LTC3850-2_15 Datasheet, PDF (19/36 Pages) Linear Technology – Dual, 2-Phase Synchronous Step-Down Switching Controller

LTC3850-2
APPLICATIONS INFORMATION
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3850-2 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 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 tempera-
ture can be estimated by using the equations given in
Note 3 of the Electrical Characteristics. For example, the
LTC3850-2 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 LTC3850-2’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:
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.
LTC3850-2 ^VIN
INTV_CC
R_VIN
5V
1Ω
C_INTVCC
+
4.7μF
C_IN
38502 F08
Figure 8. Setup for a 5V Input
38502f
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