LTM4607 datasheet(21/30 Pages) LINER

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LTC3780

3780ff

For more information www.linear.com/LTC3780

applicaTions inForMaTion

EXTVCC Connection

The LTC3780 contains an internal P-channel MOSFET

switch connected between the EXTVCC and INTVCC pins.

When the voltage applied to EXTVCC rises above 5.7V, the

internal regulator is turned off and a switch connects the

EXTVCC pin to the INTVCC pin thereby supplying internal

power. The switch remains closed as long as the voltage

applied to EXTVCC remains above 5.5V. This allows the

MOSFET driver and control power to be derived from the

output when (5.7V < VOUT < 7V) and from the internal

regulator when the output is out of regulation (start-up,

short-circuit). If more current is required through the

EXTVCC switch than is specified, an external Schottky

diode can be interposed between the EXTVCC and INTVCC

pins. Ensure that EXTVCC ≤ VIN.

The following list summarizes the three possible connec-

tions for EXTVCC:

1. EXTVCC left open (or grounded). This will cause INTVCC

to be powered from the internal 6V regulator at the cost

of a small efficiency penalty.

2. EXTVCC connected directly to VOUT (5.7V < VOUT < 7V).

This is the normal connection for a 6V regulator and

provides the highest efficiency.

3. EXTVCC connected to an external supply. If an external

supply is available in the 5.5V to 7V range, it may be

used to power EXTVCC provided it is compatible with

the MOSFET gate drive requirements.

Output Voltage

The LTC3780 output voltage is set by an external feedback

resistivedividercarefullyplacedacrosstheoutputcapacitor.

Theresultantfeedbacksignaliscomparedwiththeinternal

precision 0.800V voltage reference by the error amplifier.

The output voltage is given by the equation:

OUT =+









.•

Topside MOSFET Driver Supply (CA, DA, CB, DB)

Referring to Figure 11, the external bootstrap capacitors

CA and CB connected to the BOOST1 and BOOST2 pins

supply the gate drive voltage for the topside MOSFET

switches A and D. When the top MOSFET switch A turns

on, the switch node SW2 rises to VIN and the BOOST2

pin rises to approximately VIN + INTVCC. When the bottom

MOSFET switch B turns on, the switch node SW2 drops

to low and the boost capacitor CB is charged through DB

from INTVCC. When the top MOSFET switch D turns on,

the switch node SW1 rises to VOUT and the BOOST1 pin

rises to approximately VOUT + INTVCC. When the bottom

MOSFET switch C turns on, the switch node SW1 drops

to low and the boost capacitor CA is charged through DA

from INTVCC. The boost capacitors CA and CB need to

store about 100 times the gate charge required by the top

MOSFET switch A and D. In most applications a 0.1µF to

0.47µF, X5R or X7R dielectric capacitor is adequate.

Run Function

The RUN pin provides simple ON/OFF control for the

LTC3780. Driving the RUN pin above 1.5V permits the

controller to start operating. Pulling RUN below 1.5V puts

theLTC3780intolowcurrentshutdown.Donotapplymore

than 6V to the RUN pin.

Soft-Start Function

Soft-start reduces the input power sources’ surge cur-

rents by gradually increasing the controller’s current

limit (proportional to an internally buffered and clamped

equivalent of VITH).

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

pacitor. As the voltage on SS increases from 0V to 2.4V,

the internal current limit rises from 0V/RSENSE to 150mV/

RSENSE. The output current limit ramps up slowly, taking

1.5s/µFtoreachfullcurrent.Theoutputcurrentthusramps

up slowly, eliminating the starting surge current required

from the input power supply.

TIRMP =

2.4V

1.2µA

• CSS = 1.5s/µF

(

)•CSS

Do not apply more than 6V to the SS pin.

Current foldback is disabled during soft-start until the

voltage on CSS reaches 2V. Make sure CSS is large enough

when there is loading during start-up.

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