ltc3728legn-trpbf Linear Technology Corporation, ltc3728legn-trpbf Datasheet - Page 20

• Manufacturer Part Number: ltc3728legn-trpbf
• Description: Dual, 550khz, 2-phase Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTC3728
factor of (Duty Cycle)/(Effi ciency). For 5V regulators this
supply means connecting the EXTV
However, for 3.3V and other lower voltage regulators,
additional circuitry is required to derive INTV
from the output.
The following list summarizes the four possible connec-
tions for EXTV
1. EXTV
to be powered from the internal 5V regulator resulting in an
effi ciency penalty of up to 10% at high input voltages.
2. EXTV
connection for a 5V regulator and provides the highest
effi ciency.
3. EXTV
supply is available in the 5V to 7V range, it may be used to
power EXTV
gate drive requirements.
4. EXTV
For 3.3V and other low voltage regulators, effi ciency gains
can still be realized by connecting EXTV
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.
20
R6
R5
Figure 6a. Secondary Output Loop & EXTV
OPTIONAL EXTV
CONNECTION
5V < V
EXTV
FCB
SGND
CC
CC
CC
CC
Left Open (or Grounded). This will cause INTV
Connected to an Output-Derived Boost Network.
Connected to an External supply. If an external
Connected directly to V
CC
SEC
LTC3728
CC
< 7V
CC:
providing it is compatible with the MOSFET
PGND
CC
TG1
BG1
SW
V
IN
N-CH
N-CH
V
IN
+
C
IN
1:N
T1
OUT
CC
. This is the normal
pin directly to V
R
CC
CC
SENSE
Connection
to an output-
V
SEC
CC
+
+
power
3728 F06a
1μF
C
V
OUT
OUT
OUT
CC
.
Topside MOSFET Driver Supply (C
External bootstrap capacitors C
pins supply the gate drive voltages for the topside MOSFETs.
Capacitor C
external diode D
When one of the topside MOSFETs is to be turned on,
the driver places the C
of the desired MOSFET. This enhances the MOSFET and
turns on the topside switch. The switch node voltage, SW,
rises to V
MOSFET on, the boost voltage is above the input supply:
V
C
of the topside MOSFET(s). The reverse breakdown of the
external Schottky diode must be greater than V
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
BOOST
B
needs to be 100 times that of the total input capacitance
EXTV
= V
Figure 6b. Capacitive Charge Pump for EXTV
CC
LTC3728
IN
IN
B
and the BOOST pin follows. With the topside
in the functional diagram is charged though
+ V
PGND
BG1
TG1
V
SW
B
C
INTVCC
IN
IN
from INTV
+
N-CH
N-CH
. The value of the boost capacitor
V
B
IN
voltage across the gate-source
BAT85
CC
B
L1
when the SW pin is low.
connected to the BOOST
B
VN2222LL
, D
R
SENSE
B
0.22μF
)
+
+
CC
3728 F06b
IN(MAX)
BAT85
BAT85
1μF
C
V
OUT
OUT
3728fd
.