Datasheet

11
LTC1435
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 5V 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 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
inductive 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
compatible with the MOSFET gate drive requirements.
When driving standard threshold MOSFETs, the exter-
nal supply must always be present during operation to
prevent MOSFET failure due to insufficient gate drive.
APPLICATIONS INFORMATION
WUU
U
INTV
CC
Regulator
An internal P-channel low dropout regulator produces the
5V supply which powers the drivers and internal circuitry
within the LTC1435. The INTV
CC
pin can supply up to
15mA and must be bypassed to ground with a minimum
of 2.2µF tantalum or low ESR electrolytic. Good bypassing
is necessary to supply the high transient currents required
by the MOSFET gate drivers.
High input voltage applications, in which large MOSFETs
are being driven at high frequencies, may cause the
maximum junction temperature rating for the LTC1435 to
be exceeded. The IC supply current is dominated by the
gate charge supply current when not using an output
derived EXTV
CC
source. The gate charge is dependent on
operating frequency as discussed in the Efficiency Consid-
erations section. The junction temperature can be esti-
mated by using the equations given in Note 1 of the
Electrical Characteristics. For example, the LTC1435 is
limited to less than 17mA from a 30V supply:
T
J
= 70°C + (17mA)(30V)(100°C/W) = 126°C
To prevent maximum junction temperature from being
exceeded, the input supply current must be checked when
operating in continuous mode at maximum V
IN
.
EXTV
CC
Connection
The LTC1435 contains an internal P-channel MOSFET
switch connected between the EXTV
CC
and INTV
CC
pins.
The switch closes and supplies the INTV
CC
power when-
ever the EXTV
CC
pin is above 4.8V, and remains closed
until EXTV
CC
drops below 4.5V. This allows the MOSFET
driver and control power to be derived from the output
during normal operation (4.8V < V
OUT
< 9V) and from the
internal regulator 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,
Figure 4a. Secondary Output Loop and EXTV
CC
Connection
R6
R5
EXTV
CC

SFB
SGND
V
IN

TG



BG
PGND
LTC1435
N-CH
N-CH
+
C
IN
V
IN
1N4148
+
1µF
+
C
OUT
V
SEC
L1
1:N
R
SENSE
V
OUT
OPTIONAL
EXT V
CC
CONNECTION
5V ≤ V
SEC
≤ 9V
LTC1435 • F04a
SW
