Datasheet

LTC3855
24
3855f
applicaTions inForMaTion
INTV
CC
Regulators and EXTV
CC
The LTC3855 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 LTC3855’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 100mA and must
be bypassed to ground with a minimum of 4.7µF ceramic
capacitor or low ESR electrolytic capacitor. No matter
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 the channels.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3855 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 dependent
on operating frequency as discussed in the Efficiency
Considerations section. The junction temperature can be
estimated by using the equations given in Note 3 of the
Electrical Characteristics. For example, the LTC3855 INTV
CC
current is limited to less than 44mA from a 38V supply in
the UJ package and not using the EXTV
CC
supply:
T
J
= 70°C + (44mA)(38V)(33°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 EXT-
V
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 LTC3855’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 specified, 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
.
Significant efficiency 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 Efficiency).
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 + (44mA)(5V)(33°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
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 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 net-
work. 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.
For applications where the main input power is below 5V,
tie the V
IN
and INTV
CC
pins together and tie the combined
pins to the 5V input with a 1Ω or 2.2Ω resistor as shown
in Figure 10 to minimize the voltage drop caused by the
gate charge current. This will override the INTV
CC
linear
regulator and will prevent INTV
CC
from dropping too low