LTC3728LXCUH#TR Linear Technology, LTC3728LXCUH#TR Datasheet - Page 13

• Manufacturer Part Number: LTC3728LXCUH#TR
• Description: IC CTRLR SW REG 2-PH SYNC 32-QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3728LXCUH#TR

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 7 V
• Current - Output: 3A
• Frequency - Switching: 250kHz ~ 550kHz
• Voltage - Input: 4.5 ~ 28 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-QFN
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

OPERATION
Constant-Frequency Operation
When the FCB pin is tied to INTV
tion is disabled and the forced minimum output current
requirement is removed. This provides constant-frequency,
discontinuous current (preventing reverse inductor cur-
rent) operation over the widest possible output current
range. This constant-frequency operation is not as effi cient
as Burst Mode operation, but does provide a lower noise,
constant-frequency operating mode down to approximately
1% of the designed maximum output current.
Continuous Current (PWM) Operation
Tying the FCB pin to ground will force continuous current
operation. This is the least effi cient operating mode, but
may be desirable in certain applications. The output can
source or sink current in this mode. When sinking current
while in forced continuous operation, current will be forced
back into the main power supply potentially boosting the
input supply to dangerous voltage levels—BEWARE!
INTV
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
the EXTV
linear regulator supplies INTV
above 4.7V, the 5V regulator is turned off and an internal
switch is turned on connecting EXTV
lows the INTV
external source such as the output of the regulator itself
or a secondary winding, as described in the Applications
Information section.
Output Overvoltage Protection
An overvoltage comparator, OV, guards against transient
overshoots (>7.5%) as well as other more serious condi-
tions that may overvoltage the output. In this case, the top
MOSFET is turned off and the bottom MOSFET is turned
on until the overvoltage condition is cleared.
CC
/EXTV
CC
pin is left open, an internal 5V low dropout
CC
CC
Power
power to be derived from a high effi ciency
(Refer to Functional Diagram)
CC
power. If EXTV
CC
CC
, Burst Mode opera-
to INTV
CC
CC
CC
pin. When
. This al-
is taken
Power Good (PGOOD) Pin
The PGOOD pin is connected to an open drain of an internal
MOSFET. The MOSFET turns on and pulls the pin low when
either output is not within ± 7.5% of the nominal output
level as determined by the resistive feedback divider. When
both outputs meet the ±7.5% requirement, the MOSFET is
turned off within 10μs and the pin is allowed to be pulled
up by an external resistor to a source of up to 7V.
Foldback Current, Short-Circuit Detection
and Short-Circuit Latchoff
The RUN/SS capacitors are used initially to limit the inrush
current of each switching regulator. After the controller
has been started and been given adequate time to charge
up the output capacitors and provide full load current, the
RUN/SS capacitor is used in a short-circuit time-out circuit.
If the output voltage falls to less than 70% of its nominal
output voltage, the RUN/SS capacitor begins discharging
on the assumption that the output is in an overcurrent
and/or short-circuit condition. If the condition lasts for
a long enough period as determined by the size of the
RUN/SS capacitor, the controller will be shut down until
the RUN/SS pin(s) voltage(s) are recycled. This built-in
latchoff can be overridden by providing a >5μA pull-up
at a compliance of 5V to the RUN/SS pin(s). This current
shortens the soft start period but also prevents net dis-
charge of the RUN/SS capacitor(s) during an overcurrent
and/or short-circuit condition. Foldback current limiting
is also activated when the output voltage falls below
70% of its nominal level whether or not the short-circuit
latchoff circuit is enabled. Even if a short is present and
the short-circuit latchoff is not enabled, a safe, low output
current is provided due to internal current foldback and
actual power wasted is low due to the effi cient nature of
the current mode switching regulator.
LTC3728L/LTC3728LX
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