LTC3853 Datasheet, PDF (20/36 Pages) Linear Technology – Triple Output, Multiphase Synchronous Step-Down Controller

LTC3853
APPLICATIONS INFORMATION
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 the channels.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3853 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 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 LTC3853 INTV_CC
current is limited to less than 50mA from a 24V supply in
the UJ package and not using the EXTV_CC supply:
T_J = 85°C + (50mA)(24V)(33°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 LTC3853’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).
20
Tying the EXTV_CC pin to a 5V supply reduces the junction
temperature in the previous example from 125°C to:
T_J = 85°C + (50mA)(5V)(33°C/W) = 94°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.
LTC3853
V_IN
INTV_CC
R_VIN
1Ω
5V
+ C_INTVCC
4.7μF
C_IN
3853 F08
Figure 8. Setup for a 5V Input
3853f