Datasheet

LTC3857
21
3857fd
APPLICATIONS INFORMATION
INTV
CC
Regulators
The LTC3857 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 LTC3857’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
50mA and must be bypassed to ground with a minimum
of 4.7µF 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 maxi-
mum junction temperature rating for the LTC3857 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 LTC3857
INTV
CC
current is limited to less than 32mA from a 40V
supply when not using the EXTV
CC
supply at 70°C ambi-
ent temperature:
T
J
= 70°C + (32mA)(40V)(43°C/W) = 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 LTC3857’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 speci-
fied, 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
cur-
rent 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, 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 5.1V regulator result-
ing 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
.