LTC3890
applicaTions inForMaTion
INTV Regulators
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
CC
TheLTC3890featurestwoseparateinternalP-channellow
dropout linear regulators (LDO) that supply power at the
INTVCC pin from either the VIN supply pin or the EXTVCC
pin depending on the connection of the EXTVCC pin.
INTVCCpowersthegatedriversandmuchoftheLTC3890’s
internalcircuitry.TheVINLDOandtheEXTVCCLDOregulate
= INTV ) at maximum V .
CC
IN
When the voltage applied to EXTV rises above 4.7V, the
CC
V LDO is turned off and the EXTV LDO is enabled. The
IN
CC
EXTV LDO remains on as long as the voltage applied to
CC
INTV to 5.1V. Each of these can supply a peak current of
EXTV remains above 4.5V. The EXTV LDO attempts
CC
CC CC
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
to regulate the INTV voltage to 5.1V, so while EXTV
CC
CC
CC
CC
is less than 5.1V, the LDO is in dropout and the INTV
voltage is approximately equal to EXTV . When EXTV
CC
placed directly adjacent to the INTV and PGND pins is
is greater than 5.1V, up to an absolute maximum of 14V,
INTV is regulated to 5.1V.
CC
highlyrecommended.Goodbypassingisneededtosupply
the high transient currents required by the MOSFET gate
drivers and to prevent interaction between the channels.
CC
Using the EXTV LDO allows the MOSFET driver and
CC
control power to be derived from one of the LTC3890’s
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3890 to be
exceeded. The INTV current, which is dominated by
the gate charge current, may be supplied by either the
switching regulator outputs (4.7V ≤ V
≤ 14V) during
OUT
normal operation and from the V LDO when the output
IN
is out of regulation (e.g., start-up, short-circuit). If more
current is required through the EXTV LDO than is speci-
CC
CC
fied, an external Schottky diode can be added between the
V
LDO or the EXTV LDO. When the voltage on the
CC
EXTV and INTV pins. In this case, do not apply more
IN
CC
CC CC
EXTV pinislessthan4.7V,theV LDOisenabled.Power
than6VtotheEXTV pinandmakesurethatEXTV ≤V .
CC CC IN
IN
dissipation for the IC in this case is highest and is equal
to V • I . The gate charge current is dependent on
Significant efficiency and thermal gains can be realized
by powering INTV from the output, since the V cur-
IN INTVCC
CC
IN
operatingfrequencyasdiscussedintheEfficiencyConsid-
erationssection.Thejunctiontemperaturecanbeestimated
by using the equations given in Note 3 of the Electrical
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
Characteristics. For example, the LTC3890 INTV current
CC
the EXTV pin directly to V . Tying the EXTV pin to
CC
OUT
CC
is limited to less than 32mA from a 40V supply when not
an 8.5V supply reduces the junction temperature in the
using the EXTV supply at a 70°C ambient temperature:
CC
previous example from 125°C to:
T = 70°C + (32mA)(40V)(43°C/W) = 125°C
J
T = 70°C + (32mA)(8.5V)(43°C/W) = 82°C
J
However,for3.3Vandotherlowvoltageoutputs,additional
circuitryisrequiredtoderiveINTV powerfromtheoutput.
CC
3890fb
21
Linear Systems [ Linear Systems ]
