LTC3728LX Linear Technology, LTC3728LX Datasheet - Page 20

• Manufacturer Part Number: LTC3728LX
• Description: 2-Phase Synchronous Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3728L/LTC3728LX
Diode-connecting this pull-up resistor to INTV
Figure 7b, eliminates any extra supply current during
controller shutdown while eliminating the INTV
from preventing controller start-up.
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 C
scaled with output voltage, output capacitance and load
current characteristics. The minimum soft-start capaci-
tance is given by:
The minimum recommended soft-start capacitor of
C
Fault Conditions: Current Limit and Current Foldback
The current comparators have a maximum sense voltage
of 75mV resulting in a maximum MOSFET current of
75mV/R
ally occurs with the largest V
temperature, conditions that cause the highest power
dissipation in the top MOSFET.
Each controller 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 controller 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
each controller (typically 100ns), the input voltage and
inductor value:
20
SS
C
SS
= 0.1 F will be sufficient for most applications.
> (C
SENSE
OUT
. The maximum value of current limit gener-
)(V
OUT
U
) (10
U
– 4
) (R
IN
at the highest ambient
SENSE
W
SS
)
may need to be
U
CC
ON(MIN)
CC
loading
, as in
of
The resulting short-circuit current is:
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
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 IC 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
corresponds to a frequency of approximately 400kHz. The
nominal operating frequency range of the IC is 260kHz to
550kHz.
I
SC
I
L(SC)
R
25
= t
SENSE
O
mV
. A voltage applied to the PLLFLTR pin of 1.2V
ON(MIN)
–
2
1
(V
I
IN
L SC
(
/L)
)
OUT
w w w . D a t a S h e e t 4 U . c
returns to a safe level,
3728lxfa