LTC3784_15 Datasheet, PDF (23/38 Pages) Linear Technology – 60V PolyPhase Synchronous Boost Controller

LTC3784
Applications Information
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 in a QFN package:
T_J = 70°C + (32mA)(5V)(43°C/W) = 77°C
and from 125°C to 74°C in an SSOP package:
T_J = 70°C + (15mA)(5V)(80°C/W) = 77°C
The following list summarizes possible connections for
EXTV_CC:
EXTV_CC 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 5V to 14V range, it may be
used to provide power. Ensure that EXTV_CC is always
lower than or equal to 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 and source
of the desired MOSFET. This enhances the MOSFET and
turns on the topside switch. The switch node voltage, SW,
rises to V_OUT and the BOOST pin follows. With the topside
MOSFET on, the boost voltage is above the output voltage:
V_BOOST = V_OUT + 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_OUT(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.
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_CC short to ground), the overtemperature
shutdown circuitry will shut down the LTC3784. When the
junction temperature exceeds approximately 170°C, the
overtemperature circuitry disables the INTV_CC LDO, causing
the INTV_CC supply to collapse and effectively shut down
the entire LTC3784 chip. Once the junction temperature
drops back to approximately 155°C, the INTV_CC LDO turns
back on. Long term overstress (T_J > 125°C) should be
avoided as it can degrade the performance or shorten
the life of the part.
For more information www.linear.com/LTC3784
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