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LTC1435A
APPLICATIONS INFORMATION
WUU
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control power to be derived from the output during normal
operation (4.8V < V
OUT
< 9V) and from the internal regu-
lator when the output is out of regulation (start-up, short
circuit). Do not apply greater than 10V 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 supply 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 connections
for EXTV
CC:
1. EXTV
CC
left open (or grounded). This will cause INTV
CC
to be powered from the internal 5V regulator resulting
in an efficiency penalty of up to 10% at high input volt-
ages.
2. EXTV
CC
connected directly to V
OUT
. This is the normal
connection for a 5V regulator and provides the highest
efficiency.
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 which has been boosted to
greater than 4.8V. This can be done with either the in-
ductive boost winding as shown in Figure 4a or the
capacitive charge pump shown in Figure 4b. 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 10V range (EXTV
CC
≤ V
IN
),
it may be used to power EXTV
CC
providing it is compat-
ible with the MOSFET gate drive requirements. When
driving standard threshold MOSFETs, the external sup-
ply must always be present during operation to prevent
MOSFET failure due to insufficient gate drive.
Topside MOSFET Driver Supply (C
B
, D
B
)
An external bootstrap capacitor C
B
connected to the Boost
pin supplies the gate drive voltage for the topside MOSFET.
Capacitor C
B
in the Functional Diagram is charged through
diode D
B
from INTV
CC
when the SW pin is low. When the
Figure 4a. Secondary Output Loop and EXTV
CC
Connection
EXTV
CC
V
IN
TG
BG
PGND
LTC1435A
N-CH
N-CH
+
C
IN
V
IN
+
C
OUT
L1
R
SENSE
V
OUT
+
1µF
1435A F04b
VN2222LL
BAT85
BAT85
BAT85
0.22µF
SW
Figure 4b. Capacitive Charge Pump for EXTV
CC
topside MOSFET is to be turned on, the driver places the
C
B
voltage across the gate source of the MOSFET. This en-
hances the MOSFET and turns on the topside switch. The
switch node voltage SW rises to V
IN
and the Boost pin rises
to V
IN
+ INTV
CC
. The value of the boost capacitor C
B
needs
to be 100 times greater than the total input capacitance of
the topside MOSFET. In most applications 0.1µF is ad-
equate. The reverse breakdown on D
B
must be greater than
V
IN(MAX).
Output Voltage Programming
The output voltage is set by a resistive divider according
to the following formula:
VV
R
R
VV
OUT OUT
=+
≥119 1
2
1
119.,.