of their

nominal output levels as determined by their resistive

feedback dividers. When both outputs meet the

±7.5%

requirement, the MOSFET is turned off within 10μs and

the pin is allowed to be pulled up by an external resistor

to a source of up to 7V.

Foldback Current, Short-Circuit Detection

and Short-Circuit Latchoff

The RUN/SS capacitors are used initially to limit the inrush

current of each switching regulator. After the controller

has been started and been given adequate time to charge

up the output capacitors and provide full load current, the

RUN/SS capacitor is used in a short-circuit time-out circuit.

If the output voltage falls to less than 70% of its nominal

output voltage, the RUN/SS capacitor begins discharging

on the assumption that the output is in an overcurrent

and/or short-circuit condition. If the condition lasts for

a long enough period as determined by the size of the

RUN/SS capacitor, the controllers will be shut down until

the RUN/SS pin(s) voltage(s) are recycled. This built-in

latchoff can be overridden by providing a >5μA pull-up at

a compliance of 4.2V to the RUN/SS pin(s). This current

shortens the soft start period but also prevents net dis-

charge of the RUN/SS capacitor(s) during an overcurrent

and/or short-circuit condition. Foldback current limiting

is also activated when the output voltage falls below

70% of its nominal level whether or not the short-circuit

latchoff circuit is enabled. Even if a short is present and

the short-circuit latchoff is not enabled, a safe, low output

current is provided due to internal current foldback and

actual power dissipated is low due to the efﬁcient nature

of the current mode switching regulator.

(Refer to Functional Diagram)

LTC3707IGN Datasheet(PDF) 11 Page - Linear Technology

LTC3707IGN Datasheet(HTML) 11 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL