LTC1929 LINER [Linear Technology], LTC1929 Datasheet - Page 12

• Manufacturer Part Number: LTC1929
• Description: 2-Phase, High Efficiency, Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC1929
not present, a diode can be placed in series with the
LTC1929’s V
EXTV
V
Significant efficiency gains can be realized by powering
INTV
from the driver and control currents will be scaled by the
ratio: (Duty Factor)/(Efficiency). For 5V regulators this
means connecting the EXTV
ever, for 3.3V and other lower voltage regulators, addi-
tional circuitry is required to derive INTV
output.
The following list summarizes the four possible connec-
tions for EXTV
1. EXTV
to be powered from the internal 5V regulator resulting in
a significant efficiency penalty at high input voltages.
2. EXTV
connection for a 5V regulator and provides the highest
efficiency.
3. EXTV
supply is available in the 5V to 7V range, it may be used to
power EXTV
gate drive requirements.
4. EXTV
For 3.3V and other low voltage regulators, efficiency gains
can still be realized by connecting EXTV
derived voltage which has been boosted to greater than
4.7V but less than 7V. This can be done with either the
12
IN
Figure 5a. Secondary Output Loop with EXTV
.
CC
CC
OPTIONAL EXTV
5V < V
EXTV
and the V
CC
CC
CC
CC
from the output, since the V
CC
SEC
connected to an output-derived boost network.
left open (or grounded). This will cause INTV
connected directly to V
connected to an external supply. If an external
LTC1929
CC
< 7V
IN
CC:
providing it is compatible with the MOSFET
pin and a Schottky diode between the
CC
IN
PGND
CONNECTION
SW1
pin, to prevent current from backfeeding
BG1
TG1
U
V
IN
C
IN
N-CH
N-CH
+
U
CC
pin directly to V
1N4148
V
OUT
IN
T1
W
IN
. This is the normal
CC
current resulting
CC
power from the
CC
R
SENSE
to an output-
V
Connection
SEC
+
+
OUT
U
1929 F05a
. How-
1 F
C
V
OUT
OUT
CC
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 (C
Functional Diagram)
External bootstrap capacitors C
the BOOST1 and BOOST2 pins supply the gate drive
voltages for the topside MOSFETs. Capacitor C
Functional Diagram is charged though diode D
INTV
turns on, the driver places the C
source of the desired MOSFET. This enhances the MOSFET
and turns on the topside switch. The switch node voltage,
SW, rises to V
The value of the boost capacitor C
times that of the total input capacitance of the topside
MOSFET(s). The reverse breakdown of D
than V
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 LTC1929 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-
mon, tightly coupled pair of PC traces. The differential
CC
EXTV
IN(MAX).
when the SW pin is low. When the topside MOSFET
Figure 5b. Capacitive Charge Pump for EXTV
CC
LTC1929
IN
and the BOOST pin rises to V
PGND
SW1
TG1
BG1
V
IN
C
IN
N-CH
N-CH
+
B
B1
BAT85
V
voltage across the gate-
B
IN
and C
L1
needs to be 30 to 100
B
,D
VN2222LL
B
B
0.22 F
) (Refer to
B2
must be greater
R
SENSE
connected to
IN
+
+
+ V
CC
1929 F05b
B
BAT85
BAT85
B
INTVCC
C
V
OUT
in the
OUT
from
.