LTC3728EG#TRPBF Linear Technology, LTC3728EG#TRPBF Datasheet - Page 20

• Manufacturer Part Number: LTC3728EG#TRPBF
• Description: IC SW REG SYNC STP-DN DUAL28SSOP
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3728EG#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 5.5 V
• Current - Output: 3A
• Frequency - Switching: 250kHz ~ 550kHz
• Voltage - Input: 3.5 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

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
2. EXTV
3. EXTV
4. EXTV
20
to be powered from the internal 5V regulator resulting in
an effi ciency penalty of up to 10% at high input voltages.
connection for a 5V regulator and provides the highest
effi ciency.
supply is available in the 5V to 7V range, it may be used
to power EXTV
MOSFET gate drive requirements.
For 3.3V and other low voltage regulators, effi ciency
gains can still be realized by connecting EXTV
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.
R6
R5
Figure 6a. Secondary Output Loop and 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 Directly to V
Connected to an External Supply. If an external
CC
SEC
LTC3728
< 7V
CC:
PGND
CC
TG1
BG1
CC
SW
V
IN
providing it is compatible with the
N-CH
N-CH
V
IN
+
C
IN
1:N
T1
OUT
CC
pin directly to V
. This is the normal
R
SENSE
CC
Connection
V
SEC
CC
+
+
CC
power
3728 F06a
to an
1 F
C
V
OUT
OUT
OUT
CC
.
Topside MOSFET Driver Supply (C
External bootstrap capacitors C
pins supply the gate drive voltages for the topside MOS-
FETs. Capacitor C
though external diode D
is low. 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
and the BOOST pin follows. With the topside
+ V
PGND
BG1
TG1
V
SW
C
INTVCC
IN
B
IN
in the Functional Diagram is charged
+
N-CH
N-CH
. The value of the boost capacitor
V
IN
B
B
from INTV
voltage across the gate-source
BAT85
B
L1
connected to the BOOST
B
VN2222LL
, D
CC
R
SENSE
when the SW pin
B
0.22 F
)
+
+
CC
3728 F06b
IN(MAX)
BAT85
BAT85
1 F
C
V
OUT
OUT
3728fg
.