Datasheet

LTC3788-1
19
37881fc
APPLICATIONS INFORMATION
case, power dissipation for the IC 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 LTC3788-1
INTV
CC
current is limited to less than 15mA from a 40V
supply when not using the EXTV
CC
supply:
T
J
= 70°C + (15mA)(40V)(90°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 (PLLIN/MODE
= INTV
CC
) at maximum V
IN
.
When the voltage applied to EXTV
CC
rises above 4.8V, the
V
IN
LDO is turned off and the EXTV
CC
LDO is enabled. The
EXTV
CC
LDO remains on as long as the voltage applied to
EXTV
CC
remains above 4.55V. The EXTV
CC
LDO attempts
to regulate the INTV
CC
voltage to 5.4V, so while EXTV
CC
is less than 5.4V, the LDO is in dropout and the INTV
CC
voltage is approximately equal to EXTV
CC
. When EXTV
CC
is greater than 5.4V, up to an absolute maximum of 6V,
INTV
CC
is regulated to 5.4V.
Significant thermal gains can be realized by powering
INTV
CC
from an external supply. Tying the EXTV
CC
pin
to a 5V supply reduces the junction temperature in the
previous example from 125°C to 77°C:
T
J
= 70°C + (15mA)(5V)(90°C/W) = 77°C
If more current is required through the EXTV
CC
LDO than
is specified, an external Schottky diode can be added
between the EXTV
CC
and INTV
CC
pins. Make sure that in
all cases EXTV
CC
≤ VBIAS.
The following list summarizes possible connections for
EXTV
CC
:
EXTV
CC
Left Open (or Grounded). This will cause
INTV
CC
to be powered from the internal 5.4V regulator
resulting in an efficiency penalty at high input voltages.
EXTV
CC
Connected to an External Supply. If an external
supply is available in the 5.4V to 6V range, it may be
used to power EXTV
CC
providing it is compatible with
the MOSFET gate drive requirements. Ensure that
EXTV
CC
< VBIAS.
Topside MOSFET Driver Supply (C
B
, D
B
)
External bootstrap capacitors C
B
connected to the BOOST
pins supply the gate drive voltages for the topside MOS-
FETs. Capacitor C
B
in the Block Diagram is charged though
external diode D
B
from INTV
CC
when the SW pin is low.
When one of the topside MOSFETs is to be turned on, the
driver places the C
B
voltage across the gate-source of the
desired MOSFET. This enhances the MOSFET and turns on
the topside switch. The switch node voltage, SW, rises to
V
IN
and the BOOST pin follows. With the topside MOSFET
on, the boost voltage is above the input supply: V
BOOST
=
V
IN
+ V
INTVCC
. The value of the boost capacitor C
B
needs
to be 100 times that of the total input capacitance of the
topside MOSFET(s). The reverse breakdown of the external
Schottky diode must be greater than V
IN(MAX)
.
The external diode D
B
can be a Schottky diode or silicon
diode, but in either case it should have low leakage and fast
recovery. Pay close attention to the reverse leakage at high
temperatures where it generally increases substantially.
Each of the topside MOSFET drivers includes an internal
charge pump that delivers current to the bootstrap capaci-
tor from the BOOST pin. This charge current maintains
the bias voltage required to keep the top MOSFET on
continuously during dropout/overvoltage conditions. The
Schottky/silicon diodes selected for the topside drivers
should have a reverse leakage less than the available output
current the charge pump can supply. Curves displaying
the available charge pump current under different operat-
ing conditions can be found in the Typical Performance
Characteristics section.
A leaky diode D
B
in the boost converter can not only
prevent the top MOSFET from fully turning on but it can
also completely discharge the bootstrap capacitor C
B
and
create a current path from the input voltage to the BOOST
pin to INTV
CC
. This can cause INTV
CC
to rise if the diode
leakage exceeds the current consumption on INTV
CC
.
This is particularly a concern in Burst Mode operation
where the load on INTV
CC
can be very small. The external
Schottky or silicon diode should be carefully chosen such
that INTV
CC
never gets charged up much higher than its
normal regulation voltage.