LTC3875_15 Datasheet, PDF (25/44 Pages) Linear Technology – Dual, 2-Phase, Synchronous Controller with Low Value DCR Sensing and Temperature Compensation

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_OSNS+ pin voltage. When V_OSNS+ is higher than
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_OSNS+, whichever is lower, TG and BG are enabled. If
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_CC Regulators and EXTV_CC
The LTC3875 features a PMOS LDO that supplies power
to INTV_CC from the V_IN supply. INTV_CC powers the gate
drivers and much of the LTC3875’s internal circuitry. The
linear regulator regulates the voltage at the INTV_CC pin to
5.5V when V_IN is greater than 6V. EXTV_CC connects to
INTV_CC through a P-channel MOSFET and can supply the
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_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 LTC3875 to be
exceeded. The INTV_CC current, which is dominated by
the gate charge current, may be supplied by either the
5.5V linear regulator or EXTV_CC. When the voltage on
the EXTV_CC pin is less than 4.7V, the linear regulator is
enabled. Power dissipation for the IC in this case is high-
est 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 LTC3875
INTV_CC current is limited to less than 44mA from a 38V
supply in the UJ package and not using the EXTV_CC supply:
T_J = 70°C + (44mA)(38V)(33°C/W) = 125°C
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_IN. When the voltage applied to
EXTV_CC rises above 4.7V, the INTV_CC linear regulator is
turned off and the EXTV_CC is connected to the INTV_CC.
The EXTV_CC remains on as long as the voltage applied
to EXTV_CC remains above 4.5V. Using the EXTV_CC allows
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_CC 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 INTV_CC pins. 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).
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 + (44mA)(5V)(33°C/W) = 77°C
However, for 3.3V and other low voltage outputs, additional
circuitry is required to derive INTV_CC power from the output.
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