LTC3728LEGN-1#PBF Linear Technology, LTC3728LEGN-1#PBF Datasheet - Page 11

• Manufacturer Part Number: LTC3728LEGN-1#PBF
• Description: IC REG SYNC DUAL 28-SSOP
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3728LEGN-1#PBF

• 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: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

OPERATION
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, the controller will
cause current to fl ow back into the input fi lter capacitor.
If large enough, this element will prevent the input sup-
ply from boosting to unacceptably high levels; see C
Selection in the Applications Information Section.
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.
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
CC
/EXTV
CC
pin is left open, an internal 5V low dropout
CC
CC
power to be derived from a high effi ciency
Power
(Refer to Functional Diagram)
CC
power. If EXTV
CC
to INTV
CC
CC
CC
pin. When
. This al-
is taken
OUT
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
The RUN/SS capacitors are used initially to limit the inrush
current of each switching regulator. Foldback current limit-
ing is activated when the output voltage falls below 70%
of its nominal level. If a short is present, a safe, low output
current is provided due to the internal current foldback
and actual power wasted is low due to the effi cient nature
of the current mode switching regulator.
THEORY AND BENEFITS OF 2-PHASE OPERATION
The LTC1628 and the LTC3728L-1 family of dual high ef-
fi ciency DC/DC controllers brings the considerable benefi ts
of 2-phase operation to portable applications for the fi rst
time. Notebook computers, PDAs, handheld terminals and
automotive electronics will all benefi t from the lower input
fi ltering requirement, reduced electromagnetic interference
(EMI) and increased effi ciency associated with 2-phase
operation.
Why the need for 2-phase operation? Up until the 2-phase
family, constant-frequency dual switching regulators
operated both channels in phase (i.e., single-phase
operation). This means that both switches turned on at
the same time, causing current pulses of up to twice the
amplitude of those for one regulator to be drawn from the
input capacitor and battery. These large amplitude current
pulses increased the total RMS current fl owing from the
input capacitor, requiring the use of more expensive input
capacitors and increasing both EMI and losses in the input
capacitor and battery.
With 2-phase operation, the two channels of the dual-
switching regulator are operated 180 degrees out of phase.
This effectively interleaves the current pulses drawn by the
switches, greatly reducing the overlap time where they add
together. The result is a signifi cant reduction in total RMS
input current, which in turn allows less expensive input
capacitors to be used, reduces shielding requirements for
EMI and improves real world operating effi ciency.
LTC3728L-1
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