LTC1735_15 Datasheet, PDF (16/32 Pages) Linear Technology – High Efficiency Synchronous Step-Down Switching Regulator

LTC1735
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, 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:
V_OUT
=

0.8V_1+
R2^
R1
The resistive divider is connected to the output as shown
in Figure 4 allowing remote voltage sensing.
OPTIONAL EXTV_CC
CONNECTION
5V ≤ V_SEC ≤ 7V
V_IN
LTC1735
TG
EXTV_CC
R4
FCB
R3
SGND
SW
BG
PGND
V_IN
+
C_IN
1N4148
N-CH
V_SEC
+
6.8V
R_SENSE
L1
1:N
+
N-CH
1µF
V_OUT
C_OUT
1735 F03a
Figure 3a. Secondary Output Loop and EXTV_CC Connection
+
C_IN
V_IN
LTC1735
TG
EXTV_CC
SW
BG
V_IN
BAT85
N-CH
L1
N-CH
+
0.22µF
VN2222LL
R_SENSE
+
1µF
BAT85
BAT85
V_OUT
C_OUT
PGND
1735 F03b
Figure 3b. Capacitive Charge Pump for EXTV_CC
1735fc
16