ltc3850gn-2 Linear Technology Corporation, ltc3850gn-2 Datasheet - Page 19

• Manufacturer Part Number: ltc3850gn-2
• Description: Dual, 2-phase Synchronous Step-down Switching Controller
• Manufacturer: Linear Technology Corporation

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
by the gate charge current, may be supplied by either
the 5V linear regulator or EXTV
the EXTV
enabled. Power dissipation for the IC in this case is high-
est and is equal to V
is dependent on operating frequency as discussed in the
Effi ciency 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
from a 24V supply in the GN 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
EXTV
turned off and the EXTV
The EXTV
to EXTV
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
is out of regulation(e.g., start-up, short-circuit). If more
current is required through the EXTV
an external Schottky diode can be added between the
EXTV
the 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).
Tying the EXTV
temperature in the previous example from 125°C to:
T
T
J
J
CC
CC
CC
= 70°C + (24mA)(24V)(95°C/W) = 125°C
= 70°C + (24mA)(5V)(95°C/W) = 81°C
CC
supply:
rises above 4.7V, the INTV
and INTV
CC
CC
CC
remains above 4.5V. Using the EXTV
pin and make sure that EXTV
pin is less than 4.7V, the linear regulator is
remains on as long as the voltage applied
CC
CC
CC
CC
pin to a 5V supply reduces the junction
from the output, since the V
current is limited to less than 24mA
IN
pins. Do not apply more than 6V to
• I
CC
IN
CC
INTVCC
. When the voltage applied to
current, which is dominated
is connected to the INTV
. The gate charge current
CC
. When the voltage on
CC
CC
CC
linear regulator is
than is specifi ed,
when the output
CC
< V
IN
IN
CC
.
current
allows
CC
.
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.
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.
IN
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
LTC3850-2
CC
Figure 8. Setup for a 5V Input
:
CC
INTV
pins together and tie the combined
V
DS(ON)
CC
IN
test voltage for the MOSFET
C
4.7μF
INTVCC
CC
from dropping too low
R
1Ω
LTC3850-2
VIN
OUT
+
CC
38502 F08
. This is the
C
5V
IN
power from
CC
CC
19
voltage
linear
38502f