Datasheet

LTC3728
20
3728fg
APPLICATIONS INFORMATION
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.
Topside MOSFET Driver Supply (C
B
, D
B
)
External bootstrap capacitors C
B
connected to the BOOST
pins supply the gate drive voltages for the topside MOS-
FETs. Capacitor C
B
in the Functional Diagram is charged
though external diode D
B
from INTV
CC
when the SW pin
is low. When one of the topside MOSFETs is to be turned
on, the driver places the C
B
voltage across the gate-source
of the desired MOSFET. This enhances the MOSFET and
turns on the topside switch. The switch node voltage, SW,
rises to V
IN
and the BOOST pin follows. With the topside
MOSFET on, the boost voltage is above the input supply:
V
BOOST
= V
IN
+ V
INTVCC
. The value of the boost capacitor
C
B
needs to be 100 times that of the total input capacitance
of the topside MOSFET(s). The reverse breakdown of the
external Schottky diode must be greater than V
IN(MAX)
.
When adjusting the gate drive level, the fi nal arbiter is the
total input current for the regulator. If a change is made
and the input current decreases, then the effi ciency has
improved. If there is no change in input current, then there
is no change in effi ciency.
Output Voltage
The LTC3728 output voltages are each set by an exter-
nal feedback resistive divider carefully placed across
the output capacitor. The resultant feedback signal is
compared with the internal precision 0.800V voltage
EXTV
CC
FCB
SGND
V
IN
TG1
SW
BG1
PGND
LTC3728
R
SENSE
V
OUT
V
SEC
+
C
OUT
+
1MF
3728 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
LTC3728
R
SENSE
V
OUT
VN2222LL
+
C
OUT
3728 F06b
N-CH
N-CH
+
C
IN
+
1MF
V
IN
L1
BAT85 BAT85
BAT85
0.22MF
Figure 6a. Secondary Output Loop and EXTV
CC
Connection Figure 6b. Capacitive Charge Pump for EXTV
CC