LTC3829 Datasheet, PDF (33/40 Pages) Linear Technology – 3-Phase, Single Output Synchronous Step-Down DC/DC Controller with Diffamp

LTC3829
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 LTC3829 circuits: 1) IC V_IN current, 2) INTV_CC
regulator current, 3) I²R losses, 4) topside MOSFET
transition losses.
1. The V_IN current is the DC supply current given in the
Electrical Characteristics table, which excludes MOSFET
driver and control currents. 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 =
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 EXTV_CC 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 application, 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 and current sense
resistor. 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 and R_SENSE to ob-
tain I²R losses. For example, if each R_DS(ON) = 10mΩ,
R_L = 10mΩ, 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 for a 5V
output, or a 3% to 12% 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. Other
losses including Schottky conduction losses during
dead time and inductor core losses generally account
for less than 2% total additional loss.
3829f
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