LTC3865-1_15 Datasheet, PDF (21/38 Pages) Linear Technology – Dual, 2-Phase Synchronous DC/DC Controller with Pin Selectable Outputs

LTC3865/LTC3865-1
APPLICATIONS INFORMATION
When the master channel’s output experiences dynamic
excursion (under load transient, for example), the slave
channel output will be affected as well. For better output
regulation, use the coincident tracking mode instead of
ratiometric.
INTV_CC Regulators and EXTV_CC
The LTC3865 features a true PMOS LDO that supplies
power to INTV_CC from the V_IN supply. INTV_CC powers the
gate drivers and much of the LTC3865/LTC3865-1’s internal
circuitry. The linear regulator regulates the voltage at the
INTV_CC pin to 5V when V_IN is greater than 5.5V. 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 80mA
and must be bypassed to ground with a minimum of 4.7μF
ceramic capacitor or low ESR electrolytic capacitor. No mat-
ter 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 channels.
High input voltage applications in which large MOSFETs are
being driven at high frequencies may cause the maximum
junction temperature rating for the LTC3865/LTC3865-1
to be exceeded. The INTV_CC current, which is dominated
by the gate charge current, may be supplied by either the
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 highest and is
equal to V_IN • I_INTVCC. The gate charge current is depen-
dent 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 LTC3865 INTV_CC
current is limited to less than 42mA from a 38V supply in
the UH package and not using the EXTV_CC supply:
T_J = 70°C + (42mA)(38V)(34°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 LTC3865/LTC3865-1’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 speci-
fied, 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 current
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 + (42mA)(5V)(34°C/W) = 77°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 5V 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 regulator and provides the highest
efficiency.
3. EXTV_CC connected to an external supply. If a 5V external
supply is available, it may be used to power EXTV_CC
providing it is compatible with the MOSFET gate drive
requirements.
4. EXTV_CC connected to an output-derived boost network.
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTV_CC to an
output-derived voltage that has been boosted to greater
than 4.7V.
3865fb
21