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LTC3875
APPLICATIONS INFORMATION
the slave channel output will be affected as well. For bet-
ter output regulation, use the coincident tracking mode
instead of ratiometric.
Pre-Biased Output Start-Up
There may be situations that require the power supply to
start up with a pre-bias on the output capacitors. In this
case, it is desirable to start up without discharging that
output pre-bias. The LTC3875 can safely power up into
a pre-biased output without discharging it. The LTC3875
accomplishes this by disabling both TG and BG until the
TK/SS pin voltage and the internal soft-start voltage are
+
above the V
pin voltage. When V
OSNS
TK/SS or the internal soft-start voltage, the error amp output
is low. The control loop would like to turn BG on, which
would discharge the output. Disabling BG and TG prevents
the pre-biased output voltage from being discharged.
When TK/SS and the internal soft-start both cross 500mV
+
or V
, whichever is lower, TG and BG are enabled. If
OSNS
the pre-bias is higher than the OV threshold, the bottom
gate is turned on immediately to pull the output back into
the regulation window.
INTV
Regulators and EXTV
CC
The LTC3875 features a PMOS LDO that supplies power
to INTV
from the V
supply. INTV
CC
IN
drivers and much of the LTC3875's internal circuitry. The
linear regulator regulates the voltage at the INTV
5.5V when V
is greater than 6V. EXTV
IN
INTV
through a P-channel MOSFET and can supply the
CC
needed power when its voltage is higher than 4.7V. Each
of these can supply a peak current of 100mA and must be
bypassed to ground with a minimum of a 4.7µF ceramic
capacitor or a low ESR electrolytic capacitor. No matter
what type of bulk capacitor is used, an additional 0.1µF
ceramic capacitor placed directly adjacent to the INTV
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-
26
+
is higher than
OSNS
CC
powers the gate
CC
pin to
CC
connects to
CC
CC
For more information
mum junction temperature rating for the LTC3875 to be
exceeded. The INTV
CC
the gate charge current, may be supplied by either the
5.5V linear regulator or EXTV
the EXTV
pin is less than 4.7V, the linear regulator is
CC
enabled. Power dissipation for the IC in this case is high-
est and is equal to V
IN
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 LTC3875
INTV
current is limited to less than 44mA from a 38V
CC
supply in the UJ package and not using the EXTV
T
= 70°C + (44mA)(38V)(33°C/W) = 125°C
J
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (MODE/PLLIN
= SGND) at maximum V
EXTV
rises above 4.7V, the INTV
CC
turned off and the EXTV
The EXTV
remains on as long as the voltage applied
CC
to EXTV
remains above 4.5V. Using the EXTV
CC
the MOSFET driver and control power to be derived from
one of the LTC3875's switching regulator outputs during
normal operation and from the INTV
is out of regulation (e.g., start-up, short-circuit). If more
current is required through the EXTV
an external Schottky diode can be added between the
EXTV
and INTV
pins. Do not apply more than 6V to
CC
CC
the EXTV
pin and make sure that EXTV
CC
Significant efficiency and thermal gains can be realized
by powering INTV
from the output, since the V
CC
rent resulting from the driver and control currents will be
scaled by a factor of (Duty Cycle)/(Switcher Efficiency).
Tying the EXTV
pin to a 5V supply reduces the junction
CC
temperature in the previous example from 125°C to:
T
= 70°C + (44mA)(5V)(33°C/W) = 77°C
J
However, for 3.3V and other low voltage outputs, additional
circuitry is required to derive INTV
www.linear.com/LTC3875
current, which is dominated by
. When the voltage on
CC
• I
. The gate charge current
INTVCC
CC
. When the voltage applied to
IN
linear regulator is
CC
is connected to the INTV
CC
CC
when the output
CC
than is specified,
CC
< V
CC
IN
power from the output.
CC
supply:
.
CC
allows
.
cur-
IN
3875fb
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