LTC3727_15 Datasheet, PDF (20/32 Pages) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulators

LTC3727/LTC3727-1
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. If latchoff is
not required, the LTC3727-1 can be used.
The value of the soft-start capacitor C_SS may need to be
scaled with output voltage, output capacitance and load
current characteristics. The minimum soft-start capaci-
tance is given by:
C_SS > (C_OUT )(V_OUT) (10 ^{– 4}) (R_SENSE)
The minimum recommended soft-start capacitor of
C_SS = 0.1μF will be sufficient for most applications.
Fault Conditions: Current Limit and Current Foldback
The LTC3727 current comparator has a maximum sense
voltage of 135mV resulting in a maximum MOSFET cur-
rent of 135mV/R_SENSE. The maximum value of current
limit generally occurs with the largest V_IN at the highest
ambient temperature, conditions that cause the highest
power dissipation in the top MOSFET.
The LTC3727 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
135mV to 45mV. Under short-circuit conditions with very
low duty cycles, the LTC3727 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 t_ON(MIN) of
the LTC3727 (less than 200ns), the input voltage and
inductor value:
ΔI_L(SC) = t_ON(MIN) (V_IN/L)
The resulting short-circuit current is:
I_SC
=
45mV
R_SENSE
+
1
2
ΔI_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 (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 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 regu-
late properly with a leaky top MOSFET by altering the duty
cycle to accommodate the leakage.
Phase-Locked Loop and Frequency Synchronization
The LTC3727 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 f_O. A voltage applied to the PLLFLTR pin
of 1.2V corresponds to a frequency of approximately
380kHz. The nominal operating frequency range of the
LTC3727 is 250kHz to 550kHz.
3727fc
20