Datasheet

LTC3859
27
3859fa
APPLICATIONS INFORMATION
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (PLLIN/MODE
= INTV
CC
) at maximum V
IN
.
When the voltage applied to EXTV
CC
rises above 4.7V, the
V
BIAS
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 5.4V, so while EXTV
CC
is less than 5.4V, the LDO is in dropout and the INTV
CC
voltage is approximately equal to EXTV
CC
. When EXTV
CC
is greater than 5.4V, up to an absolute maximum of 14V,
INTV
CC
is regulated to 5.4V.
Using the EXTV
CC
LDO allows the MOSFET driver and
control power to be derived from one of the LTC3859’s
switching regulator outputs (4.7V ≤ V
OUT
≤ 14V) dur-
ing normal operation and from the V
BIAS
LDO when the
output is out of regulation (e.g., startup, short-circuit). If
more current is required through the EXTV
CC
LDO than
is specifi ed, 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
than EXTV
CC
≤ V
BIAS
.
Signifi cant effi ciency and thermal gains can be realized
by powering INTV
CC
from the buck 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).
For 5V to 14V regulator outputs, this means connecting
the EXTV
CC
pin directly to V
OUT
. Tying the EXTV
CC
pin to
a 8.5V supply reduces the junction temperature in the
previous example from 125°C to:
T
J
= 70°C + (40mA)(8.5V)(34°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
left open (or grounded). This will cause INTV
CC
to be powered from the internal 5.4V regulator result-
ing in an effi ciency penalty of up to 10% at high input
voltages.
2. EXTV
CC
connected directly to the output voltage of one
of the buck regulators. This is the normal connection
for a 5V to 14V regulator and provides the highest ef-
fi ciency.
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
.
4. EXTV
CC
connected to an output-derived boost network
off one of the buck regulators. For 3.3V and other low
voltage buck 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. This
can be done with the capacitive charge pump shown in
Figure 9. Ensure that EXTV
CC
< V
IN
.
Figure 9. Capacitive Charge Pump for EXTV
CC
3859 F09
LTC3859
TG
SW
BG
PGND
R
SENSE
MTOP
MBOT
L
EXTV
CC
BAT85 BAT85
C1
V
IN1,2
BAT85
V
OUT1,2
Topside MOSFET Driver Supply (C
B
, D
B
)
External bootstrap capacitors C
B
connected to the BOOST
pins supply the gate drive voltages for the topside MOSFETs.
Capacitor C
B
in the Functional Diagram is charged though
external diode D
B
from INTV
CC
when the SW pin is low.
When one of the topside MOSFETs is to be turned on, the
driver places the C
B
voltage across the gate-source of the
desired MOSFET. This enhances the MOSFET and turns
on the topside switch. The switch node voltage, SW, rises
to V
IN
for the buck channels (V
OUT
for the boost channel)
and the BOOST pin follows. With the topside MOSFET