LTC1628CG-SYNC#TRPBF Linear Technology, LTC1628CG-SYNC#TRPBF Datasheet - Page 20

• Manufacturer Part Number: LTC1628CG-SYNC#TRPBF
• Description: IC SW REG STEP-DOWN 28-SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1628CG-SYNC#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: Adj to 0.8V
• Current - Output: 3A
• Frequency - Switching: 140kHz ~ 310kHz
• Voltage - Input: 3.5 ~ 30 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIO S I FOR ATIO
LTC1628-SYNC
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 LTC1628-SYNC current comparator has a maximum
sense voltage of 75mV resulting in a maximum MOSFET
current of 75mV/R
limit generally occurs with the largest V
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 t
input voltage and inductor value:
20
SS
C
∆I
SS
= 0.1µF will be sufficient for most applications.
L(SC)
ON(MIN)
> (C
= t
OUT
ON(MIN)
of the LTC1628-SYNC (less than 200ns), the
)(V
OUT
SENSE
U
(V
) (10
IN
. The maximum value of current
/L)
U
– 4
) (R
SENSE
W
SS
)
IN
may need to be
at the highest
U
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 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 f
pin of 1.2V corresponds to a frequency of approximately
220kHz. The nominal operating frequency range of the
LTC1628-SYNC is 140kHz to 310kHz.
I
SC
=
R
25
SENSE
mV
+
2
1
∆
O
I
. A voltage applied to the PLLFLTR
L SC
(
)
OUT
returns to a safe level,
1628syncfa