LTC3728LX Linear Technology, LTC3728LX Datasheet - Page 18

• Manufacturer Part Number: LTC3728LX
• Description: 2-Phase Synchronous Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3728L/LTC3728LX
4. EXTV
work. For 3.3V and other low voltage regulators, efficiency
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.
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
desired MOSFET. This enhances the MOSFET and turns on
the topside switch. The switch node voltage, SW, rises to
V
on, the boost voltage is above the input supply: V
V
to be 100 times that of the total input capacitance of the
topside MOSFET(s). The reverse breakdown of the exter-
nal Schottky diode must be greater than V
adjusting the gate drive level, the final arbiter is the total
input current for the regulator. If a change is made and the
input current decreases, then the efficiency has improved.
If there is no change in input current, then there is no
change in efficiency.
18
IN
IN
R6
R5
and the BOOST pin follows. With the topside MOSFET
+ V
Figure 6a. Secondary Output Loop & EXTV
INTVCC
OPTIONAL EXTV
CONNECTION
5V < V
CC
EXTV
FCB
SGND
LTC3728LX
LTC3728L/
Connected to an Output-Derived Boost Net-
CC
B
SEC
in the functional diagram is charged though
. The value of the boost capacitor C
< 7V
B
PGND
CC
B
BG1
from INTV
TG1
SW
V
U
IN
voltage across the gate-source of the
N-CH
N-CH
U
V
IN
+
CC
B
when the SW pin is low.
C
connected to the BOOST
IN
BAT 85
1:N
T1
W
B
, D
B
R
CC
)
SENSE
IN(MAX)
Connection
V
SEC
U
CC
+
+
B
BOOST
. When
needs
to an
3728 F06a
1 F
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
amplifier. The output voltage is given by the equation:
where R1 and R2 are defined 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 flow 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 flowing out of each pair of SENSE pins is:
I
V
SENSE
OUT
EXTV
+
LTC3728LX
OUT
LTC3728L/
/SENSE
Figure 6b. Capacitive Charge Pump for EXTV
CC
+
+ I
0 8 1
resistive divider to compensate for the current
.
SENSE
PGND
V
BG1
TG1
SW
V
–
C
IN
IN
Pins
+
–
N-CH
N-CH
R
= (2.4V – V
R
2
1
V
IN
BAT85
CC
. A differential NPN input
L1
OUT
w w w . D a t a S h e e t 4 U . c
VN2222LL
CC
)/24k
R
. Continuous linear
SENSE
0.22 F
+
+
CC
3728 F06b
BAT85
1 F
BAT85
C
V
OUT
OUT
3728lxfa