LTC3613 datasheet(19/36 Pages) LINER

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LTC3613

3613fa

Top MOSFET Driver Supply (CB, DB)

An external bootstrap capacitor, CB,connectedtotheBOOST

pin supplies the gate drive voltage for the topside MOSFET.

This capacitor is charged through diode DB from INTVCC

when the switch node is low. When the top MOSFET turns

on, the switch node rises to VIN and the BOOST pin rises to

approximately PVIN + INTVCC. The boost capacitor needs to

store approximately 100 times the gate charge required by

the top MOSFET. In most applications a 0.1μF to 0.47μF, X5R

or X7R dielectric capacitor is adequate. It is recommended

that the BOOST capacitor be no larger than 10% of the

INTVCC capacitor, CVCC, to ensure that the CVCC can supply

the upper MOSFET gate charge and BOOST capacitor under

all operating conditions. Variable frequency in response

to load steps offers superior transient performance but

requires higher instantaneous gate drive. Gate charge

demands are greatest in high frequency low duty factor

applications under high dI/dt load steps and at start-up.

In order to minimize SW node ringing and EMI, connect a

5Ω to 10Ω resistor in series with the BOOST pin. Make the

CB and DB connections on the other side of the resistor. This

series resistor helps to slow down the SW node rise time,

limiting the high dI/dt current through the top MOSFET that

causes SW node ringing.

INTVCC Regulator and EXTVCC Power

The LTC3613 features a PMOS low dropout linear regulator

(LDO) that supplies power to INTVCC from the SVIN supply.

INTVCC powers much of the LTC3613’s internal circuitry.

The LDO regulates the voltage at the INTVCC pin to 5.3V.

The LDO can supply a maximum current of 50mARMS and

must be bypassed to ground with a minimum of 4.7μF

ceramic capacitor. Good bypassing is needed to supply

the high transient currents required by the power MOSFET

gate drivers.

APPLICATIONS INFORMATION

When the voltage applied to EXTVCC pin rises above 4.6V,

the INTVCC LDO is turned off and the EXTVCC is connected

to INTVCC with an internal switch. This switch remains on

as long as the voltage applied to EXTVCC remains above

4.4V. Using the EXTVCC allows the MOSFET driver and

control power to be derived from the LTC3613’s switching

regulator output during normal operation and from the

LDO when the output is out of regulation (e.g., start-up,

short circuit). If more than 50mARMS current is required

through EXTVCC, then an external Schottky diode can be

added between the EXTVCC and INTVCC pins. Do not apply

more than 6V to the EXTVCC pin and make sure that this

external voltage source is less than SVIN.

Significant efficiency and thermal gains can be realized

by powering INTVCC from the switching regulator output,

since the VIN current resulting from the driver and control

currents will be scaled by a factor of (Duty Cycle)/(Switcher

Efficiency).

The following list summarizes the four possible connec-

tions for EXTVCC:

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

to be powered from the internal 5.3V LDO resulting

in an efficiency penalty of up to 10% at high input

voltages.

2. EXTVCCconnecteddirectlytoswitchingregulatoroutput

VOUT > 4.6V. This provides the highest efficiency.

3. EXTVCC connected to an external supply. If a 4.6V or

greater external supply is available, it may be used to

power EXTVCC provided that the external supply is suf-

ficient enough for MOSFET gate drive requirements.

4. EXTVCC connected to an output-derived boost network.

For 3.3V and other low voltage converters, efficiency

gains can still be realized by connecting EXTVCC to an

output-derived voltage that has been boosted to greater

than 4.6V.

LTC3613 Datasheet(PDF) 19 Page - Linear Technology

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