LTC3707 Datasheet, PDF (23/32 Pages) Linear Technology – High Effi ciency, 2-Phase Synchronous Step-Down Switching Regulator

LTC3707
APPLICATIONS INFORMATION
Efficiency Considerations
The percent 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. Percent efficiency can
be expressed as:
%Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most
of the losses in LTC3707 circuits: 1) LTC3707 V_IN cur-
rent (including loading on the 3.3V internal regulator),
2) INTV_CC regulator current, 3) I²R losses, 4) Topside
MOSFET transition losses.
1. The V_IN current has two components: the first is the DC
supply current given in the Electrical Characteristics table,
which excludes MOSFET driver and control currents; the
second is the current drawn from the 3.3V linear regulator
output. V_IN current typically results in a small (<0.1%)
loss.
2. INTV_CC current is the sum of the MOSFET driver and
control currents. The MOSFET driver current results from
switching the gate capacitance of the power MOSFETs.
Each time a MOSFET gate is switched from low to high
to low again, a packet of charge dQ moves from INTV_CC
to ground. The resulting dQ/dt is a current out of INTV_CC
that is typically much larger than the control circuit cur-
rent. In continuous mode, I_GATECHG =f(Q_T+Q_B), where Q_T
and Q_B are the gate charges of the topside and bottom
side MOSFETs.
Supplying INTV_CC power through the EXTV_CC switch input
from an output-derived source will scale the V_IN current
required for the driver and control circuits by a factor of
(Duty Cycle)/(Efficiency). For example, in a 20V to 5V ap-
plication, 10mA of INTV_CC current results in approximately
2.5mA of V_IN current. This reduces the mid-current loss
from 10% or more (if the driver was powered directly from
V_IN) to only a few percent.
3. I²R losses are predicted from the DC resistances of
the fuse (if used), MOSFET, inductor, current sense resis-
tor, and input and output capacitor ESR. In continuous
mode the average output current flows through L and
R_SENSE, but is “chopped” between the topside MOSFET
and the synchronous MOSFET. If the two MOSFETs have
approximately the same R_DS(ON), then the resistance of
one MOSFET can simply be summed with the resistances
of L, R_SENSE and ESR to obtain I²R losses. For example, if
each R_DS(ON) = 30mΩ, R_L = 50mΩ, R_SENSE = 10mΩ and
R_ESR = 40mΩ (sum of both input and output capacitance
losses), then the total resistance is 130mΩ. This results
in losses ranging from 3% to 13% as the output current
increases from 1A to 5A for a 5V output, or a 4% to 20%
loss for a 3.3V output. Efficiency varies as the inverse
square of V_OUT for the same external components and
output power level. The combined effects of increasingly
lower output voltages and higher currents required by
high performance digital systems is not doubling but
quadrupling the importance of loss terms in the switching
regulator system!
4. Transition losses apply only to the topside MOSFET(s),
and become significant only when operating at high input
voltages (typically 15V or greater). Transition losses can
be estimated from:
Transition Loss = (1.7) V_IN2 I_O(MAX) C_RSS f
Other “hidden” losses such as copper trace and internal
battery resistances can account for an additional 5% to
10% efficiency degradation in portable systems. It is
very important to include these “system” level losses
during the design phase. The internal battery and fuse
resistance losses can be minimized by making sure that
C_IN has adequate charge storage and very low ESR at the
switching frequency. A 25W supply will typically require
a minimum of 20μF to 40μF of capacitance having a
maximum of 20mΩ to 50mΩ of ESR. The LTC3707 2-
phase architecture typically halves this input capacitance
requirement over competing solutions. Other losses
including Schottky conduction losses during dead-time
and inductor core losses generally account for less than
2% total additional loss.
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