LTC1709 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1709
Linear Technology
'LTC1709' PDF : 28 Pages View PDF
LTC1709
APPLICATIO S I FOR ATIO
external voltage source is applied to the EXTVCC pin when
the VIN supply is not present, a diode can be placed in
series with the LTC1709’s VIN pin and a Schottky diode
between the EXTVCC and the VIN pin, to prevent current
from backfeeding 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 the
ratio: (Duty Factor)/(Efficiency). For 5V regulators this
means connecting the EXTVCC pin directly to VOUT. How-
ever, for 3.3V and other lower voltage regulators, addi-
tional 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
a significant efficiency penalty 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 network.
For 3.3V and other low voltage regulators, efficiency gains
can still be realized by connecting EXTVCC to an output-
derived voltage which has been boosted to greater than
4.7V but less than 7V. This can be done with either the
inductive boost winding as shown in Figure 5a or the
capacitive charge pump shown in Figure 5b. The charge
pump has the advantage of simple magnetics.
Topside MOSFET Driver Supply (CB,DB) (Refer to
Functional Diagram)
External bootstrap capacitors CB1 and CB2 connected to
the BOOST 1 and BOOST 2 pins supply the gate drive
voltages for the topside MOSFETs. Capacitor CB in the
Functional Diagram is charged though diode DB from
INTVCC when the SW pin is low. When the topside MOSFET
turns 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 rises to VIN + VINTVCC.
The value of the boost capacitor CB needs to be 30 to 100
times that of the total input capacitance of the topside
MOSFET(s). The reverse breakdown of DB must be greater
than VIN(MAX).
The final arbiter when defining the best gate drive ampli-
tude level will be the input supply current. If a change is
made that decreases input current, the efficiency has
improved. If the input current does not change then the
efficiency has not changed either.
Output Voltage
The LTC1709 has a true remote voltage sense capablity.
The sensing connections should be returned from the load
back to the differential amplifier’s inputs through a com-
OPTIONAL EXTVCC CONNECTION
5V < VSEC < 7V
+
VIN
CIN
LTC1709 VIN
TG1
EXTVCC
N-CH
SW1
BG1
1N4148
T1
VSEC
+
RSENSE
+
1µF
VOUT
COUT
PGND
N-CH
1709 F05a
Figure 5a. Secondary Output Loop with EXTVCC Connection
16
+
VIN
+
CIN
VIN
TG1
LTC1709
N-CH
BAT85 0.22µF
VN2222LL
BAT85
BAT85
EXTVCC
SW1
BG1
RSENSE
VOUT
L1
+
COUT
PGND
N-CH
1709 F05b
Figure 5b. Capacitive Charge Pump for EXTVCC
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