LTC3615 Datasheet, PDF (24/32 Pages) Linear Technology – Dual 4MHz, 3A Synchronous Step-Down DC/DC Converter

LTC3615
Applications Information
DDR Application
The LTC3615 can be used in DDR memory power supply
applications by tying the SRLIM pin to SV_IN. In DDR mode,
the maximum slew rate is selected. The output can both
source and sink current. Current sinking is typically limited
to 1.5A, for 1MHz frequency and 1µH inductance, but can
be lower at higher frequencies and low output voltages.
If higher ripple current can be tolerated, smaller inductor
values can increase the sink current limit. See the Typical
Performance Characteristics curves for more information.
In addition, in DDR mode, lower external reference volt-
ages and tracking output voltages between channels are
possible. See the Output Voltage Tracking Input section.
Single, Low Ripple 6A Output Application
The LT3615 can generate a single, low ripple 6A output if
the outputs of the two switching regulators are tied together
and share a single output capacitor (see Figure 15 on back
of data sheet). In order to evenly share the current between
the two regulators, it is needed to connect pins FB1 to
FB2, ITH1 to ITH2 and to select forced continuous mode
at the MODE pin. To achieve lowest ripple, 90°, or better,
180°, antiphase is selected by connecting the PHASE pin
to midrail or SV_IN. There are several advantages to this
2-phase buck regulator. Ripple currents at the input and
output are reduced, reducing voltage ripple and allowing
the use of smaller, less expensive capacitors. Although
two inductors are required, each will be smaller than the
inductor required for a single-phase regulator. This may
be important when there are tight height restrictions on
the circuit.
Efficiency Considerations
The efficiency of a switching regulator is equal to the output
power divided by the input power times 100%. It is often
useful to analyze individual losses to determine what is
limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as:
Efficiency = 100% – (L1 + L2 + L3 + ...) where L1, L2, etc.
are the individual losses as a percentage of input power.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of
the losses: V_IN quiescent current and I²R losses. The V_IN
quiescent current loss dominates the efficiency loss at
very low load currents whereas the I²R loss dominates
the efficiency loss at medium to high load currents. In a
typical efficiency plot, the efficiency curve at very low load
currents can be misleading since the actual power lost is
of little consequence.
1. The V_IN quiescent current is due to two components:
the DC bias current as given in the Electrical Charac-
teristics and the internal main switch and synchronous
switch gate charge currents. The gate charge current
results from switching the gate capacitance of the
internal power MOSFET switches. Each time the gate
is switched from high to low to high again, a packet
of charge dQ moves from V_IN to ground. The resulting
dQ/dt is the current out of V_IN due to gate charge, and
it is typically larger than the DC bias current. Both the
DC bias and gate charge losses are proportional to V_IN,
thus, their effects will be more pronounced at higher
supply voltages.
2. I²R losses are calculated from the resistances of the
internal switches, R_SW, and external inductor R_L. In
continuous mode the average output current flowing
through inductor L is “chopped” between the main
switch and the synchronous switch. Thus, the series
resistance looking into the SW pin is a function of both
top and bottom MOSFET R_DS(ON) and the duty cycle
(DC), as follows:
R_SW = (R_DS(ON)TOP)(DC) + (R_DS(ON)BOT)(1 – DC)
The R_DS(ON) for both the top and bottom MOSFETs can
be obtained from the Typical Performance Characteristics
curves. To obtain I²R losses, simply add R_SW to R_L and
multiply the result by the square of the average output
current.
Other losses, including C_IN and C_OUT ESR dissipative losses
and inductor core losses, generally account for less than
2% of the total loss.
3615f
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