LTC3729_15 Datasheet, PDF (16/30 Pages) Linear Technology – 550kHz, PolyPhase, High Efficiency, Synchronous Step-Down Switching Regulator

LTC3729
APPLICATIONS INFORMATION
maximum junction temperature rating for the LTC3729 to
be exceeded. The supply current is dominated by the gate
charge supply current, in addition to the current drawn
from the differential amplifier output. The gate charge is
dependent on operating frequency as discussed in the
Efficiency Considerations section. The supply current can
either be supplied by the internal 5V regulator or via the
EXTV_CC pin. When the voltage applied to the EXTV_CC pin
is less than 4.7V, all of the INTV_CC load current is supplied
by the internal 5V linear regulator. Power dissipation for
the IC is higher in this case by (I_IN)(V_IN – INTV_CC) and
efficiency is lowered. The junction temperature can be
estimated by using the equations given in Note 1 of the
Electrical Characteristics. For example, the LTC3729 V_IN
current is limited to less than 24mA from a 24V supply:
T_J = 70°C + (24mA)(24V)(95°C/W) = 125°C
Use of the EXTV_CC pin reduces the junction temperature
to:
T_J = 70°C + (24mA)(5V)(95°C/W) = 81.4°C
The input supply current should be measured while
the controller is operating in continuous mode at maximum
V_IN and the power dissipation calculated in order to
prevent the maximum junction temperature from being
exceeded.
EXTV_CC Connection
The LTC3729 contains an internal P-channel MOSFET
switch connected between the EXTV_CC and INTV_CC pins.
When the voltage applied to EXTV_CC rises above 4.7V,
the internal regulator is turned off and the switch closes,
connecting the EXTV_CC pin to the INTV_CC pin thereby
supplying internal and MOSFET gate driving power. The
switch remains closed as long as the voltage applied to
EXTV_CC remains above 4.5V. This allows the MOSFET
driver and control power to be derived from the output
during normal operation (4.7V < V_EXTVCC < 7V) and from
the internal regulator when the output is out of regulation
(start-up, short-circuit). Do not apply greater than 7V
to the EXTV_CC pin and ensure that EXTV_CC < V_IN + 0.3V
when using the application circuits shown. If an external
voltage source is applied to the EXTV_CC pin when the V_IN
supply is not present, a diode can be placed in series
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with the LTC3729’s V_IN pin and a Schottky diode between
the EXTV_CC and the V_IN pin, to prevent current from
backfeeding V_IN.
Significant efficiency 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 the
ratio: (Duty Factor)/(Efficiency). For 5V regulators this
means connecting the EXTV_CC pin directly to V_OUT .
However, for 3.3V and other lower voltage regulators,
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 5V regulator resulting in
a significant efficiency penalty 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 an external
supply is available in the 5V to 7V range, it may be used to
power EXTV_CC providing it is compatible with the MOSFET
gate drive requirements. V_IN must be greater than or equal
to the voltage applied to the EXTV_CC pin.
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 which has been boosted to greater than
4.7V but less than 7V. This can be done with either the
inductive boost winding as shown in Figure 5a or the
capacitive charge pump shown in Figure 5b. The charge
pump has the advantage of simple magnetics.
Topside MOSFET Driver Supply (CB,DB) (Refer to
Functional Diagram)
External bootstrap capacitors C_B1 and C_B2 connected
to the BOOST1 and BOOST2 pins supply the gate drive
voltages for the topside MOSFETs. Capacitor C_B in the
Functional Diagram is charged though diode D_B from
INTV_CC when the SW pin is low. When the topside MOSFET
turns on, the driver places the C_B voltage across the
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