LTC3892_15 Datasheet, PDF (24/36 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

LTC3892/LTC3892-1
Applications Information
an 8.5V supply reduces the junction temperature in the Fault Conditions: Current Limit and
previous example from 125°C to:
Current Foldback
T_J = 70°C + (37mA)(8.5V – 6V)(34.7°C/W) = 74°C
However, for 3.3V and other low voltage outputs, additional
circuitry is required to derive DRV_CC power from the output.
The following list summarizes the four possible connec-
tions for EXTV_CC:
1. EXTV_CC grounded. This will cause DRV_CC to be powered
from the internal V_IN regulator resulting in increased
power dissipation in the LTC3892/LTC3892-1 at high
input voltages.
2. EXTV_CC connected directly to V_OUT. This is the normal
connection for a 5V to 14V regulator and provides the
highest efficiency.
3. EXTV_CC connected to an external supply. If an external
supply is available in the 5V to 14V range, it may be used to
power EXTV_CC providing it is compatible with the MOSFET
gate drive requirements. Ensure that EXTV_CC < V_IN.
4. EXTV_CC connected to an output-derived boost network.
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTV_CC to an
output-derived voltage that has been boosted to greater
than 4.7V/7.7V.
Topside MOSFET Driver Supply (C_B)
External bootstrap capacitors, C_B, connected to the BOOST
pins supply the gate drive voltage for the topside MOS-
FET. The LTC3892/LTC3892-1 features an internal switch
between DRV_CC and the BOOST pin for each controller.
These internal switches eliminate the need for external
bootstrap diodes between DRV_CC and BOOST. Capacitor C_B
in the Functional Diagram is charged through this internal
switch from DRV_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-source of the MOSFET. This
enhances the top MOSFET switch and turns it on. 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_DRVCC. 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 LTC3892/LTC3892-1 includes current foldback to help
limit load current when the output is shorted to ground. If
the output voltage falls below 70% of its nominal output
level, then the maximum sense voltage is progressively
lowered from 100% to 40% of its maximum selected
value. Under short-circuit conditions with very low duty
cycles, the channel will begin cycle skipping in order to
limit the short-circuit current. In this situation the bottom
MOSFET will be dissipating most of the power but less
than in normal operation. The short-circuit ripple cur-
rent is determined by the minimum on-time, t_ON(MIN), of
the LTC3892/LTC3892-1 (≈80ns), the input voltage and
inductor value:
∆I_L(SC)
=
t_ON(MIN)


V_IN
L


The resulting average short-circuit current is:
I_SC
=
40%
• I_{LIM(MAX )}
−
1
2
∆I_L(SC)
Fault Conditions: Overvoltage Protection (Crowbar)
The overvoltage crowbar is designed to blow a system
input fuse when the output voltage of the regulator rises
much higher than nominal levels. The crowbar causes huge
currents to flow, that blow the fuse to protect against a
shorted top MOSFET if the short occurs while the controller
is operating.
A comparator monitors the output for overvoltage condi-
tions. The comparator detects faults greater than 10%
above the nominal output voltage. When this condition
is sensed, the top MOSFET is turned off and the bottom
MOSFET is turned on until the overvoltage condition is
cleared. The bottom MOSFET remains on continuously for
as long as the overvoltage condition persists; if V_OUT returns
to a safe level, normal operation automatically resumes.
A shorted top MOSFET will result in a high current condition
which will open the system fuse. The switching regulator
will regulate properly with a leaky top MOSFET by altering
the duty cycle to accommodate the leakage.
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