ltc3787gn Linear Technology Corporation, ltc3787gn Datasheet - Page 22

• Manufacturer Part Number: ltc3787gn
• Description: Ltc3787 - Polyphase Synchronous Boost Controller
• Manufacturer: Linear Technology Corporation

applicaTions inForMaTion
LTC3787
EXTV
to regulate the INTV
is less than 5.4V, the LDO is in dropout and the INTV
voltage is approximately equal to EXTV
is greater than 5.4V, up to an absolute maximum of 6V,
INTV
Significant thermal gains can be realized by powering
INTV
to a 5V supply reduces the junction temperature in the
previous example from 125°C to 79°C in a QFN package:
and from 125°C to 74°C in an SSOP package:
If more current is required through the EXTV
is specified, an external Schottky diode can be added be-
tween the EXTV
cases EXTV
The following list summarizes possible connections for
EXTV
22
T
T
EXTV
ered from the internal 5.4V regulator resulting in an
efficiency penalty at high input voltages.
EXTV
supply is available in the 5V to 6V range, it may be used
to provide power. Ensure that EXTV
than VBIAS.
J
J
CC
CC
CC
CC
= 70°C + (32mA)(5V)(43°C/W) = 77°C
= 70°C + (15mA)(5V)(90°C/W) = 77°C
:
is regulated to 5.4V.
remains above 4.55V. The EXTV
from an external supply. Tying the EXTV
CC
CC
Connected to an External Supply. If an external
Grounded. This will cause INTV
CC
≤ VBIAS (even at start-up and shutdown).
CC
and INTV
CC
voltage to 5.4V, so while EXTV
CC
pins. Make sure that in all
CC
CC
CC
is always lower
. When EXTV
LDO attempts
CC
CC
to be pow-
LDO than
CC
pin
CC
CC
CC
Topside MOSFET Driver Supply (C
External bootstrap capacitors C
pins supply the gate drive voltages for the topside
MOSFETs. Capacitor C
though external diode D
is low. When one of the topside MOSFETs is to be turned
on, the driver places the C
source of the desired MOSFET. This enhances the MOSFET
and turns on the topside switch. The switch node volt-
age, SW, rises to V
the topside MOSFET on, the boost voltage is above the
output voltage: V
the boost capacitor C
total input capacitance of the topside MOSFET(s). The
reverse breakdown of the external Schottky diode must
be greater than V
Fault Conditions: Overtemperature Protection
At higher temperatures, or in cases where the internal
power dissipation causes excessive self heating on-chip
(such as an INTV
shutdown circuitry will shut down the LTC3787. When the
junction temperature exceeds approximately 170°C, the
overtemperature circuitry disables the INTV
the INTV
the entire LTC3787 chip. Once the junction temperature
drops back to approximately 155°C, the INTV
back on. Long term overstress (T
avoided as it can degrade the performance or shorten
the life of the part.
Since the shutdown may occur at full load, beware that
the load current will result in high power dissipation in
the body diodes of the top MOSFETs. In this case, PGOOD
output may be used to turn the system load off.
CC
supply to collapse and effectively shut down
CC
OUT(MAX)
BOOST
OUT
short to ground), the overtemperature
B
B
and the BOOST pin follows. With
needs to be 100 times that of the
B
= V
in the Block Diagram is charged
from INTV
.
B
OUT
voltage across the gate and
B
+ V
connected to the BOOST
J
B
INTVCC
, D
> 125°C) should be
CC
when the SW pin
B
)
CC
. The value of
LDO, causing
CC
LDO turns
3787fa