LTC3769_15 Datasheet, PDF (20/32 Pages) Linear Technology – 60V Low IQ Synchronous Boost Controller

LTC3769
Applications Information
In the TSSOP package, the INTV_CC current is limited to
less than 24mA from a 60V supply when not using the
EXTV_CC supply:
T_J = 70°C + (24mA)(60V)(38°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 14V,
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 75°C in a QFN package:
T_J = 70°C + (19mA)(5V)(47°C/W) = 75°C
and from 125°C to 75°C in the TSSOP package:
T_J = 70°C + (24mA)(5V)(38°C/W) = 75°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 V_BIAS 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)
An external bootstrap capacitor C_B connected to the
BOOST pin supplies the gate drive voltage for the topside
MOSFET. Capacitor C_B in the Block Diagram is charged
though external diode D_B from INTV_CC when the SW pin
is low. When the topside MOSFET 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 diode D_B 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.
The topside MOSFET driver includes an internal charge
pump that delivers current to the bootstrap capacitor 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 diode selected for the topside driver 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 operating 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
3769f
20
For more information www.linear.com/LTC3769