LTC3729L-6 Datasheet, PDF (20/28 Pages) Linear Technology – PolyPhase, Synchronous Step-Down Switching Regulator

LTC3729L-6
APPLICATIO S I FOR ATIO
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 percentage
of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC3729L-6 circuits: 1) LTC3729L-6 V_IN current
(including loading on the differential amplifier output),
2) INTV_CC regulator current, 3) I²R losses and 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 cur-
rents; the second is the current drawn from the differential
amplifier 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 current. In
continuous mode, I_GATECHG = (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 the ratio
(Duty Factor)/(Efficiency). For example, in a 20V to 5V
application, 10mA of INTV_CC current results in approxi-
mately 3mA 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 resistor,
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 approxi-
mately 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)=10mΩ, R_L=10mΩ, and R_SENSE=5mΩ, then the
total resistance is 25mΩ. This results in losses ranging
from 2% to 8% as the output current increases from 3A to
15A per output stage for a 5V output, or a 3% to 12% loss
per output stage 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 increas-
ingly 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 only when operating at high input voltages (typically
20V or greater). Transition losses can be estimated from:
Transition Loss ≈ V_IN2 (I_O(MAX) / 2) (R_DR) C_MILLER 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 in the
design of a system. The internal battery and input fuse
resistance losses can be minimized by making sure that
C_IN has adequate charge storage and a very low ESR at the
switching frequency. A 50W supply will typically require a
minimum of 200µF to 300µF of capacitance having a
maximum of 10mΩ to 20mΩ of ESR. The LTC3729L-6
PolyPhase architecture typically halves to quarters 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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