LTC3855 LINER [Linear Technology], LTC3855 Datasheet - Page 28

• Manufacturer Part Number: LTC3855
• Description: Dual, Fast, Accurate Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

LTC3838
APPLICATIONS INFORMATION
and BOOST capacitor under all operating conditions. Vari-
able frequency in response to load steps offers superior
transient performance but requires higher instantaneous
gate drive. Gate charge demands are greatest in high
frequency low duty factor applications under high load
steps and at start-up.
DRV
The LTC3838 features a PMOS low dropout (LDO) linear
regulator that supplies power to DRV
The LDO regulates its output at the DRV
The LDO can supply a maximum current of 100mA and
must be bypassed to ground with a minimum of 4.7μF
ceramic capacitor. Good bypassing is needed to supply
the high transient currents required by the MOSFET gate
drivers and to minimize 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 LTC3838 to be
exceeded, especially if the LDO is active and provides
DRV
est and is approximately equal to V
charge current is dependent on operating frequency as
discussed in the Efficiency Considerations section. The
junction temperature can be estimated by using the equa-
tion given in Note 2 of the Electrical Characteristics. For
example, when using the LDO, LTC3838’s DRV
is limited to less than 52mA from a 38V supply at T
70°C in the FE package:
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode at maximum V
When the voltage applied to the EXTV
the switchover voltage (typically 4.6V), the V
turned off and the EXTV
an internal switch. This switch remains on as long as the
voltage applied to EXTV
(around 200mV) below the switchover voltage. Using
EXTV
28
T
J
CC
CC
CC
= 70°C + (52mA)(38V)(28°C/W) = 125°C
. Power dissipation for the IC in this case is high-
Regulator and EXTV
allows the MOSFET driver and control power to
CC
CC
is connected to DRV
remains above the hysteresis
CC
Power
CC
IN
from the V
CC
• I
DRVCC
CC1
pin rises above
pin to 5.3V.
CC2
. The gate
CC
IN
IN
pin with
current
supply.
LDO is
A
IN
=
.
be derived from the LTC3838’s switching regulator output
V
output is out of regulation (e.g., start up, short circuit). If
more current is required through the EXTV
fied, an external Schottky diode can be added between the
EXTV
EXTV
Significant efficiency and thermal gains can be realized
by powering DRV
since the V
currents will be scaled by a factor of (Duty Cycle)/(Switcher
Efficiency).
Tying the EXTV
temperature in the previous example from 125°C to:
However, for 3.3V and other low voltage outputs, ad-
ditional circuitry is required to derive DRV
the converter output.
The following list summarizes the four possible connec-
tions for EXTV
1. EXTV
2. EXTV
3. EXTV
4. EXTV
OUT
T
to be powered from the internal 5.3V LDO resulting
in an efficiency penalty of up to 10% at high input
voltages.
V
(4.8V). This provides the highest efficiency.
greater external supply is available, it may be used to
power EXTV
sufficient for MOSFET gate drive requirements.
For 3.3V and other low voltage converters, efficiency
gains can still be realized by connecting EXTV
output-derived voltage that has been boosted to greater
than 4.8V.
J
OUT
CC
CC
during normal operation and from the LDO when the
= 70°C + (52mA)(5V)(28°C/W) = 77°C
CC
CC
is higher than the switchover voltage’s higher limit
CC
CC
and DRV
pin and make sure that EXTV
connected directly to switching converter output
IN
left open (or grounded). This will cause INTV
connected to an output-derived boost network.
connected to an external supply. If a 4.8V or
current resulting from the driver and control
CC
CC
CC
CC
:
CC
pin to a 5V supply reduces the junction
providing that the external supply is
pins. Do not apply more than 6V to the
from the switching converter output,
CC
is less than V
CC
CC
than is speci-
power from
CC
to an
3838fa
IN
CC
.