LTC3707 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC3707' PDF : 32 Pages View PDF
LTC3707
APPLICATIO S I FOR ATIO
derived from the output during normal operation (4.7V <
VOUT < 7V) and from the internal regulator when the output
is out of regulation (start-up, short-circuit). If more cur-
rent is required through the EXTVCC switch than is speci-
fied, an external Schottky diode can be added between the
EXTVCC and INTVCC pins. Do not apply greater than 7V to
the EXTVCC pin and ensure that EXTVCC␣ <␣ VIN.
Significant efficiency gains can be realized by powering
INTVCC from the output, since the VIN current resulting
from the driver and control currents will be scaled by a
factor of (Duty Cycle)/(Efficiency). For 5V regulators this
supply means connecting the EXTVCC pin directly to VOUT.
However, for 3.3V and other lower voltage regulators,
additional circuitry is required to derive INTVCC power
from the output.
The following list summarizes the four possible connec-
tions for EXTVCC:
1. EXTVCC Left Open (or Grounded). This will cause INTVCC
to be powered from the internal 5V regulator resulting in
an efficiency penalty of up to 10% at high input voltages.
2. EXTVCC Connected directly to VOUT. This is the normal
connection for a 5V regulator and provides the highest
efficiency.
3. EXTVCC Connected to an External supply. If an external
supply is available in the 5V to 7V range, it may be used to
power EXTVCC providing it is compatible with the MOSFET
gate drive requirements.
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
+
CIN
VIN
LTC3707
TG1
N-CH
EXTVCC
SW
T1
1:N
R6
FCB
BG1
R5
SGND
PGND
N-CH
VSEC
+
RSENSE
1µF
VOUT
+
COUT
3707 F06a
Figure 6a. Secondary Output Loop & EXTVCC Connection
18
VIN
+
CIN
VIN
LTC3707
TG1
N-CH
EXTVCC
SW
BG1
PGND
N-CH
+
1µF
BAT85
0.22µF
BAT85
VN2222LL
RSENSE
L1
+
BAT85
VOUT
COUT
3707 F06b
Figure 6b. Capacitive Charge Pump for EXTVCC
3707f
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