LTC3850-1_15 Datasheet, PDF (20/38 Pages) Linear Technology – Dual, 2-Phase Synchronous Step-Down Switching Controller

LTC3850/LTC3850-1
APPLICATIONS INFORMATION
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 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 LTC3850 INTV_CC
current is limited to less than 24mA from a 24V supply in
the GN package and not using the EXTV_CC supply:
T_J = 70°C + (24mA)(24V)(95°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 LTC3850’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 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 + (24mA)(5V)(95°C/W) = 81°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:
20
1. EXTV_CC left open (or grounded). This will cause
INTV_CC to be powered from the internal 5V regulator
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 net-
work. 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 8 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.
LTC3850 ^VIN
INTV_CC
R_VIN
5V
CINTV_CC
1Ω
+
4.7µF
C_IN
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Figure 8. Setup for a 5V Input
Topside MOSFET Driver Supply (C_B, DB)
External bootstrap capacitors C_B connected to the BOOST
pins supply the gate drive voltages for the topside MOSFETs.
Capacitor C_B in the Functional Diagram is charged though
external diode DB from INTV_CC when the SW pin is low.
When one of the topside MOSFETs is to be turned on,
the driver places the C_B voltage across the gate source
of the desired MOSFET. This enhances the MOSFET and
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