Datasheet

LTC3707
18
3707fb
APPLICATIONS INFORMATION
EXTV
CC
Connection
The LTC3707 contains an internal P-channel MOSFET
switch connected between the EXTV
CC
and INTV
CC
pins.
When the voltage applied to EXTV
CC
rises above 4.7V,
the internal regulator is turned off and the switch closes,
connecting the EXTV
CC
pin to the INTV
CC
pin thereby sup-
plying internal power. The switch remains closed as long
as the voltage applied to EXTV
CC
remains above 4.5V. This
allows the MOSFET driver and control power to be derived
from the output during normal operation (4.7V < V
OUT
<
7V) and from the internal regulator when the output is
out of regulation (start-up, short-circuit). If more current
is required through the EXTV
CC
switch than is specifi ed,
an external Schottky diode can be added between the
EXTV
CC
and INTV
CC
pins. Do not apply greater than 7V to
the EXTV
CC
pin and ensure that EXTV
CC
< V
IN
.
Signifi cant effi ciency gains can be realized by powering
INTV
CC
from the output, since the V
IN
current resulting
from the driver and control currents will be scaled by a
factor of (Duty Cycle)/(Effi ciency). For 5V regulators this
supply means connecting the EXTV
CC
pin directly to V
OUT
.
However, for 3.3V and other lower voltage regulators,
additional circuitry is required to derive INTV
CC
power
from the output.
The following list summarizes the four possible connec-
tions for EXTV
CC:
1. EXTV
CC
Left Open (or Grounded). This will cause INTV
CC
to be powered from the internal 5V regulator resulting in an
effi ciency penalty of up to 10% at high input voltages.
2. EXTV
CC
Connected directly to V
OUT
. This is the normal
connection for a 5V regulator and provides the highest
effi ciency.
3. EXTV
CC
Connected to an External supply. If an external
supply is available in the 5V to 7V range, it may be used to
power EXTV
CC
providing it is compatible with the MOSFET
gate drive requirements.
4. EXTV
CC
Connected to an Output-Derived Boost Network.
For 3.3V and other low voltage regulators, effi ciency gains
can still be realized by connecting EXTV
CC
to an output-
derived voltage that has been boosted to greater than 4.7V.
This can be done with either the inductive boost winding
as shown in Figure 6a or the capacitive charge pump
shown in Figure 6b. The charge pump has the advantage
of simple magnetics.
EXTV
CC
FCB
SGND
V
IN
TG1
SW
BG1
PGND
LTC3707
R
SENSE
V
OUT
V
SEC
+
C
OUT
+
1µF
3707 F06a
N-CH
N-CH
R6
+
C
IN
V
IN
T1
1:N
OPTIONAL EXTV
CC
CONNECTION
5V < V
SEC
< 7V
R5
EXTV
CC
V
IN
TG1
SW
BG1
PGND
LTC3707
V
OUT
VN2222LL
+
C
OUT
3707 F06b
N-CH
N-CH
+
C
IN
+
1µF
V
IN
L1
BAT85 BAT85
BAT85
0.22µF
R
SENSE
Figure 6a. Secondary Output Loop & EXTV
CC
Connection Figure 6b. Capacitive Charge Pump for EXTV
CC