Datasheet

LTC3714
18
3714f
applicaTions inForMaTion
EXTV
CC
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 EXTV
CC
pin is above 4.7V the internal 5V regulator is shut off and
an internal 50mA P-channel switch connects the EXTV
CC
pin to INTV
CC
. INTV
CC
power is supplied from EXTV
CC
until
this pin drops below 4.5V. Do not apply more than 7V to
the EXTV
CC
pin and ensure that EXTV
CC
≤ V
IN
. The follow-
ing list summarizes the possible connections for EXTV
CC
:
1. EXTV
CC
grounded. INTV
CC
is always powered from the
internal 5V regulator.
2. EXTV
CC
connected to an external supply. A high ef-
ficiency supply compatible with the MOSFET gate
drive requirements (typically 5V) can improve overall
efficiency.
3. EXTV
CC
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 will start-up using the internal linear regulator
until the boosted output supply is available.
External Gate Drive Buffers
The LTC3714 drivers are adequate for driving up to about
60nC 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 6 can
be used. Note that the bipolar devices reduce the signal
swing at the MOSFET gate, and benefit from increased
EXTV
CC
voltage of about 6V.
Figure 6. Optional External Gate Driver
Q1
FMMT619
GATE
OF M1
TG
BOOST
SW
Q2
FMMT720
Q3
FMMT619
GATE
OF M2
10Ω
BG
3714 F06
INTV
CC
PGND
Q4
FMMT720
10Ω