LTC3728LX(RevA) View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC3728LX Datasheet PDF : 32 Pages
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LTC3728L/LTC3728LX
APPLICATIO S I FOR ATIO
4. EXTVCC Connected to an Output-Derived Boost Net-
work. For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTVCC to an
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 (CB, DB)
External bootstrap capacitors CB connected to the BOOST
pins supply the gate drive voltages for the topside MOSFETs.
Capacitor CB in the functional diagram is charged though
external diode DB from INTVCC when the SW pin is low.
When one of the topside MOSFETs is to be turned on, the
driver places the CB 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
VIN and the BOOST pin follows. With the topside MOSFET
on, the boost voltage is above the input supply: VBOOST =
VIN + VINTVCC. The value of the boost capacitor CB 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 VIN(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.
OPTIONAL EXTVCC
CONNECTION
5V < VSEC < 7V
VIN
+
VIN
LTC3728L/
LTC3728LX
TG1
EXTVCC
R6
N-CH
SW
FCB
BG1
R5
SGND
N-CH
PGND
CIN
BAT 85
T1
1:N
VSEC
+
RSENSE
1µF
VOUT
+
COUT
3728 F06a
Figure 6a. Secondary Output Loop & EXTVCC Connection
18
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:
VOUT
=
0.8V1+
R2
R1
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)INTVCC. Continuous linear
operation is guaranteed throughout this range allowing
output voltage setting from 0.8V to 7.7V, depending upon
the voltage applied to EXTVCC. 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 VOUT 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:
ISENSE+ + ISENSE– = (2.4V – VOUT)/24k
VIN
+
CIN
VIN
LTC3728L/
LTC3728LX
TG1
EXTVCC
N-CH
SW
BG1
N-CH
PGND
+
1µF
BAT85
0.22µF
BAT85
VN2222LL
RSENSE
L1
BAT85
VOUT
+
COUT
3728 F06b
Figure 6b. Capacitive Charge Pump for EXTVCC
3728lxfa
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