Datasheet

LTC3891
20
3891fa
INTV
CC
Regulators
The LTC3891 features two separate internal P-channel
low dropout linear regulators (LDO) that supply power
at the INTV
CC
pin from either the V
IN
supply pin or the
EXTV
CC
pin depending on the connection of the EXTV
CC
pin. INTV
CC
powers the gate drivers and much of the
LTC3891’s internal circuitry. The V
IN
LDO and the EXTV
CC
LDO regulate INTV
CC
to 5.1V. Each of these can supply a
peak current of at least 50mA and must be bypassed to
ground with a minimum of 2.2μF ceramic capacitor. No
matter what type of bulk capacitor is used, an additional
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 maximum
junction temperature rating for the LTC3891 to be exceeded.
The INTV
CC
current, which is dominated by the gate charge
current, may be supplied by either the V
IN
LDO or the
EXTV
CC
LDO. When the voltage on the EXTV
CC
pin is less
than 4.7V, the V
IN
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.
For example, the LTC3891 INTV
CC
current is limited to less
than 32mA from a 40V supply when not using the EXTV
CC
supply at a 70°C ambient temperature:
T
J
= 70°C + (32mA)(40V)(43°C/W for QFN) = 125°C
To prevent the maximum junction temperature from be-
ing exceeded, the input supply current must be checked
while operating in forced continuous mode (PLLIN/MODE
= INTV
CC
) at maximum V
IN
.
When the voltage applied to EXTV
CC
rises above 4.7V, the
V
IN
LDO is turned off and the EXTV
CC
LDO is enabled. The
EXTV
CC
LDO remains on as long as the voltage applied to
EXTV
CC
remains above 4.5V. The EXTV
CC
LDO attempts
to regulate the INTV
CC
voltage to 5.1V, so while EXTV
CC
is less than 5.1V, the LDO is in dropout and the INTV
CC
voltage is approximately equal to EXTV
CC
. When EXTV
CC
is greater than 5.1V, up to an absolute maximum of 14V,
INTV
CC
is regulated to 5.1V.
Using the EXTV
CC
LDO allows the MOSFET driver and control
power to be derived from one of the LTC3891’s switching
regulator outputs (4.7V ≤ V
OUT
≤ 14V) during normal
operation and from the V
IN
LDO when the output is out of
regulation (e.g., start-up, short-circuit). If more current
is required through the EXTV
CC
LDO than is specified, an
external Schottky diode can be added between the EXTV
CC
and INTV
CC
pins. In this case, 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).
For 5V to 14V regulator outputs, this means connecting
the EXTV
CC
pin directly to V
OUT
. Tying the EXTV
CC
pin to
an 8.5V supply reduces the junction temperature in the
previous example from 125°C to:
T
J
= 70°C + (32mA)(8.5V)(43°C/W) = 82°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
Grounded. This will cause INTV
CC
to be powered
from the internal 5.1V 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 to 14V regulator and provides the
highest efficiency.
3. EXTV
CC
Connected to an External Supply. If an external
supply is available in the 5V to 14V range, it may be
used to power EXTV
CC
providing it is compatible with
the MOSFET gate drive requirements. Ensure that
EXTV
CC
< V
IN
.
APPLICATIONS INFORMATION