ltc3853euj Linear Technology Corporation, ltc3853euj Datasheet - Page 20

• Manufacturer Part Number: ltc3853euj
• Description: Triple Output, Multiphase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

LTC3853
APPLICATIONS INFORMATION
ceramic capacitor placed directly adjacent to the INTV
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
gate charge current, may be supplied by either the 5V linear
regulator or EXTV
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
on operating frequency as discussed in the Effi ciency
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
current is limited to less than 50mA from a 24V supply in
the UJ package and not using the EXTV
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
rises above 4.7V, the INTV
and the EXTV
remains on as long as the voltage applied to EXTV
above 4.5V. Using the EXTV
and control power to be derived from one of the LTC3853’s
switching regulator outputs during normal operation and
from the INTV
(e.g., start-up, short-circuit). If more current is required
through the EXTV
diode can be added between the EXTV
Do not apply more than 6V to the EXTV
sure that EXTV
Signifi cant effi ciency and thermal gains can be realized by
powering INTV
resulting from the driver and control currents will be scaled
by a factor of (Duty Cycle)/(Switcher Effi ciency).
20
T
J
IN
= 85°C + (50mA)(24V)(33°C/W) = 125°C
• I
INTVCC
CC
CC
CC
CC
. The gate charge current is dependent
is connected to the INTV
CC
CC
< V
from the output, since the V
when the output is out of regulation
CC
. When the voltage on the EXTV
than is specifi ed, an external Schottky
IN
IN
current, which is dominated by the
. When the voltage applied to EXTV
.
CC
CC
linear regulator is turned off
allows the MOSFET driver
CC
CC
and INTV
CC
CC
supply:
pin and make
. The EXTV
CC
IN
remains
CC
current
CC
pins.
pin
CC
CC
CC
CC
Tying the EXTV
temperature in the previous example from 125°C to:
However, for 3.3V and other low voltage outputs, addi-
tional circuitry is required to derive INTV
the output.
The following list summarizes the four possible connec-
tions for EXTV
1. EXTV
2. EXTV
3. EXTV
4. EXTV
For applications where the main input power is 5V, tie
the V
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
regulator and will prevent INTV
due to the dropout voltage. Make sure the INTV
is at or exceeds the R
which is typically 4.5V for logic-level devices.
T
INTV
resulting in an effi ciency penalty of up to 10% at high
input voltages.
normal connection for a 5V regulator and provides
the highest effi ciency.
external supply is available, it may be used to power
EXTV
gate drive requirements.
work. For 3.3V and other low voltage regulators,
effi ciency gains can still be realized by connecting
EXTV
boosted to greater than 4.7V.
J
IN
= 85°C + (50mA)(5V)(33°C/W) = 94°C
CC
and INTV
CC
CC
CC
CC
CC
CC
to be powered from the internal 5V regulator
providing it is compatible with the MOSFET
connected to an output-derived boost net-
to an output-derived voltage that has been
connected to an external supply. If a 5V
left open (or grounded). This will cause
connected directly to V
CC
CC
Figure 8. Setup for a 5V Input
LTC3853
:
CC
pin to a 5V supply reduces the junction
INTV
pins together and tie the combined
V
DS(ON)
CC
IN
test voltage for the MOSFET
C
4.7μF
INTVCC
R
1Ω
CC
VIN
from dropping too low
+
3853 F08
C
OUT
5V
IN
CC
. This is the
power from
CC
CC
voltage
linear
3853f