Datasheet

LTC3829
28
3829fa
For more information www.linear.com/LTC3829
APPLICATIONS INFORMATION
INTV
CC
(LDO) and EXTV
CC
The LTC3829 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 LTC3829’s internal circuitry. The
LDO 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 LTC3829 to be
exceeded. The INTV
CC
current, which is dominated by the
gate charge current, may be supplied by either the 5V LDO
or EXTV
CC
. When the voltage on the EXTV
CC
pin is less
than 4.7V, the LDO 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
tables. For example, the LTC3829 INTV
CC
current is limited
to less than 42mA from a 38V supply in the UHF 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 be-
ing exceeded, the input supply current must be checked
while operating in continuous conduction mode (MODE
= SGND) at maximum V
IN
. When the voltage applied to
EXTV
CC
rises above 4.7V, the INTV
CC
LDO 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 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
cur-
rent 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 + (42mA)(5V)(34°C/W) = 77°C
However, for low voltage outputs, 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 LDO 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.