LTC1735
U
W U U
APPLICATIO S I FOR ATIO
derived from the output or other external source during
normal operation. When the output is out of regulation
(start-up,shortcircuit)powerissuppliedfromtheinternal
regulator. Do not apply greater than 7V to the EXTVCC pin
and ensure that EXTVCC ≤ VIN.
Output Voltage Programming
The output voltage is set by an external resistive divider
according to the following formula:
R2
R1
V
= 0.8V 1+
OUT
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 a
factor of (Duty Cycle)/(Efficiency). For 5V regulators this
The resistive divider is connected to the output as shown
in Figure 4 allowing remote voltage sensing.
simply means connecting the EXTVCC pin directly to VOUT
.
However, for 3.3V and other lower voltage regulators,
additional circuitry is required to derive INTVCC power
from the output.
V
IN
OPTIONAL EXTV
CONNECTION
CC
5V ≤ V
≤ 7V
+
SEC
C
IN
1N4148
V
V
SEC
IN
The following list summarizes the four possible connec-
tions for EXTVCC:
+
+
LTC1735
6.8V
1µF
N-CH
N-CH
TG
SW
BG
R
SENSE
1. EXTVCC left open (or grounded). This will cause INTVCC
to be powered from the internal 5.2V regulator resulting in
an efficiency penalty of up to 10% at high input voltages.
V
OUT
L1
1:N
EXTV
CC
R4
R3
C
FCB
OUT
2. EXTVCC connected directly to VOUT. This is the normal
connection for a 5V output regulator and provides the
highest efficiency. For output voltages higher than 5V,
EXTVCC is required to connect to VOUT so the SENSE pins’
absolute maximum ratings are not exceeded.
SGND
PGND
1735 F03a
Figure 3a. Secondary Output Loop and EXTVCC Connection
3. 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 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.
+
V
IN
1µF
+
C
IN
0.22µF
BAT85
BAT85
BAT85
V
IN
LTC1735
VN2222LL
N-CH
TG
SW
BG
R
SENSE
V
OUT
L1
EXTV
CC
4. EXTVCC connected to an external supply. If an external
supply is available in the 5V to 7V range (EXTVCC ≤ VIN),
such as notebook main 5V system power, it may be used
to power EXTVCC providing it is compatible with the
MOSFET gate drive requirements. This is the typical case
asthe5Vpowerisalmostalwayspresentandisderivedby
another high efficiency regulator.
+
C
OUT
N-CH
PGND
1735 F03b
Figure 3b. Capacitive Charge Pump for EXTVCC
1735fc
16
Linear [ Linear ]
