LTC3867_15 Datasheet, PDF (26/36 Pages) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

LTC3867
APPLICATIONS INFORMATION
INTV_CC (LDO) and EXTV_CC
The LTC3867 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 LTC3867’s internal circuitry.
The LDO regulates the voltage at the INTV_CC pin to 5.3V
when V_IN is greater than 5.8V. 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. Either of these
can supply a peak current of 100mA and must be bypassed
to ground with a minimum of 4.7µF ceramic capacitor or
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 sup-
ply the high transient currents required by the MOSFET
gate drivers. High input voltage applications in which
large MOSFETs are being driven at high frequencies may
cause the maximum junction temperature rating for the
LTC3867 to be exceeded. The INTV_CC current, which is
dominated by the gate charge current, may be supplied by
either the 5.3V LDO or EXTV_CC. When the voltage on the
EXTV_CC pin is less than 4.5V, the 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 2 of the
Electrical Characteristics tables. For example, the LTC3867
INTV_CC current is limited to less than 30mA from a 38V
supply in the UF package and not using the EXTV_CC supply
with a 70°C ambient temperature:
T_J = 70°C + (30mA)(38V)(47°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 = SGND)
at maximum V_IN. When the voltage applied to EXTV_CC rises
above 4.7V, the INTV_CC LDO 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 an efficient switching regulator
output during normal operation. If more current is required
through the EXTV_CC than is specified, an external Schottky
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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 EXTV_CC, 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 + (30mA)(5V)(47°C/W) = 77°C
However, for 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 LDO resulting
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.
For applications where the main input power is 5V, tie
the V_IN and INTV_CC pins together and tie the combined
pins to the 5V input with a 1Ω or 2.2Ω resistor as shown
in Figure 12 to minimize the voltage drop caused by the
gate charge current. This will override the INTV_CC linear
regulator and will prevent INTV_CC from dropping too low
due to the dropout voltage. Make sure the INTV_CC voltage
is at or exceeds the R_DS(ON) test voltage for the MOSFET
which is typically 4.5V for logic-level devices
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