Datasheet

16
LTC1735
1735fc
APPLICATIO S I FOR ATIO
WUU
U
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
CC
pin
and ensure that EXTV
CC
≤ V
IN
.
Significant efficiency gains can be realized by powering
INTV
CC
from the output, since the V
IN
current resulting
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
CC
pin directly to V
OUT
.
However, for 3.3V and other lower voltage regulators,
additional circuitry is required to derive INTV
CC
power
from the output.
The following list summarizes the four possible connec-
tions for EXTV
CC:
1. EXTV
CC
left open (or grounded). This will cause INTV
CC
to be powered from the internal 5.2V regulator resulting in
an efficiency penalty of up to 10% at high input voltages.
2. EXTV
CC
connected directly to V
OUT
. This is the normal
connection for a 5V output regulator and provides the
highest efficiency. For output voltages higher than 5V,
EXTV
CC
is required to connect to V
OUT
so the SENSE pins’
absolute maximum ratings are not exceeded.
3. EXTV
CC
connected to an output-derived boost network.
For 3.3V and other low voltage regulators, efficiency gains
can still be realized by connecting EXTV
CC
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 3a or the capacitive charge
pump shown in Figure 3b. The charge pump has the
advantage of simple magnetics.
4. EXTV
CC
connected to an external supply. If an external
supply is available in the 5V to 7V range (EXTV
CC
≤ V
IN
),
such as notebook main 5V system power, it may be used
to power EXTV
CC
providing it is compatible with the
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:
VV
R
R
OUT
=+






08 1
2
1
.
The resistive divider is connected to the output as shown
in Figure 4 allowing remote voltage sensing.
Figure 3a. Secondary Output Loop and EXTV
CC
Connection
Figure 3b. Capacitive Charge Pump for EXTV
CC
EXTV
CC
FCB
SGND
V
IN
TG
SW
BG
PGND
LTC1735
R
SENSE
V
OUT
V
SEC
6.8V
+
C
OUT
+
1µF
1735 F03a
N-CH
N-CH
R4
+
C
IN
V
IN
L1
1:N
1N4148
OPTIONAL EXTV
CC
CONNECTION
5V ≤ V
SEC
≤ 7V
R3
EXTV
CC
V
IN
TG
SW
BG
PGND
LTC1735
R
SENSE
V
OUT
VN2222LL
+
C
OUT
1735 F03b
N-CH
N-CH
+
C
IN
+
1µF
V
IN
L1
BAT85 BAT85
BAT85
0.22µF