LTC1628I View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1628I' PDF : 32 Pages View PDF
LTC1628/LTC1628-PG
U
OPERATIO (Refer to Functional Diagram)
overcurrent and/or short-circuit condition. Foldback cur-
rent 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 efficient nature
of the current mode switching regulator.
THEORY AND BENEFITS OF 2-PHASE OPERATION
The LTC1628 dual high efficiency DC/DC controller brings
the considerable benefits of 2-phase operation to portable
applications for the first time. Notebook computers, PDAs,
handheld terminals and automotive electronics will all
benefit from the lower input filtering requirement, reduced
electromagnetic interference (EMI) and increased effi-
ciency associated with 2-phase operation.
Why the need for 2-phase operation? Up until the LTC1628,
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 capaci-
tor and battery. These large amplitude current pulses
increased the total RMS current flowing 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 significant reduction in
total RMS input current, which in turn allows less expen-
sive input capacitors to be used, reduces shielding re-
quirements for EMI and improves real world operating
efficiency.
Figure 3 compares the input waveforms for a representa-
tive single-phase dual switching regulator to the new
LTC1628 2-phase dual switching regulator. An actual
measurement of the RMS input current under these con-
ditions shows that 2-phase operation dropped the input
current from 2.53ARMS to 1.55ARMS. While this is an
impressive reduction in itself, remember that the power
losses are proportional to IRMS2, meaning that the actual
power wasted is reduced by a factor 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. Im-
provements 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
duty cycles which, in turn, are dependent upon the input
voltage VIN (Duty Cycle = VOUT/VIN). Figure 4 shows how
12
5V SWITCH
20V/DIV
3.3V SWITCH
20V/DIV
INPUT CURRENT
5A/DIV
INPUT VOLTAGE
500mV/DIV
IIN(MEAS) = 2.53ARMS
(a)
DC236 F03a
IIN(MEAS) = 1.55ARMS
(b)
DC236 F03b
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 Efficiency
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