LTC1929CG View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1929CG' PDF : 24 Pages View PDF
LTC1929/LTC1929-PG
APPLICATIO S I FOR ATIO
EXTVCC Connection
The LTC1929 contains an internal P-channel MOSFET
switch connected between the EXTVCC and INTVCC pins.
When the voltage applied to EXTVCC rises above 4.7V, the
internal regulator is turned off and the switch closes,
connecting the EXTVCC pin to the INTVCC pin thereby
supplying internal and MOSFET gate driving power. The
switch remains closed as long as the voltage applied to
EXTVCC remains above 4.5V. This allows the MOSFET
driver and control power to be derived from the output
during normal operation (4.7V < VEXTVCC < 7V) and from
the internal regulator when the output is out of regulation
(start-up, short-circuit). Do not apply greater than 7V to
the EXTVCC pin and ensure that EXTVCC < VIN + 0.3V when
using the application circuits shown. If an 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
LTC1929’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 BOOST1 and BOOST2 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,
OPTIONAL EXTVCC CONNECTION
5V < VSEC < 7V
+
VIN
CIN
VIN
TG1
LTC1929
EXTVCC
SW1
N-CH
BG1
1N4148
T1
VSEC
+
RSENSE
+
1µF
VOUT
COUT
PGND
N-CH
1929 F05a
Figure 5a. Secondary Output Loop with EXTVCC Connection
+
CIN
VIN
TG1
LTC1929
N-CH
EXTVCC
SW1
BG1
PGND
N-CH
VIN
+
BAT85 0.22µF
BAT85
VN2222LL
BAT85
RSENSE
VOUT
L1
+
COUT
1929 F05b
Figure 5b. Capacitive Charge Pump for EXTVCC
16
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