LTC1778IGN datasheet(15/24 Pages) LINER

15 / 24 page

LTC1778/LTC1778-1

1778fb

INTVCC Regulator

An internal P-channel low dropout regulator produces the

5V supply that powers the drivers and internal circuitry

within the LTC1778. The INTVCC pin can supply up to

50mA RMS and must be bypassed to ground with a

minimum of 4.7µF low ESR tantalum capacitor. Good

bypassing is necessary to supply the high transient cur-

rents required by the MOSFET gate drivers. Applications

using large MOSFETs with a high input voltage and high

frequency of operation may cause the LTC1778 to exceed

its maximum junction temperature rating or RMS current

rating. Most of the supply current drives the MOSFET

gates unless an external EXTVCC source is used. In con-

tinuous mode operation, this current is IGATECHG =f(Qg(TOP)

+ Qg(BOT)). The junction temperature can be estimated

from the equations given in Note 2 of the Electrical

Characteristics. For example, the LTC1778CGN is limited

to less than 14mA from a 30V supply:

TJ = 70°C + (14mA)(30V)(130°C/W) = 125°C

For larger currents, consider using an external supply with

the EXTVCC pin.

EXTVCC Connection

The EXTVCC pin can be used to provide MOSFET gate drive

and control power from the output or another external

source during normal operation. Whenever the EXTVCC

pin is above 4.7V the internal 5V regulator is shut off and

an internal 50mA P-channel switch connects the EXTVCC

pin to INTVCC. INTVCC power is supplied from EXTVCC until

this pin drops below 4.5V. Do not apply more than 7V to

the EXTVCC pin and ensure that EXTVCC ≤ VIN. The follow-

ing list summarizes the possible connections for EXTVCC:

1. EXTVCC grounded. INTVCC is always powered from the

internal 5V regulator.

2. EXTVCC connected to an external supply. A high effi-

ciency supply compatible with the MOSFET gate drive

requirements (typically 5V) can improve overall

efficiency.

3. EXTVCC connected to an output derived boost network.

The low voltage output can be boosted using a charge

pump or flyback winding to greater than 4.7V. The system

APPLICATIO S I FOR ATIO

will start-up using the internal linear regulator until the

boosted output supply is available.

External Gate Drive Buffers

The LTC1778 drivers are adequate for driving up to about

30nC into MOSFET switches with RMS currents of 50mA.

Applications with larger MOSFET switches or operating at

frequencies requiring greater RMS currents will benefit

from using external gate drive buffers such as the LTC1693.

Alternately, the external buffer circuit shown in Figure 7

can be used. Note that the bipolar devices reduce the

signal swing at the MOSFET gate, and benefit from an

increased EXTVCC voltage of about 6V.

Figure 7. Optional External Gate Driver

FMMT619

GATE

OF M1

BOOST

FMMT720

FMMT619

GATE

OF M2

1778 F07

INTVCC

PGND

FMMT720

10Ω

Soft-Start and Latchoff with the RUN/SS Pin

The RUN/SS pin provides a means to shut down the

LTC1778 as well as a timer for soft-start and overcurrent

latchoff. Pulling the RUN/SS pin below 0.8V puts the

LTC1778 into a low quiescent current shutdown (IQ <

30µA). Releasing the pin allows an internal 1.2µA current

source to charge up the external timing capacitor CSS. If

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

delay before starting of about:

Cs F C

DELAY

=µ

()

When the voltage on RUN/SS reaches 1.5V, the LTC1778

begins operating with a clamp on ITH of approximately

0.9V. As the RUN/SS voltage rises to 3V, the clamp on ITH

is raised until its full 2.4V range is available. This takes an

additional 1.3s/µF, during which the load current is folded

back until the output reaches 75% of its final value. The pin

can be driven from logic as shown in Figure 7. Diode D1

LTC1778IGN Datasheet(PDF) 15 Page - Linear Technology

LTC1778IGN Datasheet(HTML) 15 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

Datasheet Download

Link URL