LTC3728 Linear Technology, LTC3728 Datasheet - Page 18

• Manufacturer Part Number: LTC3728
• Description: 2-Phase Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3728
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.
Output Voltage
The LTC3728 output voltages are each set by an external
feedback resistive divider carefully placed across
the output capacitor. The resultant feedback signal is
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
EXTV
FCB
SGND
CC
B
SEC
LTC3728
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
1:N
T1
W
B
, D
B
R
CC
)
SENSE
IN(MAX)
Connection
V
SEC
U
+
+
B
BOOST
. When
needs
3728 F06a
1µF
C
V
OUT
OUT
=
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:
Since V
can choose R1 in Figure 2 to have a maximum value to
absorb this current.
I
V
SENSE
OUT
EXTV
+
OSENSE
OUT
/SENSE
Figure 6b. Capacitive Charge Pump for EXTV
CC
LTC3728
=
+
+ I
0 8 1
resistive divider to compensate for the current
.
SENSE
PGND
V
is servoed to the 0.8V reference voltage, we
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
0.22µF
+
+
CC
3728 F06b
BAT85
1µF
BAT85
C
V
OUT
OUT
3728fb