Datasheet

LTC3727A-1
18
3727a1fa
2. EXTV
CC
Connected directly to V
OUT
. This is the normal
connection for a 7.5V regulator and provides the highest
effi ciency.
3. EXTV
CC
Connected to an External supply. If an external
supply is available in the 7.5V to 8.5V range, it may be
used to power EXTV
CC
providing it is compatible with the
MOSFET gate drive requirements.
4. EXTV
CC
Connected to an Output-Derived Boost Network.
For 3.3V and other low voltage regulators, effi ciency
gains can still be realized by connecting EXTV
CC
to an
output-derived voltage that has been boosted to greater
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
APPLICATIONS INFORMATION
Figure 6. Secondary Output Loop & EXTV
CC
Connection
EXTV
CC
FCB
SGND
V
IN
TG1
SW
BG1
PGND
LTC3727A-1
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
of the topside MOSFET(s). The reverse breakdown of the
external Schottky diode must be greater than V
IN(MAX)
.
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 LTC3727A-1 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
amplifi er. The output voltage is given by the equation:
VV
R
R
OUT
=+
⎛
⎝
⎜
⎞
⎠
⎟
08 1
2
1
.
where R1 and R2 are defi ned 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 voltage
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 fl ow 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 fl owing 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
VV
MAX
OUT
OUT
124
08
24
24
()
.
.–
.
=
⎛
⎝
⎜
⎞
⎠
⎟
<for