Datasheet

LTC3788
20
3788fc
applicaTions inForMaTion
INTV
CC
Regulators
The LTC3788 features two separate internal P-channel
low dropout linear regulators (LDO) that supply power at
the INTV
CC
pin from either the VBIAS supply pin or the
EXTV
CC
pin depending on the connection of the EXTV
CC
pin. INTV
CC
powers the gate drivers and much of the
LTC3788’s internal circuitry. The VBIAS LDO and the
EXTV
CC
LDO regulate INTV
CC
to 5.4V. Each of these can
supply a peak current of 50mA 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 prevent in-
teraction 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 LTC3788 to be
exceeded. The INTV
CC
current, which is dominated by the
gate charge current, may be supplied by either the VBIAS
LDO or the EXTV
CC
LDO. When the voltage on the EXTV
CC
pin is less than 4.8V, the VBIAS LDO is enabled. In this
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
INTV
CC
current is limited to less than 40mA from a 40V
supply when not using the EXTV
CC
supply:
T
J
= 70°C + (40mA)(40V)(34°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 + (40mA)(5V)(34°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.