參數(shù)資料
型號(hào)： LTC3728LZEUH
廠商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： 3 A DUAL SWITCHING CONTROLLER, 590 kHz SWITCHING FREQ-MAX, PQCC28
封裝： 5 X 5 MM, PLASTIC, MO-220WHHD, QFN-28
文件頁(yè)數(shù)： 10/32頁(yè)
文件大?。?/td> 467K
代理商： LTC3728LZEUH
LTC3728LZ
18
3728lzf
4. EXTVCCConnectedtoanOutput-DerivedBoostNetwork.
For 3.3V and other low voltage regulators, efciency 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
external Schottky diode must be greater than VIN(MAX).
When adjusting the gate drive level, the nal arbiter is the
total input current for the regulator. If a change is made
and the input current decreases, then the efciency has
improved. If there is no change in input current, then there
is no change in efciency.
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
amplier. The output voltage is given by the equation:
VV
R
OUT
=+
08
1
2
1
.
where R1 and R2 are dened 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 ow 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 owing out of each pair of SENSE pins is:
ISENSE
+ + ISENSE– = (2.4V – VOUT)/24k
EXTVCC
FCB
SGND
VIN
TG1
SW
BG1
PGND
LTC3728LZ
RSENSE
VOUT
VSEC
+
COUT
+
1
F
3728LZ F06a
N-CH
R6
+
CIN
VIN
T1
1:N
OPTIONAL EXTVCC
CONNECTION
5V < VSEC < 7V
R5
BAT 85
EXTVCC
VIN
TG1
SW
BG1
PGND
LTC3728LZ
RSENSE
VOUT
VN2222LL
+
COUT
3728LZ F06b
N-CH
+
CIN
+
1
F
VIN
L1
BAT85
0.22
F
Figure 6a. Secondary Output Loop & EXTVCC Connection
Figure 6b. Capacitive Charge Pump for EXTVCC
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