Datasheet

18
LTC3727/LTC3727-1
3727fc
than 7.5V. This can be done with the inductive boost
winding as shown in Figure 6.
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 MOSFETs.
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 exter-
nal Schottky diode must be greater than V
IN(MAX)
. When
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.
Figure 6. Secondary Output Loop & EXTV
CC
Connection
Output Voltage
The LTC3727 output voltages are each set by an external
feedback resistive divider carefully placed across the
output capacitor. 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:
VV
R
R
OUT
=+
⎛
⎝
⎜
⎞
⎠
⎟
08 1
2
1
.
where R1 and R2 are defined in Figure 2.
SENSE
+
/SENSE
–
Pins
The common mode input range of the current comparator
sense pins is from 0V to 14V. Continuous linear operation
is guaranteed throughout this range allowing output volt-
age setting from 0.8V to 14V. A differential NPN input
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
OUT
resistive divider to compensate for the current
comparator’s negative input bias current. The maximum
current flowing out of each pair of SENSE pins is:
I
SENSE
+
+ I
SENSE
–
= (2.4V – V
OUT
)/24k
Since V
OSENSE
is servoed to the 0.8V reference voltage, we
can choose R1 in Figure 2 to have a maximum value to
absorb this current.
Rk
V
VV
MAX
OUT
124
08
24
()
.
.–
=
⎛
⎝
⎜
⎞
⎠
⎟
for V
OUT
< 2.4V
APPLICATIO S I FOR ATIO
WUUU
EXTV
CC
FCB
SGND
V
IN
TG1
SW
BG1
PGND
LTC3727
R
SENSE
V
OUT
V
SEC
+
C
OUT
+
1μF
3727 F06
N-CH
N-CH
R6
+
C
IN
V
IN
T1
1:N
OPTIONAL EXTV
CC
CONNECTION
7.5V < V
SEC
< 8.5V
R5