LTC3728LXCUH Linear Technology, LTC3728LXCUH Datasheet - Page 14

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

Specifications of LTC3728LXCUH

• 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
LTC3728L/LTC3728LX
THEORY AND BENEFITS OF 2-PHASE OPERATION
The LTC1628 and the LTC3728L family of dual high effi -
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 regula-
tors 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.
14
Figure 3. Input Waveforms Comparing Single-Phase (a) and 2-Phase (b) Operation for Dual Switching Regulators
Converting 12V to 5V and 3.3V at 3A Each. The Reduced Input Ripple with the LTC1628 2-Phase Regulator Allows
Less Expensive Input Capacitors, Reduces Shielding Requirements for EMI and Improves Effi ciency
(Refer to Functional Diagram)
I
IN(MEAS)
= 2.53A
(a)
RMS
DC236 F03a
INPUT CURRENT
INPUT VOLTAGE
3.3V SWITCH
5V SWITCH
500mV/DIV
20V/DIV
20V/DIV
5A/DIV
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.
Figure 3 compares the input waveforms for a representa-
tive single-phase dual switching regulator to the LTC1628
2-phase dual switching regulator. An actual measurement of
the RMS input current under these conditions shows that
2-phase operation dropped the input current from 2.53A
to 1.55A
remember that the power losses are proportional to I
meaning that the actual power wasted is reduced by a fac-
tor of 2.66. The reduced input ripple voltage also means
less power is lost in the input power path, which could
include batteries, switches, trace/connector resistances
and protection circuitry. Improvements in both conducted
and radiated EMI also directly accrue as a result of the
reduced RMS input current and voltage.
Of course, the improvement afforded by 2-phase opera-
tion is a function of the dual switching regulator’s relative
RMS
. While this is an impressive reduction in itself,
I
IN(MEAS)
= 2.53A
(b)
RMS
DC236 F03b
RMS
3728lxfe
RMS
2
,