LTC3728LXCUH#TR Linear Technology, LTC3728LXCUH#TR Datasheet - Page 21

• Manufacturer Part Number: LTC3728LXCUH#TR
• Description: IC CTRLR SW REG 2-PH SYNC 32-QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3728LXCUH#TR

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 7 V
• Current - Output: 3A
• Frequency - Switching: 250kHz ~ 550kHz
• Voltage - Input: 4.5 ~ 28 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-QFN
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS INFORMATION
4. EXTV
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.
BOOST
B
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.
needs to be 100 times that of the total input capacitance
R6
R5
Figure 6a. Secondary Output Loop and EXTV
OPTIONAL EXTV
CONNECTION
5V < V
CC
EXTV
FCB
SGND
= V
IN
Connected to an Output-Derived Boost Network.
LTC3728LX
LTC3728L/
CC
IN
SEC
and the BOOST pin follows. With the topside
+ V
< 7V
PGND
INTVCC
CC
B
TG1
BG1
SW
V
IN
in the Functional Diagram is charged
N-CH
N-CH
. The value of the boost capacitor
B
B
V
from INTV
IN
voltage across the gate-source
+
B
C
IN
connected to the BOOST
BAT 85
1:N
T1
B
, D
CC
B
when the SW pin
R
)
SENSE
CC
Connection
V
SEC
+
+
CC
IN(MAX)
3728 F06a
to an
1μF
C
V
OUT
OUT
.
Output Voltage
The output voltages are each set by an external feedback
resistive divider carefully placed across the output capaci-
tor. The resultant feedback signal is compared with the
internal precision 0.800V voltage reference by the error
amplifi er. The output voltage is given by the equation:
where R1 and R2 are defi ned in Figure 2.
SENSE
The common mode input range of the current comparator
sense pins is from 0V to (1.1)INTV
operation is guaranteed throughout this range allowing
output voltage setting from 0.8V to 7.7V, depending upon
the voltage applied to EXTV
stage is biased with internal resistors from an internal 2.4V
source, as shown in the Functional Diagram. This requires
that current either be sourced or sunk from the SENSE
pins depending on the output voltage. If the output voltage
is below 2.4V, current will fl ow out of both SENSE pins to
the main output. The output can be easily preloaded by
the V
comparator’s negative input bias current. The maximum
current fl owing out of each pair of SENSE pins is:
I
V
SENSE
OUT
EXTV
OUT
+
LTC3728LX
LTC3728L/
/SENSE
CC
Figure 6b. Capacitive Charge Pump for EXTV
= 0.8V 1+
+
resistive divider to compensate for the current
+ I
SENSE
LTC3728L/LTC3728LX
PGND
BG1
TG1
SW
V
–
C
IN
IN
Pins
+
–
N-CH
N-CH
R2
= (2.4V – V
R1
V
IN
BAT85
CC
. A differential NPN input
L1
OUT
VN2222LL
CC
)/24k
R
. Continuous linear
SENSE
0.22μF
+
+
CC
3728 F06b
BAT85
1μF
BAT85
C
V
21
OUT
OUT
3728lxfe