LTC3707-SYNC Ver la hoja de datos (PDF) - Linear Technology

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LTC3707-SYNC

High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator

Linear Technology 

LTC3707-SYNC

APPLICATIO S I FOR ATIO

INTVCC Regulator

An internal P-channel low dropout regulator produces 5V

at the INTVCC pin from the VIN supply pin. INTVCC powers

the drivers and internal circuitry within the LTC3707-

SYNC. The INTVCC pin regulator can supply a peak current

of 50mA and must be bypassed to ground with a mini-

mum of 4.7µF tantalum, 10µF special polymer, or low

ESR type electrolytic capacitor. A 1µF ceramic capacitor

placed directly adjacent to the INTVCC and PGND IC pins

is highly recommended. Good bypassing is necessary to

supply the high transient currents required by the␣ MOSFET

gate drivers and to prevent interaction between channels.

Higher input voltage applications in which large MOSFETs

are being driven at high frequencies may cause the maxi-

mum junction temperature rating for the IC to be ex-

ceeded. The system supply current is normally dominated

by the gate charge current. Additional external loading of

the INTVCC and 3.3V linear regulators also needs to be

taken into account for the power dissipation calculations.

The total INTVCC current can be supplied by either the 5V

internal linear regulator or by the EXTVCC input pin. When

the voltage applied to the EXTVCC pin is less than 4.7V, all

of the INTVCC current is supplied by the internal 5V linear

regulator. Power dissipation for the IC in this case is

highest: (VIN)(IINTVCC), and overall efficiency is lowered.

The gate charge current is dependent on operating fre-

quency as discussed in the Efficiency Considerations

section. The junction temperature can be estimated by

using the equations given in Note 2 of the Electrical

Characteristics. For example, the IC VIN current is limited

to less than 24mA from a 24V supply when not using the

EXTVCC pin as follows:

TJ = 70°C + (24mA)(24V)(95°C/W) = 125°C

Use of the EXTVCC input pin reduces the junction tempera-

ture to:

TJ = 70°C + (24mA)(5V)(95°C/W) = 81°C

Dissipation should be calculated to also include any added

current drawn from the internal 3.3V linear regulator. To

prevent maximum junction temperature from being ex-

ceeded, the input supply current must be checked operat-

ing in continuous mode at maximum VIN.

EXTVCC Connection

The IC contains an internal P-channel MOSFET switch

connected between the EXTVCC and INTVCC pins. When

the voltage applied to EXTVCC rises above 4.7V, the inter-

nal regulator is turned off and the switch closes, connect-

ing 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 4.5V. This

allows the MOSFET driver and control power to be derived

from the output during normal operation (4.7V < 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 added between the EXTVCC

and INTVCC pins. Do not apply greater than 7V to the

EXTVCC pin and ensure that EXTVCC␣ <␣ VIN.

Significant efficiency gains can be realized by powering

INTVCC from the output, since the VIN current resulting

from the driver and control currents will be scaled by a

factor of (Duty Cycle)/(Efficiency). For 5V regulators this

supply means connecting the EXTVCC pin directly to VOUT.

However, for 3.3V and other lower voltage regulators,

additional circuitry is required to derive INTVCC power

from the output.

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 5V regulator resulting in

an efficiency penalty of up to 10% at high input voltages.

2. EXTVCC Connected directly to VOUT. This is the normal

connection for a 5V regulator and provides the highest

efficiency.

3. EXTVCC Connected to an External supply. If an external

supply is available in the 5V to 7V range, it may be used to

power EXTVCC providing it is compatible with the MOSFET

gate drive requirements.

4. EXTVCC Connected to an Output-Derived Boost Net-

work. For 3.3V and other low voltage regulators, efficiency

gains can still be realized by connecting EXTVCC to an

output-derived voltage that has been boosted to greater

than 4.7V. This can be done with either the inductive boost

3707sf

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