LTC1628CG-SYNC Ver la hoja de datos (PDF) - Linear Technology
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LTC1628CG-SYNC Datasheet PDF : 32 Pages
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LTC1628-SYNC
APPLICATIO S I FOR ATIO
Why should you defeat overcurrent latchoff? During the
prototyping stage of a design, there may be a problem
with noise pickup or poor layout causing the protection
circuit to latch off. Defeating this feature will easily allow
troubleshooting of the circuit and PC layout. The internal
short-circuit and foldback current limiting still remains
active, thereby protecting the power supply system from
failure. After the design is complete, a decision can be
made whether to enable the latchoff feature.
The value of the soft-start capacitor CSS may need to be
scaled with output voltage, output capacitance and load
current characteristics. The minimum soft-start capaci-
tance is given by:
CSS > (COUT )(VOUT) (10 – 4) (RSENSE)
The minimum recommended soft-start capacitor of
CSS = 0.1µF will be sufficient for most applications.
Fault Conditions: Current Limit and Current Foldback
The LTC1628-SYNC current comparator has a maximum
sense voltage of 75mV resulting in a maximum MOSFET
current of 75mV/RSENSE. The maximum value of current
limit generally occurs with the largest VIN at the highest
ambient temperature, conditions that cause the highest
power dissipation in the top MOSFET.
The LTC1628-SYNC includes current foldback to help
further limit load current when the output is shorted to
ground. The foldback circuit is active even when the
overload shutdown latch described above is overridden. If
the output falls below 70% of its nominal output level, then
the maximum sense voltage is progressively lowered from
75mV to 25mV. Under short-circuit conditions with very
low duty cycles, the LTC1628-SYNC 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 current is determined by the minimum on-
time tON(MIN) of the LTC1628-SYNC (less than 200ns), the
input voltage and inductor value:
∆IL(SC) = tON(MIN) (VIN/L)
The resulting short-circuit current is:
ISC
=
25mV
RSENSE
+
1
2
∆IL(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 (OV) detects overvoltage faults
greater than 7.5% 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 overvolt-
age condition is cleared. The output of this comparator is
only latched by the overvoltage condition itself and will
therefore allow a switching regulator system having a poor
PC layout to function while the design is being debugged.
The bottom MOSFET remains on continuously for as long
as the OV condition persists; if VOUT 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 regu-
late properly with a leaky top MOSFET by altering the duty
cycle to accommodate the leakage.
Phase-Locked Loop and Frequency Synchronization
The LTC1628-SYNC has a phase-locked loop comprised
of an internal voltage controlled oscillator and phase
detector. This allows the top MOSFET turn-on to be locked
to the rising edge of an external source. The frequency
range of the voltage controlled oscillator is ±50% around
the center frequency fO. A voltage applied to the PLLFLTR
pin of 1.2V corresponds to a frequency of approximately
220kHz. The nominal operating frequency range of the
LTC1628-SYNC is 140kHz to 310kHz.
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