Datasheet

LTC3865/LTC3865-1
21
3865fb
APPLICATIONS INFORMATION
When the master channel’s output experiences dynamic
excursion (under load transient, for example), the slave
channel output will be affected as well. For better output
regulation, use the coincident tracking mode instead of
ratiometric.
INTV
CC
Regulators and EXTV
CC
The LTC3865 features a true PMOS LDO that supplies
power to INTV
CC
from the V
IN
supply. INTV
CC
powers the
gate drivers and much of the LTC3865/LTC3865-1’s internal
circuitry. The linear regulator regulates the voltage at the
INTV
CC
pin to 5V when V
IN
is greater than 5.5V. EXTV
CC
connects to INTV
CC
through a P-channel MOSFET and can
supply the needed power when its voltage is higher than
4.7V. Each of these can supply a peak current of 80mA
and must be bypassed to ground with a minimum of 4.7µF
ceramic capacitor or low ESR electrolytic capacitor. No mat-
ter what type of bulk capacitor is used, an additional 0.1µF
ceramic capacitor placed directly adjacent to the INTV
CC
and PGND pins is highly recommended. Good bypassing
is needed to supply the high transient currents required
by the MOSFET gate drivers and to prevent interaction
between channels.
High input voltage applications in which large MOSFETs are
being driven at high frequencies may cause the maximum
junction temperature rating for the LTC3865/LTC3865-1
to be exceeded. The INTV
CC
current, which is dominated
by the gate charge current, may be supplied by either the
5V linear regulator or EXTV
CC
. When the voltage on the
EXTV
CC
pin is less than 4.7V, the linear regulator is enabled.
Power dissipation for the IC in this case is highest and is
equal to V
IN
• I
INTVCC
. The gate charge current is depen-
dent on operating frequency as discussed in the Effi ciency
Considerations section. The junction temperature can be
estimated by using the equations given in Note 3 of the
Electrical Characteristics. For example, the LTC3865 INTV
CC
current is limited to less than 42mA from a 38V supply in
the UH package and not using the EXTV
CC
supply:
T
J
= 70°C + (42mA)(38V)(34°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (MODE/PLLIN
= SGND) at maximum V
IN
. When the voltage applied to
EXTV
CC
rises above 4.7V, the INTV
CC
linear regulator is
turned off and the EXTV
CC
is connected to the INTV
CC
.
The EXTV
CC
remains on as long as the voltage applied to
EXTV
CC
remains above 4.5V. Using the EXTV
CC
allows the
MOSFET driver and control power to be derived from one
of the LTC3865/LTC3865-1’s switching regulator outputs
during normal operation and from the INTV
CC
when the
output is out of regulation (e.g., start-up, short-circuit). If
more current is required through the EXTV
CC
than is speci-
fi ed, an external Schottky diode can be added between the
EXTV
CC
and INTV
CC
pins. Do not apply more than 6V to
the EXTV
CC
pin and make sure that EXTV
CC
< V
IN
.
Signifi cant effi ciency and thermal 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)/(Switcher Effi ciency).
Tying the EXTV
CC
pin to a 5V supply reduces the junction
temperature in the previous example from 125°C to:
T
J
= 70°C + (42mA)(5V)(34°C/W) = 77°C
However, for 3.3V and other low voltage outputs, addi-
tional 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 result-
ing in an effi ciency 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
effi ciency.
3. EXTV
CC
connected to an external supply. If a 5V external
supply is available, it may be used to power EXTV
CC
providing it is compatible with the MOSFET gate drive
requirements.
4. EXTV
CC
connected to an output-derived boost network.
For 3.3V and other low voltage regulators, effi ciency
gains can still be realized by connecting EXTV
CC
to an
output-derived voltage that has been boosted to greater
than 4.7V.