Datasheet

LTC3876
27
3876f
When the voltage applied to the EXTV
CC
pin rises above
4.7V, the V
IN
LDO is turned off and the EXTV
CC
is connected
to DRV
CC2
pin with an internal switch. This switch remains
on as long as the voltage applied to EXTV
CC
remains
above 4.5V. Using EXTV
CC
allows the MOSFET driver and
control power to be derived from the LTC3876’s switching
regulator output V
OUT
during normal operation and from
the LDO 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 DRV
CC
pins. Do not
apply more than 6V to the EXTV
CC
pin and make sure that
EXTV
CC
is less than V
IN
.
Significant efficiency and thermal gains can be realized
by powering DRV
CC
from the switching converter 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 + (52mA)(5V)(28°C/W) = 77°C
However, for 3.3V and other low voltage outputs, ad-
ditional circuitry is required to derive DRV
CC
power from
the converter 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 5.3V LDO resulting
in an efficiency penalty of up to 10% at high input
voltages.
2. EXTV
CC
connected directly to switching converter output
V
OUT
> 4.7V. This provides the highest efficiency.
3. EXTV
CC
connected to an external supply. If a 4.7V or
greater external supply is available, it may be used to
power EXTV
CC
providing that the external supply is
sufficient for MOSFET gate drive requirements.
4. EXTV
CC
connected to an output-derived boost network.
For 3.3V and other low voltage converters, 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 never exceeds
5.3V, tie the DRV
CC1
and DRV
CC2
pins to the V
IN
input
through a small resistor, (such as 1 to 2) as shown
in Figure 7 to minimize the voltage drop caused by the
gate charge current. This will override the LDO and will
prevent DRV
CC
from dropping too low due to the dropout
voltage. Make sure the DRV
CC
voltage exceeds the R
DS(ON)
test voltage for the external MOSFET which is typically at
4.5V for logic-level devices.
APPLICATIONS INFORMATION
Figure 7. Setup for V
IN
≤ 5.3V
Input Undervoltage Lockout (UVLO)
The LTC3876 has two functions that help protect the con-
troller in case of input undervoltage conditions. An internal
UVLO comparator constantly monitors the INTV
CC
and
DRV
CC
voltages to ensure that adequate voltages are pres-
ent. The comparator enables internal UVLO signal, which
locks out the switching action of both channels, until the
INTV
CC
and DR
VCC1,2
pins are all above their respective
UVLO thresholds. The rising threshold (to release UVLO)
of the INTV
CC
is typically 4.2V, with 0.5V falling hysteresis
(to re-enable UVLO). The UVLO thresholds for DR
VCC1,2
are
lower than that of INTV
CC
but higher than typical threshold
voltages of power MOSFETs, to prevent them from turning
on without sufficient gate drive voltages.
Generally for V
IN
> 6V, a UVLO can be set through monitoring
the V
IN
supply by using external voltage dividers at the RUN
pins from V
IN
to SGND. To design the voltage divider, note
that both RUN pins have two levels of threshold voltages.
The precision gate-drive-enable threshold voltage of 1.2V
DRV
CC2
LTC3876
DRV
CC1
C
DRVCC
R
DRVCC
3876 F07
V
IN
C
IN