LTC1735 Linear Technology, LTC1735 Datasheet - Page 15

• Manufacturer Part Number: LTC1735
• Description: High Efficiency Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
EXTV
The LTC1735 contains an internal P-channel MOSFET
switch connected between the EXTV
Whenever the EXTV
regulator shuts off, the switch closes and INTV
supplied via EXTV
allows the MOSFET gate drive and control power to be
derived from the output or other external source during
normal operation. When the output is out of regulation
(start-up, short circuit) power is supplied from the internal
regulator. Do not apply greater than 7V to the EXTV
and ensure that EXTV
Significant efficiency gains can be realized by powering
INTV
from the driver and control currents will be scaled by a
factor of (Duty Cycle)/(Efficiency). For 5V regulators this
simply means connecting the EXTV
However, for 3.3V and other lower voltage regulators,
additional circuitry is required to derive INTV
from the output.
The following list summarizes the four possible connec-
tions for EXTV
1. EXTV
to be powered from the internal 5.2V regulator resulting in
an efficiency penalty of up to 10% at high input voltages.
2. EXTV
connection for a 5V output regulator and provides the
highest efficiency. For output voltages higher than 5V,
EXTV
absolute maximum ratings are not exceeded.
3. EXTV
For 3.3V and other low voltage regulators, efficiency gains
can still be realized by connecting EXTV
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 3a or the capacitive charge
pump shown in Figure 3b. The charge pump has the
advantage of simple magnetics.
4. EXTV
supply is available in the 5V to 7V range (EXTV
such as notebook main 5V system power, it may be used
to power EXTV
CC
CC
CC
CC
CC
CC
CC
from the output, since the V
is required to connect to V
Connection
left open (or grounded). This will cause INTV
connected to an output-derived boost network.
connected directly to V
connected to an external supply. If an external
CC:
CC
CC
U
providing it is compatible with the
CC
until EXTV
CC
pin is above 4.7V the internal 5.2V
U
V
IN
.
CC
drops below 4.5V. This
OUT
OUT
CC
W
CC
IN
pin directly to V
. This is the normal
so the SENSE pins’
and INTV
current resulting
CC
to an output-
CC
U
CC
CC
power is
CC
power
CC
pins.
V
OUT
IN
pin
CC
),
.
MOSFET gate drive requirements. This is the typical case
as the 5V power is almost always present and is derived by
another high efficiency regulator.
Output Voltage Programming
The output voltage is set by an external resistive divider
according to the following formula:
The resistive divider is connected to the output as shown
in Figure 4 allowing remote voltage sensing.
Figure 3a. Secondary Output Loop and EXTV
V
R4
R3
OUT
EXTV
OPTIONAL EXTV
CONNECTION
5V V
Figure 3b. Capacitive Charge Pump for EXTV
EXTV
FCB
SGND
CC
LTC1735
0 8
CC
SEC
LTC1735
.
PGND
V
7V
V
SW
BG
TG
IN
PGND
1
C
CC
IN
SW
V
BG
TG
IN
+
R
R
2
1
V
IN
N-CH
N-CH
V
IN
N-CH
N-CH
+
BAT85
C
L1
IN
1N4148
1:N
L1
VN2222LL
0.22 F
LTC1735
R
CC
SENSE
R
6.8V
Connection
SENSE
V
+
+
SEC
CC
+
+
1735 F03b
BAT85
15
BAT85
1 F
C
V
OUT
OUT
1735 F03a
1 F
C
V
OUT
OUT