Datasheet

18
LTC1628-SYNC
1628syncfa
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
B
, D
B
)
External bootstrap capacitors C
B
connected to the BOOST
pins supply the gate drive voltages for the topside MOS-
FETs. 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
external 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.
Output Voltage
The LTC1628-SYNC output voltages are each set by an
external feedback resistive divider carefully placed across
Figure 6a. Secondary Output Loop & EXTV
CC
Connection
Figure 6b. Capacitive Charge Pump for EXTV
CC
the output capacitor. The resultant feedback signal is
compared with the internal precision 0.800V voltage ref-
erence 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 (1.1)INTV
CC
. Continuous linear
operation is guaranteed throughout this range allowing
output voltage setting from 0.8V to 7.7V, depending upon
the voltage applied to EXTV
CC
. 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.
EXTV
CC
FCB
SGND
V
IN
TG1
SW
BG1
PGND
LTC1628-SYNC
R
SENSE
V
OUT
V
SEC
+
C
OUT
+
1µF
1628 F06a
N-CH
N-CH
R6
+
C
IN
V
IN
T1
1:N
OPTIONAL EXTV
CC
CONNECTION
5V < V
SEC
< 7V
R5
EXTV
CC
V
IN
TG1
SW
BG1
PGND
LTC1628-SYNC
R
SENSE
V
OUT
VN2222LL
+
C
OUT
1628 F06b
N-CH
N-CH
+
C
IN
+
1µF
V
IN
L1
BAT85 BAT85
BAT85
0.22µF
APPLICATIO S I FOR ATIO
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